In a study of 181 patients with the novel coronavirus (COVID-19), the median incubation period was found to be 5.1 days, according to researchers. Furthermore, they established that 101 out of every 10,000 cases will develop symptoms after 14 days of active monitoring or quarantine.

These findings could affect current practices during the COVID-19 pandemic, which are largely based on the assumption of a 14-day incubation period.

The study authors noted that earlier research based on 88 cases in China found that incubation periods can range from 2.1 days to 11.1 days, whereas a different study of 158 cases found a median period of 5 days. These findings are largely in line with the incubation periods of other human coronaviruses, according to the researchers.

Gaining a more specific understanding of the COVID-19 incubation period could have notable benefits for public health recommendations, including active monitoring, surveillance, control, and modeling practices.

“Understanding the length of active monitoring needed to limit the risk for missing severe acute respiratory syndrome (SAR)- coronavirus (CoV)-2 infections is necessary for health departments to effectively use limited resources,” the study authors said.

The researchers collected data from 181 total cases with confirmed SARS-CoV-2 infection detected outside of Hubei Province prior to February 24, 2020. Of those included, 38% were female, 60% were male, and 2% were of unknown sex. The median age was 44.5 years.

Most of the cases (161) had a confirmed recent travel history to, or residence in, Wuhan, and others had evidence of contact with travelers from Hubei or people with confirmed infections. Notably, among the patients who developed symptoms in the community, the median time from symptom onset to hospitalization was just 1.2 days. 

Based on the data from these 181 cases, the authors estimated the median incubation period of COVID-19 to be 5.1 days. Furthermore, they found that fewer than 2.5% of infected patients will show symptoms within 2.2 days of exposure. For 97.5% of patients, the researchers found that symptom onset will occur within 11.5 days.

Because the period of possible exposure may be less defined within mainland China, the researchers also performed a separate analysis for cases detected outside the region. For these cases, the mean incubation period was 5.5 days. For cases detected inside mainland China, the median incubation period was 4.8 days.

Finally, the researchers used their findings to estimate the number of symptomatic infections that could be expected to be missed over the course of an active monitoring program. For a 7-day monitoring program, approximately 21.2 cases could be expected to be missed for every 10,000 high-risk people monitored. After 14 days, the authors said it is highly unlikely to miss further symptomatic infections among high-risk persons.

The authors noted several limitations to their study, including the fact that there may be inaccuracies in the reported timeline between patients’ exposure and symptom onset. Still, the authors said their work provides additional evidence for a median incubation period of approximately 5 days, a fact that they said could be used by public health officials to set evidence-based COVID-19 control policies.

REFERENCE
Lauer S, Grantz K, Bi Q, Jones F, et al. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application. Annals of Internal Medicine; March 10, 2020. https://annals.org/aim/fullarticle/2762808/incubation-period-coronavirus-disease-2019-covid-19-from-publicly-reported. Accessed March 18, 2020.