Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Cholera Vaccines
HISTORY OF DISEASE
Cholera is a rapidly dehydrating, watery diarrheal disease caused by intestinal infection with the bacterium Vibrio cholerae serogroups O1 and O139. Cholera has probably existed on the Indian subcontinent for thousands of years. Ancient manuscripts describe the physical symptoms of what we now recognize as cholera gravis: rapid-onset vomiting, abdominal pain, explosive diarrhea, dehydration, and death. Cholera is one of the dreaded epidemic and pandemic diseases that have had the power to alter history. The disease has the unusual ability to rapidly infect large numbers of people, to spread internationally, and to kill a high proportion of those affected. Before the development of effective rehydration therapy with intravenous and oral fluids, cholera epidemics were associated with case-fatality rates that exceeded 40% and led to tens of thousands of deaths. John Snow is credited with understanding the importance of water as a key vehicle for the spread of the disease, but it was not until the early 1880s that Robert Koch isolated and defined the causative agent from the fecal specimen of a patient with the disease. ,
WHY THE DISEASE IS IMPORTANT
Cholera has been thought to exist on the Indian subcontinent since antiquity. From 1817 to 1923, six pandemics occurred worldwide. Earlier pandemics are known or presumed to have been caused by classical V. cholerae O1, while the seventh pandemic is ascribed to the El Tor biotype, which has replaced the classical biotype. Disease caused by V. cholerae O1 or O139 continues to be a serious threat to human health, particularly in the developing countries of Asia, Africa, and, recently, the Caribbean. Although formerly it was a major problem in South and Central America, relatively few cases have been reported in this region since 1998. Since the beginning of the seventh pandemic, cholera has become endemic in many parts of Africa, in Haiti, and recently in Yemen, causing large, recurrent, and deadly epidemics. In addition to the threat posed to residents of these regions, the disease is also a risk to travelers to these areas. Indeed, the cases of cholera reported to occur in developed countries are almost always acquired during foreign travel. ,
CLINICAL DESCRIPTION AND COMPLICATIONS
Severe cholera, or cholera gravis, is characterized by acute diarrhea and, usually, vomiting, which lead rapidly (within 4–18 hours) to moderate or profound dehydration. Typically, a previously healthy person is suddenly stricken with watery diarrhea and copious vomiting. Ten or more voluminous stools may be passed within a few hours, at first liquid in consistency and then becoming like rice water. The complications from cholera arise from the loss of fluid volume and electrolytes, especially sodium, potassium, and bicarbonate, in the stool and vomitus. These losses can result in hypovolemia, metabolic acidosis, and potassium deficiency. Secondary complications may arise from hypovolemia or from inadequate or inappropriate fluid and electrolyte replacement. Such complications can include renal failure, hypokalemia, arterial occlusions (especially in the elderly), pulmonary edema, and, in pregnant women, premature delivery or abortion. Most patients experience a decrease in blood glucose levels during treatment, but some young children experience profound hypoglycemia and seizures.
Not all patients with V. cholerae infections experience severe cholera gravis. Most infected people are asymptomatic or have only mild diarrhea. Past studies have estimated the case-to-infection ratio (number of symptomatic cases per number of people infected) to range from 1 in 3, to 1 in 100, depending on the geographic region, biotype, phase of epidemic, and size of inoculum. The nature of the epidemic appears to affect the case-to-infection ratio, because explosive epidemics tend to have more severe cases, probably because higher inoculum sizes result from the large number of infected persons whose stools contaminate drinking water sources.
BACTERIOLOGY
Formerly, epidemic cholera was caused only by toxigenic strains of V. cholerae serogroup O1. However, in 1992, another serogroup, O139, emerged as a cause of epidemic cholera in Asia, with a clinical syndrome and epidemiologic spread identical to infections caused by serogroup O1. This new O139 strain was derived from the El Tor biotype of O1 strains and was characterized by the presence of a capsule and a modified lipopolysaccharide (LPS). Thus, there are two serogroups of V. cholerae that can cause epidemic cholera, O1 and O139. The new serogroup is relevant to vaccine development because immunity to serogroup O1 Vibrio does not confer immunity to serogroup O139. Since its first recognition in Bangladesh and India, V. cholerae O139 has spread to other countries in Asia but not outside this region. The incidence of cholera caused by O139 strains waned after 1992, but in 2002 there was a resurgence of this serogroup in Bangladesh. In the Indian subcontinent, the serogroup continues to be isolated, though rather uncommonly, with variations in the proportion of cases caused by this strain depending on the season and location.
There are other serogroups of V. cholerae that can cause sporadic cases of diarrhea, or may cause local or systemic infections, but they do not cause epidemic disease. These are called non-O1–non-O139 V. cholerae (formerly called nonagglutinating [NAG] vibrios because they did not agglutinate with the O1 antiserum). Some strains of O1 or O139 V. cholerae , especially those found in the environment, do not produce cholera toxin. These also do not cause epidemic disease. Thus, the epidemic strains always belong to either O1 or O139 serogroups and also produce cholera toxin. The serogroup O1 V. cholerae is subdivided into two serotypes based on specific antigens in the O antigen: Ogawa and Inaba. V. cholerae O1 is also divided into two biotypes based on biochemical reactions and other phenotypic features: classical and El Tor. More recently, genetic markers for the classical and El Tor strains have been identified. Classical strains, which appear to cause more severe disease than El Tor strains, have largely disappeared in recent years, with the last case detected in Bangladesh in the mid-1990s. The current seventh pandemic strain is an El Tor biotype, and it includes both Ogawa and Inaba serotypes.
Variants of El Tor strains that share phenotypic characteristics of both El Tor and classical biotypes were originally described in Bangladesh, and they are termed hybrid biotype strains . Newer molecular markers have also revealed the emergence of cholera strains with phenotypic characteristics that classify them as El Tor, but that elaborate classical-type enterotoxin. These new strains, termed El Tor variants , have been found to have displaced typical El Tor cholera in Bangladesh since 2002. Subsequently, they have been found in Kolkata, India, and in Beira, Mozambique, where they have also displaced typical El Tor cholera, and elsewhere in Asia, in Africa, and in Haiti. It has been asserted that variant El Tor strains, like classical biotype strains, are associated with more severe disease than that caused by the earlier El Tor strains.
Phage typing was formerly used to further classify strains, but molecular methods are now used. Multilocus enzyme electrophoresis can distinguish between classical and El Tor strains and has grouped the toxigenic El Tor biotype strains into four major clonal groups, or electrophoretic types (ETs), representing broad geographic areas. These include the Australian clone (ET1), the U.S. Gulf Coast clone (ET2), the seventh pandemic clone (ET3), and the Latin American clone (ET4). In addition, a standardized ribotyping scheme for V. cholerae O1 and for O139 can distinguish seven different ribotypes. , It has also been proposed that the biotyping of El Tor be refined to account for the newly emergent hybrid biotype and El Tor variant strains. Recently, using advanced genomic sequencing, it has been possible to study in detail the molecular evolution and global spread of cholera in at least three distinct waves during the current seventh pandemic. Studies of the whole organism genomes revealed that the emergence of cholera in Haiti was derived from a Nepali strain, likely from Nepali UN peacekeeping forces in Haiti, and that the historic spread of cholera to the Americas and to Africa was via human carriage of cholera organisms from the Ganges delta. In aggregate, these findings have pinpointed the source of outbreaks in these regions as being human travel, rather than emergence from local environmental sources. Although invaluable for molecular epidemiology, these methods and improved classifications are not necessary in the clinical laboratory.
PATHOGENESIS AS IT RELATES TO PREVENTION
Crucial steps in the pathogenesis of cholera include colonization of the small intestinal mucosa and the elaboration of the enterotoxin (cholera toxin [CT]) ( Fig. 15.1 ). Colonization depends on production of pilus structures such as the toxin-coregulated pilus (TCP). Bacterial motility allows the bacteria to penetrate the mucous lining of the mucosa and come into close association with the epithelium. Colonization of the small intestine depends on three critical virulence factors: TCP, hemagglutinin/protease, and the bacterium’s single flagellum. Lee and colleagues presented a thorough discussion of the temporal expression of the virulence factors of V. cholerae and their role in pathogenesis.
If the bacteria colonize the mucosa in sufficient numbers, they are able to secrete quantities of CT close to the mucosa and thereby cause disease. CT, a protein with a molecular weight of 84,000, has a subunit structure consisting of a central active subunit (A subunit) and a surrounding pentameric binding subunit (B subunit). , The A subunit is responsible for the physiologic and toxic activity of the toxin, and the B subunit for the characteristic tight binding of toxin to GM 1 ganglioside receptors on the intestinal epithelial cell surface. The CT genes ( ctxAB ) are carried on a lysogenic filamentous phage of V. cholerae , called CTXPhi. CTXPhi can transfer between toxigenic and nontoxigenic strains of V. cholerae . CTXPhi integrates into the bacterial chromosome and forms stable lysogens. After attachment to the surface of the cells via the B subunit, the A subunit (or, more specifically, its released A1 polypeptide) of CT stimulates the enzyme adenylate cyclase, which initiates a cascade of biochemical events that lead to hypersecretion of chloride and bicarbonate with concomitant water loss following the resulting ionic gradient, far in excess of the absorptive capacity of the gut. The excess fluid is thus passed out as diarrheal fluid.
In cholera, the activity of CT is limited to the intestinal mucosa because CT is not systemically absorbed. Antibacterial and antitoxic intestinal secretory antibodies, primarily secretory immunoglobulin (Ig) A, act synergistically to prevent colonization by cholera vibrios and binding of CT, thereby preventing disease. The predominant antigen inducing protective antibacterial immunity is LPS. The data are contradictory about whether immune responses to TCP contribute to protective immunity. Protection against CT is directed toward its B subunit, not only protecting against cholera but also cross-protecting against diarrhea caused by heat-labile toxin (LT) producing enterotoxigenic Escherichia coli (ETEC).
MODES OF TRANSMISSION
Transmission occurs through consumption of contaminated water or food and is often the result of combined contamination. Contamination may be fecal or from a natural marine reservoir of the organism. Contaminated water, for example, is often used to rinse fresh food, and thus the food becomes contaminated, serving as the vehicle of transmission. Contaminated food, if kept at room temperature, supports the growth of V. cholerae and may lead to common-source outbreaks. ,
DIAGNOSIS
The diagnosis of cholera can be suspected clinically but is confirmed by culturing the organism from a fecal culture. V. cholerae is preserved well in routine fecal transport media, although Cary-Blair medium is preferred. Primary bacteriologic plates should include selective media such as thiosulfate-citrate-bile salts-sucrose or taurocholate-tellurite-gelatin agar. , Bacteriologic confirmation of initial cases in a region is essential because of the epidemic implications, but during major epidemics, laboratory confirmation is not needed except on a sample basis. Rapid immunologic diagnostic tests, using immunochromatographic dipsticks, have been developed for both O1 and O139 V. cholerae strains. , These tests have acceptable sensitivity, but have suffered from poor specificity, although specificity of these tests appears to increase after a 6-hour enrichment period of samples in alkaline peptide water. A more recent rapid lateral-flow assay, using a monoclonal antibody to the O-specific antigen of V. cholerae 01 lipopolysaccharide, was found to have sensitivity and specificity above 95% when used for patients with acute watery diarrhea in Bangladesh. These tests will have greatest application as point of care diagnostics, especially in epidemic contexts where sensitivity and speed are greatly valued. Newer molecular methods, such as polymerase chain reaction (PCR) are increasingly being developed, but still are not practical for most situations where a reliable cholera diagnostic test result is needed quickly.
Clinical management of the individual case does not depend on laboratory confirmation because treatment is aimed at rehydration and only secondarily toward eradicating the bacteria. Acute and convalescent sera can be tested for vibriocidal or agglutinating antibodies to confirm the diagnosis of cholera caused by serogroup O1; a fourfold rise in titer is diagnostic of recent cholera infection. A similar assay is possible for serogroup O139, but it is more difficult because the capsule found on this strain makes it less sensitive to complement-mediated killing, and the assay is performed in only a few research laboratories. The O139 vibriocidal assay generally uses as the target strain a mutant strain lacking the capsule. Antitoxin antibodies are also stimulated during infection and can be measured; however, this measurement is not completely specific because other vibrios, as well as enterotoxigenic Escherichia coli (ETEC), can produce an immunologically similar cholera-like toxin and stimulate antitoxin responses. In general, acute and convalescent sera are not needed to diagnose an individual patient; however, the measurement of serum titers can be helpful during epidemiologic evaluations.
CASE MANAGEMENT
Appropriate case management of diarrhea using intravenous and oral rehydration solutions has reduced deaths from diarrhea dramatically over the last 20 years. Oral rehydration salt solution should be considered as the first treatment for cholera, although intravenous fluids with a polyelectrolyte solution (e.g., Ringer lactate) are needed in cases of severe dehydration or shock. The most important elements of therapy are the rapid replacement of fluids to correct the dehydration, base administration to correct the acidosis, and provision of potassium to correct the potassium deficiency. With proper treatment, no patient should die of cholera, but if treatment is delayed, given too slowly, or inappropriately, case-fatality rates exceed 5%, even today.
In severe cholera cases, an effective antibiotic can be used to reduce the volume of diarrhea and the length of time the organism is excreted in the stool. Doxycycline or tetracycline is the antibiotic of choice for sensitive strains, but resistance to this class of drug may occur. Unlike many other enteric organisms, the predominant strains often lose their resistance, so antibiotic resistance is a characteristic that varies from time to time and from place to place. , Thus, one must monitor antibiotic resistance during cholera epidemics and choose another antibiotic when necessary. Other clinically effective antibiotics include ciprofloxacin, cotrimoxazole, erythromycin, azithromycin, chloramphenicol, and furazolidone.
EPIDEMIOLOGY
Incidence
Endemic cholera denotes the repeated occurrence of cholera in a population over time, without the need for exogenous reintroduction of the pathogen. Practically speaking, the World Health Organization (WHO) defines a cholera-endemic population as one that has experienced cholera in at least 3 of the past 5 years. In such populations, cholera incidence may still vary considerably from year to year. For example, in Matlab, Bangladesh, the annual incidence of treated cholera episodes varied from 0.2 cases to 5.1 cases per 1000 population over a 15-year period. In cholera-endemic populations, the incidence of cholera is typically greatest in children younger than 5 years, presumably because of lower levels of preexisting natural immunity to cholera in this young age group, reflecting fewer past exposures to cholera vibrios. It had been thought that infants and young toddlers were at low risk in these populations because of the protective effects of breastfeeding, , but studies in Kolkata, India, and in North Jakarta, Indonesia, have shown a very high incidence in this age group. An important recent observation is that even in countries with endemic cholera, cholera does not affect entire populations but appears to be concentrated in smaller populations, or “hot spots.” This observation implies that a focus of interventions for cholera in these hot-spot populations, such as vaccination and provision of improved water, sanitation, and hygiene (WASH), may be sufficient to interrupt cholera transmission in the much larger regions in which the hot spots are located. , ,
In contrast to endemic cholera, the term epidemic cholera refers to cholera that requires exogenous introduction of cholera vibrios into a population and is not recurrent in time and place. In such populations, which are typically immunologically naïve to cholera because of a lack of past exposures, incidence rates of cholera are usually age independent, and the clinical spectrum of cholera illnesses tends to be more severe. This is well illustrated by the recent outbreak of cholera in Haiti, where high rates have been observed in adults and case-fatality rates have been high, and where an investigation by a WHO team determined that accidental introduction of cholera by a local UN peacekeeping force from South Asia occurred in a setting with poor water-sanitation infrastructure and with a population with no background natural immunity to cholera. Although useful conceptually, the terms endemic cholera and epidemic cholera represent two ends of the epidemiologic spectrum, and, in practice, large outbreaks termed epidemics may occur in populations with endemic cholera.
Risk Groups
Cases of cholera occur in both children and adults. In addition to the consumption of specific high-risk foods, other risk factors for cholera include low socioeconomic status, which correlates with the use of impure water, poor sanitation, and poverty. Resettlement camps of internally or externally displaced persons or refugees also pose a key risk for cholera, with approximately 75% of such camps in Africa having an outbreak at some point. Similarly, populations are commonly at risk for cholera outbreaks following natural disasters, as illustrated by the recent major cholera epidemic in Haiti following a severe earthquake and a hurricane in close succession.
Beyond these environmental risk factors, several host factors influence susceptibility to cholera, including being nonbreastfed for infants and young toddlers and gastric hypochlorhydria. , , The protection infants receive from breastfeeding may result from antibodies in the breast milk, but it may also result from less exposure to contaminated food and liquids. Individuals of blood type O are more likely to develop severe illness, although this increased risk appears to apply to El Tor but not to classical cholera. A study in Mozambique implicated HIV coinfection as a risk factor for cholera, raising the possibility of the convergence of the ongoing pandemics of these two infections. Recent data also suggest that commensal flora of the gut microbiome influence host susceptibility to cholera after exposure to cholera vibrios. ,
Whether or not pregnancy is a risk factor for cholera is not clear, but cholera in pregnancy has been linked with high rates of abortions, stillbirths, premature childbirths, and maternal deaths. Fetal loss rates from cholera cases reaching up to 50% have been reported, for which the degree of dehydration and hypovolemia are the strongest factors associated with fetal loss.
Reservoirs of Infection
Although V. cholerae is generally thought to be spread by the fecal–oral route, it can also persist in environmental waters without continued contamination by human feces. During epidemics, environmental waters may become heavily contaminated with V. cholerae from infected human feces, and this contamination further spreads the epidemic. Once the surface waters become contaminated, an environmental reservoir may maintain the bacterium without further fecal contamination if appropriate conditions of salinity and temperature exist, as illustrated by the persistent environmental reservoir for V. cholerae O1 off the coast of Louisiana in the Gulf of Mexico. As demonstrated in the United States, the environmental reservoir can lead to additional sporadic primary cases from contaminated seafood, but if sanitation is adequate, secondary cases do not occur. If sanitation is poor, the sporadic cases may be followed by additional secondary cases and may lead to another epidemic through amplification of the inoculum and further contamination of the waters.
In the environment, the Vibrio organisms associate with certain plankton (known as copepods), chitinous shelled animals (e.g., crabs), and vegetation. These associations appear to be crucial for the long-term survival of Vibrio organisms, and they may also enhance horizontal gene transmission among vibrios. Importantly, chitin induces natural competence in V. cholerae , enabling bacterial uptake of extracellular DNA from the environment, thereby facilitating genetic transformation. Furthermore, the association with chitinous shells increases the risk of transmission via consumption of crabs, shrimp, and other shellfish, many of which are eaten only partially cooked. In some cultures, raw fish may also become a vehicle of transmission. Unfortunately, even if the fish are caught in uncontaminated waters, they may become contaminated when washed in harbor water as the catch is brought to market, and cooked food may become recontaminated in the kitchen with utensils exposed to uncooked food.
Significance as a Public Health Problem
An analysis of the annual global disease burden of cholera estimated that there are up to 4.0 million cholera cases and 143,000 cholera deaths in endemic countries, and 87,000 cases and 2500 deaths in cholera epidemics with more than half the cases detected in Africa. , More than 1 million cases occurred in Latin America during the first 3 years of the explosive 1991 epidemic, causing approximately 9000 deaths. However, the number of reported cases greatly diminished after 1998, and the last reported cholera death occurred in 2001. ,
Cholera is often underreported to WHO. For example, the actual rates of cholera in Bangladesh, which does not report cholera to WHO, have generally exceeded two per 1000 population annually, suggesting that approximately 300,000 cases occur in this country alone. Bearing in mind these limitations in reporting, the worldwide occurrence of massive and prolonged outbreaks seems to be increasing. Since 2005, such outbreaks, each with a total of more than 10,000 cases, were reported in Afghanistan, Angola, DRC, Ethiopia, Ghana, Guinea Bissau, Haiti, Iraq, Kenya, Nigeria, Sierra Leone, Somalia, South Africa, South Sudan, Yemen, Zambia, and Zimbabwe. The outbreak in Haiti, which began in October 2010, has been responsible as of this writing for approximately 730,000 reported cases and 9000 deaths, and that in Yemen, which began in 2016, has reported more than 2.3 million cases and 3900 deaths.
In addition to the endemic and epidemic cases occurring in developing countries, sporadic cases occur along the U.S. Gulf Coast and are often associated with undercooked shellfish, especially crabs. After the U.S. Gulf Coast hurricanes of 2005, two cases of toxigenic V. cholerae infection that occurred in a Louisiana couple were attributed to consumption of undercooked or contaminated seafood. Most other cases in the United States and other developed countries arise as a consequence of travel to endemic areas.
IMMUNITY AND IMMUNE MECHANISMS
Epidemiologic data from cholera endemic settings as well as studies in experimentally infected volunteers have clearly shown that a clinical cholera infection confers significant protection for at least 3 years against a new cholera episode. Immune protection appears to be more pronounced after cholera caused by the classical biotype than after El Tor cholera, and more long-lasting after Inaba than after Ogawa serotype infection. Also repeated, usually nonclinical natural exposure to V. cholerae O1 can confer a level of immune protection. This is evident from many epidemiologic studies in cholera endemic settings demonstrating a marked inverse relationship between cholera incidence and age. Such age-related decreased susceptibility in endemic settings is in sharp contrast to the practically age-independent attack rate found when cholera outbreaks occur in previously nonexposed populations, such as the massive recent cholera outbreak in Haiti beginning in 2010.
Seroepidemiologic studies have shown that in cholera endemic areas vibriocidal antibodies, which are largely IgM antibodies against the bacterial cell wall lipopolysaccharide (LPS) O antigen, increase with age and that the risk of disease is inversely proportional to the vibriocidal antibody titer. , However, it should be noted that the vibriocidal antibodies are not the protective immune effector molecules, but only a surrogate marker for the intestinal mucosal immune status. Thus, as discussed further below, parenteral vaccines confer only limited and short-lived protection even though they can induce extremely high serum vibriocidal antibody titers.
Instead, immune protection in cholera is mediated by locally produced secretory IgA antibodies in the gut directed against mainly the bacterial cell wall LPS O antigen and the B subunit part of the cholera toxin. The O1 LPS contains major group-specific A epitopes shared between the Inaba and Ogawa serotypes and in addition less prominent serotype-specific B epitopes (Ogawa) or C epitopes (Inaba). Antibodies against mainly the cross-reactive but also the serotype-specific epitopes contribute to protection. Similarly, protective immunity against V cholerae O139 also seems to be mediated predominantly by antibodies to the O139 LPS. The protective relevance, if any, of other antibacterial antibodies, including antibodies against the colonization-promoting toxin coregulated pilus antigen (TCP) or fimbrial protein antigen remains to be defined.
Findings from studies in both animals and human beings have shown a protective role also for intestinal IgA antitoxic immunity and specifically for mucosal IgA antibodies directed against the B subunit moiety of the toxin. An important observation guiding the design of the whole-cell plus cholera toxin B subunit oral cholera vaccine was the synergistic protective effect of intestinal antibodies against LPS and cholera toxin B subunit.
Mucosal immunologic memory providing rapid anamnestic SIgA responses to renewed exposure to V. cholerae LPS and toxin antigens is of critical importance for long-term protection after cholera infection or vaccination. Thus, while the acute SIgA anti-LPS and anti-CTB responses peak within the first 7–30 days and have largely vanished after 6–9 months significant immune protection lasts for at least 3 years after either a clinical cholera episode or effective OCV immunization. In contrast, intestinal mucosal immunologic memory is of very long duration, as evidenced by rapid strong SIgA responses induced by single-dose OCV boosting in Swedish volunteers as long as 10–14 years after an initial two-dose OCV immunization. ,
PASSIVE IMMUNIZATION
Passive immunization against cholera, although theoretically possible, is not used and would not be practical as a public health tool. Natural passive immunization in the form of breast-feeding reduces the risk of cholera in the breast-fed child, and a study in Bangladesh found that the magnitude of the effect correlated with the levels of specific SIgA antibodies against the LPS O as well as cholera toxin B subunit antigens.
ACTIVE IMMUNIZATION
History of Vaccine Development
The development of vaccines against cholera dates to shortly after the discovery of the bacterial etiology of cholera by Robert Koch in the late 19th century. In the late 20th century, a modern vaccine against cholera was one of the first to exploit the new tools of molecular biology. Yet despite these efforts, cholera remains an important public health problem in much of the world. Until recently, vaccines against cholera have generally not been used to control endemic or epidemic disease. This is in part because interest in using limited resources to improve water quality and sanitation was greater than in using resources to vaccinate populations.
Preparations of parenteral killed whole-cell cholera vaccine were produced shortly after Koch’s discovery. In 1884 in Spain, Ferran produced a killed bacterial vaccine and inoculated thousands of people in an area experiencing an epidemic at the time. Of those inoculated, 1.3% came down with cholera, compared with 7.7% of those who were not vaccinated.
Shortly after Ferran, Haffkine began working on a cholera vaccine. His work was stimulated by the cholera epidemics in his native Russia; however, unable to return to his homeland, he went to India, where he began giving vaccines to large numbers of persons living in the Delhi and Kolkata areas. He became convinced of the success of his vaccine in 1894 when none of the 116 immunized persons in a Calcutta slum developed cholera, whereas nine cases were reported among the 84 unimmunized. The popularity of the vaccine grew during the early part of the 1900s as the cholera problem continued and no effective therapy existed. Notable is the account of Russell, who carried out large-scale trials of parenteral cholera vaccines in the 1920s. When vaccine efficacy was assessed in a trial in which more than 8000 persons received two doses and 17,000 received one dose of the parenteral vaccine, and 25,000 persons were not immunized, the vaccines were associated with a protective efficacy of approximately 80% during a 3-month follow-up period. Similarly, in further studies involving up to 3 million persons in uncontrolled trials in India, the injectable vaccine appeared to show excellent efficacy. As well, there were numerous anecdotal reports concerning the effectiveness of the parenteral vaccine.
A killed oral cholera vaccine (OCV) was also tested by Russell in the 1920s in India. In retrospect, this was an especially interesting vaccine because of developments in the 1980s with another killed OCV (described later). The first oral vaccine, termed Bezredka’s bilivaccine because it was developed at the Pasteur Institute by Bezredka and contained ox bile in addition to killed V. cholerae O1, was tested in India along with the parenteral vaccine described earlier. The oral vaccine provided protection that was approximately equal (82%) to that of the injectable vaccine and appeared to be highly protective against cholera-related deaths. Possibly because of the bile, the vaccine also caused diarrhea in some persons, and thus it was not acceptable to potential recipients who feared that the vaccine teams were in fact spreading cholera. Unfortunately, this apparent success with an OCV was not pursued for more than 50 years.
Because of the apparent efficacy of vaccines from these early but poorly designed studies, the panic that accompanied cholera epidemics, and the lack of consistently effective treatment, the parenteral vaccines became widely used. For expatriates, such as colonists living in areas that were endemic for cholera, this was probably wise at the time, especially in the absence of safe water and refrigeration. The requirement to receive booster doses every 6 months was not a serious constraint for these expatriates, because cholera was an ever-present risk, and the vaccine probably prevented many cases during this early colonial period. With the accompanying panic stimulated by cholera epidemics, many countries began requiring proof of vaccination for travelers crossing international boundaries, in the mistaken belief that vaccination would prevent the international traveler from spreading the bacterium between countries.
The parenteral killed whole-cell cholera vaccine ultimately fell from favor for several reasons. First, during the 1960s, several controlled studies from East Pakistan (now Bangladesh), India, the Philippines, and Indonesia showed that the vaccine had only limited efficacy (approximately 50%) and that protection was brief in duration. , Some vaccine preparations were associated with higher efficacy, but these tended to be associated with higher rates of side effects. Side effects were similar to those of killed whole-cell typhoid vaccines, although perhaps somewhat less severe. More important, however, was that the injectable vaccine had to be given frequently (every 6–12 months) to maintain clinically significant protection. Resources for intensive vaccination programs were not available in areas endemic for cholera and, if mobilized, would have taken resources from other more effective interventions. Killed parenteral whole-cell vaccines are no longer recommended by the WHO, and the manufacture and sale of these vaccines in the United States have been discontinued.
Despite the demise of conventional, parenterally administered killed whole-cell cholera vaccines, interest in cholera vaccines continued, but it shifted to the development of orally administered vaccines. The oral approach was motivated by several observations. Persons who have been infected with V. cholerae develop a protective immune response. This conclusion is based on both volunteers who were challenged 5 years after an original experimental challenge with virulent V. cholerae O1 and on analysis of cohorts from endemic areas of Bangladesh. , There is some evidence that classical strains may induce a more solid immunity than El Tor strains. Also, with the recognition that immune protection against cholera is the result of intestinal secretory antibacterial and antitoxic antibodies came the observation that oral administration of antigens is the most efficient method of eliciting this mucosal immunity. , , , In addition to this biological rationale, oral vaccines also have the major advantages of being easy to administer, not requiring physicians or nurses, and being free of the risk of needle-borne infections. Today’s licensed OCVs consist either of killed organisms and antigens, or of live, genetically attenuated strains.
Immunization With Modern, Licensed Oral Vaccines
In recent years, three oral vaccines, two killed and one live, have been developed and licensed in various countries ( Table 15.1 ). A killed vaccine known as Dukoral consists of V. cholerae O1 killed whole cells combined with the B subunit of CT (CTB); another vaccine consisting only of killed whole cells, but of both the O1 and O139 serogroups, goes by the trade names Shanchol in India, mORCVAX in Vietnam, and Euvichol (and Euvichol Plus) in Korea. The live vaccine, formerly produced as Orochol (also known as Mutacol in the Americas) and recently licensed as Vaxchora, is derived from a strain of V. cholerae O1 from which the active moiety of the CT gene ctxA is deleted. These are summarized in the following paragraphs.
TABLE 15.1
Comparison of Killed WC-rCTB Vaccine, Live CVD103-HgR Vaccine, and Reformulated Killed WC-Only Licensed Vaccines
| Characteristic | WC-rCTB | CVD103-HgR | Reformulated WC-Only |
|---|---|---|---|
| Commercial name | Dukoral | Vaxchora (formerly produced as Orochol and Mutacol) | Vietnam: mORCVAX India: Shanchol Korea: Euvichol and Euvichol Plus |
| Vaccine type/composition | Killed whole cells of Vibrio cholerae O1 (Inaba and Ogawa, classical and El Tor) plus recombinant cholera toxin B subunit (monovalent) | Live attenuated V. cholerae O1 classical Inaba strain 596B (monovalent) | Killed whole cells of V. cholerae O1 (Inaba and Ogawa, classical and El Tor) and O139 (bivalent) |
| Regimen | Two doses given 7–14 days apart (three doses for children 2–5 y) | One dose | Two doses given at least 14 days apart |
| Age range for vaccination | ≥2 y | 18–64 y | Vietnam: ≥2 y India and Korea: ≥1 y |
| Duration of protection | 2 y (6 mo for children 2–5 y) | 3 mo (established only in North American volunteers) | At least 5 y |
| Booster dose requirements | Every 2 y (every 6 mo for children 2–5 y) | Unknown | Every 3–5 y |
| Requirement for oral buffer | Yes | Yes | No |
| Storage temperature | 2–8°C | −25°C to −15°C | 2–8°C |
| Shelf life | 3 y | 18 mo | 2 y |
| International acceptance | WHO prequalified | Not prequalified by WHO | WHO prequalified |
| Price to the public sector per dose | Will depend on production volume (2008 negotiated price to WHO: ∼$5.25/dose for small quantity of 250,000 doses) | Vaccine price not currently available | Vietnam: ∼$0.75/dose India: $1.85 per dose Korea: Unknown |
WC, whole cell; WC-rCTB, whole cell–recombinant cholera toxin B.
Modified from Abramson JS, Chair; Ad-hoc Cholera Vaccine Working Group. Background Paper on the Integration of Oral Cholera Vaccines Into Global Cholera Control Programmes. Presented to the WHO Strategic Advisory Group of Experts (SAGE) in October 2009. Final draft available at: http://www.who.int/immunization/sage/1_Background_Paper_Cholera_Vaccines_FINALdraft_13_oct_v2.pdf .
Killed Whole-Cell Vaccine Plus Cholera Toxin B Subunit
Constituents of the Vaccine
This vaccine contains a mixture of recombinant CTB plus killed strains of V. cholerae O1 representing both serotypes (Inaba and Ogawa) and both biotypes (El Tor and classical). Two of the strains are heat-killed (classical Inaba strain Cairo 48 and classical Ogawa strain Cairo 50) to better express the LPS antigens, and two are formalin killed (El Tor Inaba strain Phil 6973 and classical Ogawa strain Cairo 50) to better preserve the protein antigens. The CTB is produced by a genetically engineered V. cholerae strain that hyperproduces this antigen. The vaccine is free of the cholera toxin A (CTA) subunit and its toxicity. The recombinant CTB retains its ability to bind to the GM 1 ganglioside of the cell membrane. Because the heat-LT of toxigenic E. coli cross-reacts with CT, this antigen provides cross-protection against diarrhea caused by LT-ETEC. As a killed vaccine, whole cell–recombinant cholera toxin B (WC-rCTB) has no potential for genetic reversion to virulence. CTB is sensitive to stomach acid, so the vaccine is given with a buffer. The protective whole-cell antigens, and especially LPS, are not acid sensitive.
The vaccine is prepared as a liquid formulation, and it comes as a whitish, 3-mL suspension in a single-dose glass vial. Each dose of the vaccine consists of a total dose of 10 11 killed bacteria with 1 mg of recombinant CTB. The vaccine vial comes with a sachet of buffer consisting of sodium hydrogen carbonate, citric acid, sodium carbonate, saccharin, sodium citrate, and raspberry flavor. The buffer is supplied as white effervescent granules and is to be dissolved in a glass of water, to which the vaccine is added and then taken by mouth.
Manufacture of the Vaccine
Batches of V. cholerae 01 strains are grown in large fermenters. The harvested bacteria are then killed with either formalin or heat, and the different batches are combined to make a mixture of the batches in equal concentrations of bacteria. All fermentation, harvesting, and mixing, and all transfers between these steps are performed in a closed, steam-sterilizable system to facilitate processing and to eliminate the risk of contamination. The CTB is prepared from a strain of V. cholerae O1 that is genetically engineered to produce this antigen, but it lacks the genes for producing cholera A subunit. After fermentation, CTB is purified and then added to the whole-cell vaccine.
Producer
WC-rCTB (Dukoral) is produced by SBL Vaccin AB (now owned by Valneva) and is packaged in a two-dose package with two unit-dose vials of vaccine and two sachets with buffer. WC-rCTB is registered in more than 60 countries, but it remains experimental in the United States. It has been prequalified by WHO for purchase by United Nations agencies.
Dosage and Route
For adults, the vaccine is given as two oral doses, coadministered with liquid oral buffer, with an interval of 1–2 weeks. Children 2–6 years of age should receive three doses with an interval of 1–2 weeks between doses. For persons at continued risk for cholera, booster doses should be given every 6 months to children 6 years and younger, and every 2 years to older persons.
Availability in Combinations
The vaccine is not available in combination with any other vaccine.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here