What we know about epidemiology
COVID-19 is caused by a novel new coronavirus that shares some genetic overlap with the viruses that caused Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS).2 The first case of COVID-19 was reported on December 1, 2019, from Wuhan, China.1 Within a very short period of time the disease has spread throughout the world, and on March 11, 2020, the World Health Organization (WHO) declared the infection to be a true pandemic. The countries with the highest prevalence of COVID-19 include China, South Korea, Iran, Italy, France, Spain, and the United States. However, more than 100 other countries and regions have reported cases. As of the first week of April, approximately 1 million persons in the world have been diagnosed with COVID-19. Of those infected, slightly more than 50,000 deaths have occurred. At the time of this writing, 234,483 cases have been documented in the United States, and current estimates indicate that approximately 7% of the population in the country could become infected.1,3,4
The virus responsible for COVID-19 is a single-stranded, enveloped RNA virus. Like its counterparts that caused SARS and MERS, this virus originates in animals, primarily bats. The early cases seem to have resulted from patient contact with exotic animals displayed in the Huanan Seafood Wholesale Market.1
The virus is transmitted directly by respiratory droplets and by close surface-to-hand contact with infected respiratory secretions. The virus appears to remain viable on environmental surfaces for 1 to 3 days, although the degree of infectivity over time is not well delineated. With direct exposure to respiratory droplets, the infectivity is relatively high; approximately 2 to 3 individuals become infected as the result of contact with an infected patient. By contrast, the “reproduction number (R)” for influenza is closer to 1.2,5
Certain persons appear to be at increased risk for developing infection and becoming seriously ill2,6:
- persons older than age 60
- persons with underlying medical illness
- persons who are immunosuppressed.
The reported range in the case fatality rate (CFR) varies from 1% to 13%, with the higher rates concentrated in older patients with comorbidities.3 These initial reports of high CFRs may be misleading because in the initial phases of this pandemic many patients with mild or no symptoms were not tested, and, thus, the overall prevalence of infection is not clear. By way of comparison, the CRF for influenza A and B is about 0.1%.2
Of note, the number of reported cases in the pediatric population is low, and the outcomes in these individuals are much better than in the older population.2,3,6 At present, there are only two reports of COVID-19 in pregnancy; these two studies include 18 women and 19 infants.7,8 The frequency of preterm delivery was 50% in these reports. Sixteen of the 18 patients were delivered by cesarean delivery; at least 6 of these procedures were performed for a non-reassuring fetal heart rate tracing. No maternal deaths were identified, and no cases of vertical transmission occurred.
We must remember that the number of patients described in these two reports is very small. Although the initial reports are favorable, in other influenza epidemics, pregnant women have not fared so well and have experienced disproportionately higher rates of morbidity and mortality.2
Reported clinical manifestations
The incubation period of COVID-19 ranges from 2 to 14 days; the median is 5.2 days. Many patients with proven COVID-19 infection are asymptomatic. When clinical findings are present, they usually are relatively mild and include low-grade fever, myalgias, arthralgias, sore throat, mild dyspnea, and a dry nonproductive cough. Some patients also may experience diarrhea. Of course, these findings are also consistent with influenza A or B or atypical pneumonia. One key to differentiation is the patient’s history of recent travel to an area of high COVID-19 prevalence or contact with a person who has been in one of these areas and who is clinically ill.2,3,9,10
In some patients, notably those who are older than 65 years of age and/or who have underlying medical illnesses, the respiratory manifestations are more prominent.6 These patients may develop severe dyspnea, pneumonia, adult respiratory distress syndrome (ARDS), multiorgan failure, and septic shock. Interestingly, the more severe manifestations tend to occur during the second week of the illness. In this group of more severely ill patients requiring hospitalization, 17% to 29% develop ARDS, and 23% to 32% require admission to the intensive care unit.2,6
Pregnant patients who become severely ill may be at risk for spontaneous miscarriage and preterm labor. With profound maternal hypoxia, fetal heart rate abnormalities may become apparent. To date, no clearly proven cases of vertical transmission of infection to the newborn have been identified. However, as noted above, current reports only include 18 pregnancies and 19 infants.2,3,7,8,11
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