Baltimore, MD—Pharmacists are fielding a lot of questions about the new coronavirus outbreak, including queries about the incubation period of SARS-CoV-2, the newly-identified virus that causes coronavirus disease, COVID-19.

A report in Annals of Internal Medicine offers an answer. Johns Hopkins Bloomberg School of Public Health researchers suggest that patients infected with the virus might be symptom-free for about 5 days on average but should expect to experience symptoms within 12 days. Those results are in line with current recommendations by the CDC to actively monitor patients for 14 days after an assumed exposure to SARS-CoV-2, according to the authors, who add that the time span could have important implications for informing control activities.

“This work provides additional evidence for a median incubation period for COVID-19 of approximately five days, similar to SARS,” the authors write. “Assuming infection occurs at the initiation of monitoring, our estimates suggest that 101 out of every 10,000 cases will develop symptoms after 14 days of active monitoring or quarantine. Whether this rate is acceptable depends on the expected risk for infection in the population being monitored and considered judgment about the cost of missing cases. Combining these judgments with the estimates presented here can help public health officials to set rational and evidence-based COVID-19 control policies.”

Background information in the article notes that, in December 2019, a cluster of severe pneumonia cases of unknown etiology was reported in Wuhan, Hubei province, China. Investigators attributed the illnesses to a novel strain of coronavirus belonging to the same family of viruses causing Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), and the four human coronaviruses associated with the common cold.

The authors point out that infection with the virus can be asymptomatic or result in mild-to-severe symptomatic disease. Until now, little has been understood about the incubation period for COVID-19.

To help provide guidance, the study team conducted an analysis of news reports, public health reports, and press releases from 50 provinces, regions, and countries outside of Wuhan, China, to estimate the length of the incubation period of COVID-19, the time from exposure to symptom onset. The study found information on 181 confirmed cases with identifiable exposure and symptom-onset windows. Based on the available data, the researchers estimated the median incubation period of COVID-19 to be 5.1 days, adding that 97.5% of patients who develop symptoms appeared to do so within 11.5 days of infection.

Researchers suggest that their estimates imply that, under conservative assumptions, 101 out of every 10,000 cases will develop symptoms after 14 days of active monitoring or quarantine, which supports current CDC recommendations.

The authors caution, however, that given recent evidence of SARS-CoV-2 transmission by mildly symptomatic and asymptomatic individuals, the latent period could be shorter than the estimated incubation period, with important implications for transmission dynamics.

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