New study reports on incubation period of COVID-19
Thursday, March 12, 2020
The global outbreak of SARS-CoV-2 infection emerged in December 2019 in Wuhan, a city of 11 million in central China, and has resulted in 118,322 officially confirmed cases around the world and 4,292 deaths from pneumonia caused by the virus, according to the March 11 Situation Report by the World Health Organization.
The first case of COVID-19 in the United States was reported on Jan. 21, 2020. Since then, every day brings new information about the spread of the virus. Although the virus that causes COVID-19 probably emerged from an animal source, we now know that the virus is spreading from person to person, mainly between people who are in close contact with one another (within about 6 feet) through respiratory droplets produced when an infected person coughs or sneezes.
Less commonly, people may also get COVID-19 by touching a surface or object contaminated with the virus and then touching their own mouths, noses, or possibly their eyes. The Centers for Disease Control and Prevention has been clear about precautions, including handwashing with soap and water for at least 20 seconds and using an alcohol-based hand sanitizer that contains at least 60% alcohol if soap and water are unavailable.
A new study on COVID-19, led by researchers at Johns Hopkins Bloomberg School of Public Health, estimates 5.1 days for incubation period and suggests that about 97.5% of people who develop symptoms of infection will do so within 11.5 days of exposure.
The researchers estimated that for every 10,000 individuals quarantined for 14 days, only about 101 would develop symptoms after being released from quarantine. These estimates imply that, under conservative assumptions, in 101 out of every 10,000 cases, people will develop symptoms after 14 days of active monitoring or quarantine.
For the study, the researchers analyzed 181 cases from China and other countries that were detected before Feb. 24, 2020, were reported in the media, and included likely dates of exposure and symptom onset. Most of the cases involved travel to or from Wuhan, China, or exposure to individuals who had been to Hubei, the province for which Wuhan is the capital. According to Justin Lessler, an associate professor in the Bloomberg School's Department of Epidemiology, the current recommendation of 14 days for active monitoring or quarantine is reasonable, although with that period some cases would be missed over the long term.
The new estimate of 5.1 days for the median incubation period of SARS-CoV-2 is similar to estimates from the earliest studies of this new virus, which were based on fewer cases. This incubation period for SARS-CoV-2 is in the same range as SARS-CoV, a different human-infecting coronavirus that caused a major outbreak centered in southern China and Hong Kong from 2002-04. For MERS-CoV, a coronavirus that has caused hundreds of cases in the Middle East, with a relatively high fatality rate, the estimated mean incubation period is 5-7 days.
This study provides additional evidence for a median incubation period for COVID-19 of approximately 5 days, like SARS. Study results support current proposals for the length of quarantine or active monitoring of persons potentially exposed to SARS-CoV-2, although longer monitoring periods might be justified in extreme cases.
Lessler and colleagues have published an online tool that allows public health officials and members of the public to estimate how many cases would be caught and missed under different quarantine periods.
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