Coronavirus Incubation Period is About 5 Days, Study Estimates

The median incubation period for the novel coronavirus is about 5 days—which is similar to SARS, according to a paper published in the Annals of Internal Medicine.

Investigators from Johns Hopkins Bloomberg School of Public Health analyzed published information about the coronavirus in order to estimate the length of the incubation period from exposure to symptom onset. The investigators used news reports, public health reports, and press releases from 50 provinces, regions, and countries outside of the outbreak’s epicenter in Wuhan, China.

Current knowledge about the coronavirus and its incubation period is limited. However, both SARS and MERS are related to the coronavirus and estimates suggest patients can be asymptomatic or result in mild to severe symptoms with COVID-19. And one early study done in Wuhan based on 88 confirmed cases estimated that the median incubation period was 6.4 days, the study authors noted, with a range from 2 to 11 days. Another study using 158 cases of coronavirus estimated that the range was between 2 and 14 days, they added.

The study authors found information about 181 confirmed cases of coronavirus that had details about exposure and symptom onset windows. Of these cases identified prior to February 24, 2020, 69 were female, 108 were male, and 4 were of unknown sex. Cases were noted in 24 countries and regions outside of mainland China (108) as well as within provinces of China (73). Most of them had recent travel or residence of Wuhan, though some had evidence of exposure to travelers or others with known infections.

They assumed that exposure always preceded symptom onset and allowed for continued exposure among families known to be traveling together and other cluster-type transmission when the ordering of transmission was not clear.

Using that data, the investigators found that the median incubation period of coronavirus was about 5.1 days. Additionally, they said that 97.5% of those who develop symptoms did so within 11.5 days of infection.

“The incubation period can inform several important public health activities for infectious diseases, including active monitoring, surveillance, control, and modeling,” the study authors wrote. “Active monitoring requires potentially exposed persons to contact local health authorities to report their health status every day.”

The study authors believe this can extrapolate to mean that 101 out of every 10,000 cases will develop symptoms after 14 days of active monitoring or quarantine. This finding aligns with U.S. Centers for Disease Control and Prevention recommendations that say patients should be actively monitored for 14 days after an assumed coronavirus exposure.

“We have exclusively considered reported, confirmed cases of COVID-19, which may overrepresent hospitalized persons and others with severe symptoms, although we note that the proportion of mild cases detected has increased as surveillance and monitoring systems have been strengthened,” they said of their study’s limitations, noting that their analysis does not classify people as being at high, medium or low risk for being symptomatic. For their next analysis, the study authors have created an application that aims to estimate the proportion of missed coronavirus cases across active monitoring durations for up to 100 days at various population risk levels.

For the most recent case counts in the COVID-19 outbreak, check out the Contagion® Outbreak Monitor.