An analysis of data on infections from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel human coronavirus that causes the respiratory illness COVID-19, yielded an estimate of 5.1 days for the median disease incubation period. This median time from exposure to onset of symptoms suggests that the 14-day quarantine period used by the U.S. Centers for Disease Control and Prevention (CDC) for individuals with likely exposure to SARS-CoV-2 is reasonable.

In December 2019, a cluster of severe pneumonia cases of unknown cause was reported in Wuhan, Hubei province, China.

The initial cluster was epidemiologically linked to a seafood wholesale market in Wuhan, although many of the initial 41 cases were later reported to have no known exposure to the market.

A novel strain of coronavirus, SARS-CoV-2, belonging to the same family of viruses that SARS and MERS, as well as the 4 human coronaviruses associated with the common cold, was subsequently isolated from lower respiratory tract samples of 4 cases on January 7, 2020.

Infection with the virus can be asymptomatic or can result in mild to severe symptomatic COVID-19 disease.

There is limited support for many of its key epidemiologic features, including the incubation period for clinical disease, which has important implications for surveillance and control activities.

To estimate the length of the incubation period of COVID-19 and describe its public health implications, Dr. Justin Lessler from the Johns Hopkins Bloomberg School of Public Health and colleagues analyzed 181 cases from China and other countries that were detected prior to February 24, were reported in the media, and included likely dates of exposure and symptom onset.

Most of the cases involved travel to or from Wuhan or exposure to individuals who had been to Hubei province.

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.

The researchers estimated that for every 10,000 individuals quarantined for 14 days, only about 101 would develop symptoms after being released from quarantine.

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,” Dr. Lessler said.

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.

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

The team’s paper was published in the journal Annals of Internal Medicine.

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Stephen A. Lauer et al. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application Free. Ann Intern Med, published online March 10, 2020; doi: 10.7326/M20-0504