New study casts doubt on 14-day quarantine standard for COVID-19

Keeping adult patients with coronavirus disease 2019 (COVID-19) in isolation for 14 days may not be enough, according to a new study. Extending the quarantine period to 18–21 days appears to be more effective in controlling the spread of the disease.

“Thus far, the incubation time studies have been focused on hospitalized patients who were highly selected based on medical conditions,” and have likely missed many patients who presented with mild symptoms and were not admitted, researchers said.

Drawing from official local health agencies in China, the research team curated an unselected cohort of 2,015 lab-confirmed COVID-19 patients, enrolling regardless of medical and hospitalization status. The sample was predominantly male and adult. More than a third were imported cases, commonly reporting a travel history to, or residence in, Wuhan. [medRxiv 2020;doi:10.1101/2020.03.15.20036533]

Viral incubation took anywhere from 1–9 days in majority of the adult participants, with most needing only 1–5 days. The resulting median incubation period was 7 days, 1.8 days longer than what had been established in previous reports. This was consistent even after disaggregating for sex. The full range of incubation periods in the overall cohort was 0–33 days. [N Engl J Med 2020;doi:10.1056/NEJMoa2001316; N Engl J Med 2020;doi:10.1056/NEJMoa2002032]

Children, in comparison, demonstrated a significantly longer median incubation period of 9 days (p=0.02).

In 233 patients (11.5 percent of the total sample), incubation of the virus took longer than the 14-day quarantine period set by the World Health Organization, with most needing an additional week.

“These data suggested that the official 14-day quarantine period only captured 88.5 percent of the population developing COVID-19,” the researchers said.

“[I]mplementation of the 21-day quarantine period for everybody will capture 98.3 percent of the cases,” they suggested, adding that extending the current isolation guidelines by 3 or 4 days would be enough to capture around 95 percent of cases.

Thirty-nine patients (2.0 percent of the total sample) had an incubation period of 0 days, and the researchers reported instances of asymptomatic transmission, indicating that the virus might be contagious even during incubation. Six positive COVID-19 patients were free of symptoms and were much younger than most of the other cases.

These findings should be considered in light of methodological limitations, including the exclusively Chinese study sample, which might have implications on the generalizability of the findings, and the lack of data from Hubei province, including Wuhan.

“The incubation period is essential in making intervention strategies to control infectious diseases,” the researchers said. “Thus, understanding of incubation time of a wide-range spectrum of COVID-19 for establishing an optimal quarantine period is crucial to designing public health interventions.”

“An extension of the adult quarantine period to 18 days or 21 days could be more effective in preventing virus-spreading and controlling the disease,” they added.

This study is a preprint and is still awaiting peer review.