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COVID-19: Epidemiology, Clinical Manifestations, Diagnosis, Community Prevention, and Prognosis
Epidemiology
On December 31, 2019, the World Health Organization (WHO) was informed of a cluster of cases of pneumonia of unknown cause in Wuhan City, Hubei Province, China. On January 9, 2020, a novel coronavirus (named severe acute respiratory syndrome coronavirus-2 [SARS-CoV-2]) was identified in samples obtained from these patients, whose disease is now called coronavirus 2019 (COVID-19). Although the precise source of the COVID-19 pandemic is uncertain, data suggest recombination from several related species in bats in South China ( Chapter 335 ). SARS-CoV-2 likely jumped from a bat to an unknown animal, possibly animals being sold at wet markets to humans in Wuhan, China.
Despite aggressive mitigation strategies, the infection rapidly spread and, on March 11, 2020, the World Health Organization (WHO) declared COVID-19 a global pandemic. Cases have occurred in every country.
As of March 2023, the 1.1 million COVID-19 deaths in the United States represented the second highest number of deaths per capita in the world, trailing only Brazil. Total mortality rates in the United States during the COVID epidemic have been about 20% above expected, with excess deaths higher among non-Hispanic Blacks than among other ethnic groups. Overall, about one-third of the increase in all-cause U.S. deaths during the pandemic have been attributable to increases in non-COVID-19 deaths, from causes such as cardiovascular disease, cancer, Alzheimer disease, and diabetes.
The reported number of cases underestimates the overall burden of COVID-19. Seroprevalence surveys suggest that infections exceed the number of reported cases by approximately 10-fold or more. By March 2023, over 6.9 million deaths worldwide were reported from COVID-19, but COVID-19 deaths are likely underreported in some countries. Moreover, up to 12 million excess deaths occurred worldwide related to other medical problems (e.g., cardiovascular disease, cancer, strokes, and other infectious diseases) not adequately treated during the pandemic as well as from other indirect consequences of the pandemic. Worldwide, these 18 million excess deaths represented about a 15% increase in global deaths. By the third quarter of 2022, an estimated 96% of U.S. adults had SARS-CoV-2 antibodies: about 23% from infection alone, about 26% from vaccination alone, and about 48% from both infection and vaccination.
SARS-CoV-2 does not have features of an eradicable virus, because of the presence of animal reservoirs, because of its transmission from asymptomatic or presymptomatic individuals, and because its symptoms resemble those of other respiratory pathogens. The infection therefore is expected to become endemic, thereby leading to an ongoing need for vaccination and therapeutics.
The United States has the highest absolute number of documented COVID-19 deaths of any country worldwide for multiple reasons, including a relatively low rate of use of available vaccines compared with other countries of similar economic status. Moreover, COVID-19 has disproportionately affected racial and ethnic minority groups in the United States, with Black, Hispanic, and Native American individuals comprising a disproportionately high number of infections and deaths. As in other countries, indigenous populations (American Indian and Alaska Native) have the highest number of per capita COVID deaths in the United States. The factors that explain these disparities include more comorbid conditions, distrust of the medical system, and socioeconomic determinants of health, such as poverty, education, housing inequalities, use of public transportation, types of employment, and access to health care ( Chapter 4 ).
Transmission
SARS-CoV-2 is primarily transmitted directly from person to person through respiratory particles (droplet and, on occasion, aerosol). Transmission principally occurs when respiratory particles containing virus are released when the infected person coughs, sneezes, or talks in close contact (within approximately 6 feet or 2 meters) with an uninfected person. SARS-CoV-2 also can be airborne in enclosed, poorly ventilated spaces. By comparison, COVID-19 acquisition through contact with contaminated surfaces is not thought now to be a major route of infection. , The virus is shed in stool, but the role of fecal-oral transmission is minimal even though surveillance of wastewater can track the prevalence of COVID.
The risk of transmission is higher among household contacts (above 50%) than among work or social contacts (about 1.3%). For all contacts, prolonged (>30-minute) conversations or closer contacts (e.g., sharing a bedroom or vehicle) markedly increase transmission. Asymptomatic transmission is thought to account for about 20% of all transmissions. Transmission outdoors is much less common due to ventilation. For example, a study in Wuhan, China, showed that just 1 of over 7000 infections was linked to outdoor transmission. Indeed, ventilation has emerged as one of the most important nonpharmaceutical mitigation strategies for COVID-19. Finally, global rates of human infection are somewhat lower when the maximum ambient temperature is above 52˚ F and when the ultraviolet light index is higher.
It is also possible for humans to transmit SARS-CoV-2 to other mammals, and infection has been documented in about 30 species of animals, including dogs, cats, gorillas, farmed mink, ferrets, hamsters, rabbits, otters, mice, lions, tigers, deer, and other mammals ( Chapter 335 ). However, the risk these infected animals pose to humans is still being investigated.
After exposure, the incubation period for COVID-19 ranges from 2 to 14 days, with about 98% of cases having the onset of symptoms by day 12, and a relatively constant median of 5 days among persons without prior immunity or vaccination. Viral shedding appears to begin about 2 to 3 days before the onset of symptoms, to be greatest in the first week of infection, to be uncommon more than 5 days after the onset of symptoms, and to be very unusual by 10 days after the onset of symptoms in mild-to-moderate disease, 15 days in severely ill or immunocompromised patients, or 2 days after clinical resolution. Vaccination reduces the time of infectivity after a subsequent breakthrough infection, with implications for shortening the period of isolation when vaccinated individuals become infected.
As SARS-CoV-2 has evolved, variants have arisen ( Chapter 335 ). Most changes have little to no impact, but some mutations can change transmissibility, the severity of disease, the performance of diagnostic tests, the efficacy of therapeutics such as monoclonal antibodies, or the effectiveness of public health and social mitigation measures. If a variant demonstrates increased transmissibility, increased virulence, or immune evasion, it is classified as a “variant of concern.” The alpha (B.1.1.7), beta (B.1.351), gamma (P.1), delta (B.1.617.2), and omicron (B.1.1.529) variants have been designated as variants of concern, and omicron has a number of subvariants (BA.1, BA.2 [including its offshoot BA.2.12.1], BA.3, BA.4, and BA.5). For example, the delta variant was more transmissible than the alpha variant but equally virulent. The omicron variant is more transmissible than delta but less virulent.
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