People infected with coronavirus are symptom-free for an average of five days, according to a study that reinforces the need for strict quarantine measures.
The analysis found that 5.1 days was the median length of time before people started showing signs of illness, although there was a wide range of incubation periods, with a tiny minority of people taking up to two weeks.
Justin Lessler, of the Johns Hopkins Bloomberg School of Public Health and senior author of the report, said: “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.”
The findings did not reveal the extent to which people can transmit the illness during this symptom-free period, but preliminary evidence suggests there is at least a short window before people start feeling ill when they can pass on the virus to others.
Lessler and colleagues found that about 98% of people who develop symptoms of Covid-19 will do so within 11.5 days of exposure. For every 10,000 individuals quarantined for 14 days, only about 101 would develop symptoms after being released from quarantine, the analysis estimated.
The study, published in the journal Annals of Internal Medicine, looked at 181 cases from China and other countries that were detected before 24 February, reported in the media, and included likely dates of exposure and symptom onset. Most of the cases involved travel to or from Wuhan, the Chinese city where the epidemic began, or exposure to other people who had been there.
The 5.1-day average incubation period is similar to that seen in Sars and Mers. Coronaviruses that cause common colds typically have a three-day symptom-free period after infection.
The NHS and 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.
The latest findings suggest this strikes a good balance between minimising chances of transmission and not causing unnecessary disruption – particularly as health workers are among those being quarantined.
“It’s very reassuring that by 14 days, while it might not be 100%, it will be close,” said Graham Cooke, professor of infectious diseases at Imperial College London.
Cooke added that people should not interpret the findings as a clear bill of health if they have not shown any signs of illness within five days of a potential exposure. “That’s absolutely the wrong interpretation,” he said. “At five days, half of people won’t yet have developed symptoms.”