New analysis estimates 5.1 days for COVID-19 incubation

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An analysis of publicly available data on infections from the new coronavirus, SARS-CoV-2, that causes the respiratory illness COVID-19 yielded an estimate of 5.1 days for the median disease incubation period, according to a new study led by researchers at Johns Hopkins Bloomberg School of Public Health. This median time from exposure to onset of symptoms suggests that the 14-day quarantine period used by the US Centres for Disease Control and Prevention for individuals with likely exposure to the coronavirus is reasonable.

The analysis suggests that about 97.5% of people who develop symptoms of SARS-CoV-2 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.

For the study, the researchers analysed 181 cases from China and other countries that were detected prior to 24 February, 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, the city at the centre of the epidemic, or exposure to individuals who had been to Hubei, the province for which Wuhan is the capital.

The CDC and many other public health authorities around the world have been using a 14-day quarantine or active-monitoring period for individuals who are known to be at high risk of infection due to contact with known cases or travel to a heavily affected area.

“Based on our analysis of publicly available data, 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,” says study senior author Justin Lessler, an associate professor in the Bloomberg School’s department of epidemiology.

The global outbreak of SARS-CoV-2 infection emerged in December 2019 in Wuhan, a city of 11m in central China, and has resulted in 95,333 officially confirmed cases around the world and 3,282 deaths from pneumonia caused by the virus, according to the World Health Organisation’s 5 March Situation Report. The majority of the cases are from Wuhan and the surrounding Hubei province, although dozens of other countries have been affected, including the US, but chiefly South Korea, Iran, and Italy.

An accurate estimate of the disease incubation period for a new virus makes it easier for epidemiologists to gauge the likely dynamics of the outbreak, and allows public health officials to design effective quarantine and other control measures. Quarantines typically slow and may ultimately stop the spread of infection, even if there are some outlier cases with incubation periods that exceed the quarantine period.

Lessler notes that sequestering people in a way that prevents them from working has costs, both personal and societal, which is perhaps most obvious when health care workers and first responders like firefighters are quarantined.

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 centred 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.

Human coronaviruses that cause common colds have mean illness-incubation periods of about three days.

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.

Abstract
Background: A novel human coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was identified in China in December 2019. There is limited support for many of its key epidemiologic features, including the incubation period for clinical disease (coronavirus disease 2019 [COVID-19]), which has important implications for surveillance and control activities.
Objective: To estimate the length of the incubation period of COVID-19 and describe its public health implications.
Design: Pooled analysis of confirmed COVID-19 cases reported between 4 January 2020 and 24 February 2020.

Setting: News reports and press releases from 50 provinces, regions, and countries outside Wuhan, Hubei province, China.
Participants: Persons with confirmed SARS-CoV-2 infection outside Hubei province, China.
Measurements: Patient demographic characteristics and dates and times of possible exposure, symptom onset, fever onset, and hospitalization.
Results: There were 181 confirmed cases with identifiable exposure and symptom onset windows to estimate the incubation period of COVID-19. The median incubation period was estimated to be 5.1 days (95% CI, 4.5 to 5.8 days), and 97.5% of those who develop symptoms will do so within 11.5 days (CI, 8.2 to 15.6 days) of infection. These estimates imply that, under conservative assumptions, 101 out of every 10 000 cases (99th percentile, 482) will develop symptoms after 14 days of active monitoring or quarantine.

Limitation: Publicly reported cases may overrepresent severe cases, the incubation period for which may differ from that of mild cases.
Conclusion: This work provides additional evidence for a median incubation period for COVID-19 of approximately 5 days, similar to SARS. Our 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.
Primary Funding Source: US Centres for Disease Control and Prevention, National Institute of Allergy and Infectious Diseases, National Institute of General Medical Sciences, and Alexander von Humboldt Foundation.

Authors
Stephen Lauer, Kyra Grantz, Qifang Bi, Forrest Jones, Qulu Zheng, Hannah Meredith, Andrew Azman, Nicholas Reich, Justin Lessle

Johns Hopkins Bloomberg School of Public Health

Annals of Internal Medicine abstract

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