Vaccines to Prevent Mpox

Incidence and Prevalence in Africa

From 1970 to 2021, human mpox cases occurred in individuals originating from Cameroon, Central African Republic, Cote d’Ivoire, DRC, Gabon, Liberia, Nigeria, Republic of Congo, and Sierra Leone; during the latter years, cases have been recognized in several countries despite decades of no reported human disease. Reasons for this are not fully understood but have been hypothesized to be multifactorial (e.g., human, animal, and environmental factors, and waning individual and population level immunity afforded by routine smallpox vaccination). During 2017–2018 a large outbreak occurred in urban centers within Nigeria (Clade II) further raising questions about the reason for the increased case counts, particularly in urban locations. Clade I historically occurred in central Africa and Clade II in west Africa, with only Cameroon reporting case-patients infected by both clades of the virus.

Occurrence Outside of Africa

Outside of Africa, cases of mpox had been identified in only 4 countries until 2022. In 2003, the United States experienced an outbreak of 46 confirmed and probable cases of human mpox attributed to imported wild mammals from Ghana. Those mammals had been co-housed in a facility with pet prairie dogs, which infected humans via direct contact. No secondary cases occurred. During 2018–2021, there were eight instances where travelers from Nigeria presented with illness in nonendemic countries (Israel [1], United Kingdom [4], United States [2], and Singapore [1]). Secondary cases (3) occurred in two instances; one involved a healthcare worker who may have been exposed while changing a hospitalized patient’s linens and the other involved two household contacts who had direct contact with the same patient or their contaminated fomites. ^,

In May 2022, a global outbreak occurred involving rapid human-to-human transmission in over 100 countries which predominantly (but not solely) affected gay, bisexual, and other men who have sex with men (GBMSM). Transmission via close contact during sexual activity has been attributed to many cases in this 2022 multinational mpox outbreak. Most viral genetic sequences belong to Clade II, variant B.1. A minor variant (Clade II, A.2) was also found. Both variants were identified in ill people exposed in an endemic country as early as 2021.

Animal Reservoirs and Modes of Transmission

Mpox is a zoonotic disease characterized by primary transmission from an infected animal to a human followed by secondary human-to-human transmission. The definitive animal reservoir species has not been identified but several species of small mammals, including African rodent species, demonstrate susceptibility to infection and are the likely reservoirs. Nonhuman primates are not the reservoir species of MPXV, although many primate species exhibit similar susceptibility and clinical presentation as humans.

The infected individual is considered infectious from the time of symptom onset until all lesions, including crusts, have healed and a fresh layer of skin is formed. There is limited evidence suggesting potential for infectiousness up to 4 days prior to symptom onset for some human mpox cases. Transmission may occur via contact with infectious respiratory secretions, bodily fluids, or lesions and generally requires close, sustained direct physical contact with a person infected with MPXV. Transplacental transmission may occur. Fomite contamination can pose an additional hazard, as OPXV are double-stranded DNA viruses that may retain viability on an environmental surface for extended periods of time. Because some persons may be at occupational risk of exposure to OPXV (including MPXV), the U.S. Advisory Committee on Immunization Practices (ACIP) recommends vaccination of certain laboratorians, healthcare personnel, and public health responders. ^,

Signs and Symptoms

Among cases prior to the 2022 multinational mpox outbreak, most clinical features of mpox have been similar to those of smallpox (discrete ordinary type or modified type) with the notable addition of lymphadenopathy exhibited by many mpox patients. Infections associated with the 2022 multinational mpox outbreak have features consistent with classic mpox but there are also several notable exceptions ( Table 87.1 ). Cases can be mistaken for alternate etiologies (e.g., herpes simplex virus, syphilis, molluscum contagiosum, drug eruptions, allergic reactions) or occur concurrently with other infections, most commonly sexually transmitted infections (2022 outbreak) or varicella (in endemic areas). Pain has been a particularly prominent symptom in the 2022 outbreak, particularly when lesions have occurred in the rectum ; lesions have often involved the genitals. For cases during the outbreak, the incubation period has been shorter than was previously recognized, and the prodrome has not reliably occurred. During the 2003 U.S. mpox outbreak, pediatric patients were more likely to be hospitalized (Clade II) ; historically, children have also been more likely to have severe disease from mpox Clade I. However, during the 2022 mpox outbreak, pediatric cases have been mild. More severe manifestations have occurred among moderately to severely immunocompromised persons, most commonly, those with advanced HIV infection (AIDS).