As case numbers around the world continue to rise, scientists are working hard to better understand the new coronavirus, SARS-CoV-2, that causes the respiratory illness COVID-19. The latest research to be published has estimated a median incubation period, ie the time between exposure to the new coronavirus and when symptoms are displayed, of 5.1 days. This means it takes on average five days before people show any signs they may have been infected after coming into contact with someone who has the virus, lending support to the recommended 14-day quarantine period that is currently being used by the US Centers for Disease Control and Prevention and other public health bodies.
Led by researchers at Johns Hopkins Bloomberg School of Public Health, the study analyzed data from media reports of 181 cases of the new coronavirus from China and other countries, which were detected prior to February 24. From this data, they inferred that about 97.5 percent of people will develop symptoms of the infection within 11.5 days of exposure.
Further estimates in the study published in the journal Annals of Internal Medicine suggest that under conservative assumptions 101 out of every 10,000 cases will develop symptoms after 14 days of active monitoring or quarantine.
“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,” study senior author Justin Lessler, an associate professor in the Bloomberg School’s Department of Epidemiology, said in a statement.
Therefore, the authors suggest that “longer monitoring periods might be justified in extreme cases.”
The authors also noted that their estimated median incubation period of five days for COVID-19 was similar to that of SARS, a different human-infecting coronavirus that caused a major outbreak from 2002-2004. However, coronaviruses that cause common colds have a mean illness-incubation time of around three days.
Understanding the disease incubation period can help epidemiologists gauge the likely dynamics of the outbreak, and inform public health officials who are designing effective quarantine and control measures. Whilst the incubation estimates from this study support the estimates from earlier research of the virus, the authors caution that the period may in fact be shorter.
Their estimates assume people become infected as soon as they come into contact with the virus, but that may not always be the case. Furthermore, the publicly reported cases used in the study may over-represent severe cases, therefore the incubation period for those with mild cases may differ.
“I think it is really important when dealing with this outbreak that we fully understand the limitations of studies and their findings and also base any intervention or policy on norms, not extremes,” Jonathan Ball, Professor of Molecular Virology at the University of Nottingham, who was not involved in the study, pointed out. However, he continued that “as it stands there is little evidence to suggest that a quarantine or self-isolation period of 14 days is not suitable.”