How long is the incubation period for COVID-19?

A new study provides more evidence for a median incubation period of about 5 days for coronavirus disease 2019 (COVID-19), which is similar to that applied for severe acute respiratory syndrome (SARS) in the past. This supports current proposals for the length of quarantine or active monitoring of individuals who are potentially exposed to SARS coronavirus 2 (SARS-CoV-2).

“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,” the researchers said. “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.” [Sci Rep 2018;8:1093]

A pooled analysis was conducted on reports of COVID-19 cases from 50 provinces, regions and countries outside Wuhan, Hubei province, in China between 4 January 2020 and 24 February 2020. Data on patient demographics and dates and times of possible exposure, symptom onset, fever onset, and hospitalization were obtained.

A total of 181 individuals with confirmed SARS-CoV-2 infection were analysed to estimate the incubation period of COVID-19. These patients had identifiable exposure and symptom onset windows. [Ann Intern Med 2020;doi:10.7326/M20-0504]

The estimated median incubation period of SARS-CoV-2 was 5.1 days (95 percent confidence interval [CI], 4.5–5.8 days). Of the patients who developed symptoms, 97.5 percent did so within 11.5 days (95 percent CI, 8.2–15.6 days). Under conservative assumptions, this indicated that 101 out of every 10,000 cases (99th percentile, 482) would develop symptoms after 14 days of quarantine or active monitoring.

“We found that the current period of active monitoring recommended by the US Centers for Disease Control and Prevention (14 days) is well supported by the evidence,” the researchers said. [www.whitehouse.gov/briefings-statements/press-briefing-members-presidents-coronavirus-task-force]

These results are also “broadly consistent” with estimates of the incubation period by other studies. [Lancet 2020;395:497-506; Euro Surveill 2020;doi:10.2807/1560-7917.ES.2020.25.5.2000062; J Clin Med 2020;doi:10.3390/jcm9020538; N Engl J Med 2020;doi:10.1056/NEJMoa2001316]

“Symptomatic disease is frequently associated with transmissibility of a pathogen,” the researchers said.

“However, given recent evidence of SARS-CoV-2 transmission by mildly symptomatic and asymptomatic persons, we note that time from exposure to onset of infectiousness may be shorter than the incubation period estimated here, with important implications for transmission dynamics,” they added. [Lancet 2020;395:514-523; N Engl J Med 2020;doi:10.1056/NEJMc2001468]

This study was limited by the potential over-representation of severe cases in publicly reported cases. The incubation period for these patients might differ from those with mild severity.

“Combining these judgments with the estimates presented here can help public health officials to set rational and evidence-based COVID-19 control policies,” the researchers said.

SARS-CoV-2, the virus responsible for COVID-19, was first identified in China in 2019. It has now affected nearly 152,000 persons across the globe, with more than 5,000 deaths recorded as of March 15.