US study says coronavirus usually takes 5 days to show symptoms

Most people infected with the novel coronavirus will show symptoms within five days, and almost everyone will be symptomatic by 12, according to a new study by American researchers that examines the outbreak’s spread in China over the past few months.

The finding suggests the 14-day quarantine imposed by Israel and other nations for those who had contact with infected people is the correct length.

But, the authors warn, the virus is contagious before the symptoms appear, a key factor in the difficulty authorities in China and elsewhere have had in containing the pathogen, despite quarantine measures imposed by many governments.

“If somebody is in their incubation period [before symptoms appear], that is the window when somebody who’s already been infected can walk into the country and not be detected by symptom-based surveillance,” Johns Hopkins Bloomberg School of Public Health associate professor of epidemiology Justin Lessler, the author of the study, told CNN.

The research was published Monday in the journal Annals of Internal Medicine.

The five-day incubation period was discovered when Lessler examined over 180 coronavirus cases from areas without the pathogen in individuals who had just arrived from the disease’s early epicenter in Wuhan, China.

Examining that spread in the earliest days of the outbreak, when transmission was moving almost exclusively from Wuhan outward, allowed Lessler to set good estimates for the moment of infection — the time spent in Wuhan — and then to track the amount of time it took for the traveler to show symptoms. Most were symptomatic by five days. Only in the rarest cases did the incubation period last longer than 12.

“We have sort of a narrow window at the beginning of the epidemic to really tease out what’s going,” Lessler told CNN this week. “If it’s everywhere, you don’t know where people got infected.”

Both Israel and the US have used the two-week standard for quarantine ever since initial Chinese findings from the virus’s first major outbreak in Wuhan suggested it was an effective period for verifying if an individual was infected.

CNN noted in reporting on the research that it does not relate to when the virus is infectious.

Global health authorities have warned that the pathogen appears able to spread itself before symptoms appear — and not, as in many other diseases, only with the symptoms through droplets released in sneezes and the like.

“What matters is if a person can transmit before they are aware they might be infectious,” Harvard T.H. Chan School of Public Health associate professor of epidemiology Bill Hanage, who was not involved in Lessler’s study, told the network.

The answer “is an unambiguous yes,” based on the speed at which the disease appears to be spreading through population in dozens of countries — far too fast to be delayed till the end of the incubation period.

For most people, the virus causes only mild or moderate symptoms such as fever and cough. For some, especially the elderly and people with existing health problems, it can cause more severe illness, including pneumonia. Most people recover in a matter of weeks, as has happened with three-quarters of those infected in China.

The virus has infected more than 800 people in the US and killed at least 30, with one state after another recording its first infections in quick succession.

Israel, which had 76 coronavirus cases as of Wednesday, has imposed a mandatory two-week quarantine on anyone entering the country and banned public gatherings of more than 2,000 people, among other measures.