The coronavirus' average incubation period is 5 days, a study shows — but in 1% of cases, it may last longer than the standard 2-week quarantine

Tayfun Coskun/Anadolu Agency via Getty ImagesNew York City on March 3.
  • The new coronavirus has killed at least 4,000 people and infected more than 116,000.
  • In a new study, scientists found that the virus’ average incubation period – the length of time between when a person gets infected and when they test positive for the illness – is five days.
  • The new research also showed that 97.5% of people who are infected develop symptoms within 11.5 days.
  • About 1% of patients, however, show symptoms after 14 days – outside the window of the CDC’s quarantine guidelines.
  • Visit Business Insider’s homepage for more stories.

Since the start of the coronavirus outbreak, researchers have been racing to answer a critical question: How much time passes between when a patient gets infected and when their coronavirus test comes back positive?

The estimates have been wide-ranging. According to the US Centres for Disease Control and Prevention, the coronavirus’ incubation period is believed to be two to 14 days. One study published in January reported six days. Another study (which has yet to be peer-reviewed) suggested it could be as long as 24 days.

Now a group of researchers led by epidemiologists at Johns Hopkins University may finally have an answer.

Their new study, published Monday in the journal Annals of Internal Medicine, suggests the coronavirus’ average incubation time is five days.

Typically, the end of the incubation period is marked by the onset of symptoms like coughing and fever. Figuring out how long it takes for those symptoms to appear “is particularly relevant when you’re thinking about how long you actually have to monitor people to make sure that they’re not infected,” Lauren Ancel Meyers, a professor of integrative biology at the University of Texas at Austin, previously told Business Insider.

Nearly all infected patients show symptoms within 12 days

Italy coronavirusIvan Romano/Getty ImagesTrain passengers are given a sanitary check in Salerno, Italy, on March 8.

The Johns Hopkins researchers analysed 181 coronavirus cases detected between January 4 and February 24 outside China’s Hubei province (where the outbreak started). They focused on cases in which a patient got sick after travelling to that province but whose diagnosis was confirmed somewhere else, since that made it easier to parse out each patient’s incubation period.

They found that on average, the coronavirus incubation period is 5.1 days.

That estimate aligns well with the known incubation periods for other coronaviruses found in humans, like SARS, which also has a five-day average.

The researchers also reported that “nearly all infected persons who have symptoms” would show them within 12 days of infection.

US Coronavirus 16David Ryder/Getty ImagesA healthcare worker with a stretcher at Life Care Centre of Kirkland, Washington, on February 29.

Identifying a virus’ incubation period in this way is critical for modelling its spread.

“It gives you an idea of how quickly the virus could propagate, if you want to project how many patients you’re going to see,” Stephen Morse, an epidemiologist at Columbia University, previously told Business Insider.

It also helps inform health departments about how long they need to actively monitor at-risk people without missing cases, the study authors wrote.

Is a 14-day quarantine long enough?

The US and many other countries have formulated their quarantine policies based on a 14-day incubation period. About 800 US citizens evacuated from Wuhan, China, were brought to military bases in nine states and put under a mandatory two-week quarantine.

But there have been concerns that this two-week period is insufficient.

Coronavirus quarantineRiverside County Department of Public HealthDozens of the US citizens who had been evacuated from Wuhan, China, after their quarantine ended.

A study of six family members infected with the virus, published February 21 in the Journal of the American Medical Association, showed that one patient’s incubation period was 19 days long.

Zhong Nanshan, who discovered the SARS virus, published research on February 6 that suggested the coronavirus incubation period could be up to 24 days. That study has not yet been peer-reviewed, however.

But the Johns Hopkins researchers estimated that for every 10,000 people quarantined for 14 days, 101 would develop symptoms after their release from quarantine – about 1%.

“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,” Justin Lessler, a senior author of the new study, said in a press release.

Lessler and his coauthors wrote that “longer monitoring periods might be justified in extreme cases.”

Experts still aren’t sure if people are contagious during the incubation period

Some research shows that on rare occasions, people can pass the coronavirus to others without ever becoming ill. One woman from Wuhan spread the virus to five of her family members without showing symptoms.According to the CDC, 392 of the 705 positive coronavirus cases on the Diamond Princess cruise ship were asymptomatic.

But health experts still aren’t sure whether people are contagious during the virus’ incubation period.

A member of the medical staff measures the temperature of a traveller at a autobahn park place near Gries am Brenner, Austrian province of Tyrol, at border crossing with Italy on Tuesday, March 10, 2020. Austria authorities started on random checks of arriving vehicles at the border crossings with Italy in reaction to the outbreak of the new coronavirus in Europe, particularly in Italy. As part of the move, officials measure the temperatures of some passengers in cars, trucks and buses. For most people, the new coronavirus causes only mild or moderate symptoms, such as fever and cough. For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia. (AP Photo/Kerstin Joensson )Kerstin Joensson/APA medical staff member measures the temperature of a traveller in Austria near the Italian border on March 10.

“The role of pre-symptomatic transmission (infection detection during the incubation period prior to illness onset) is unknown,”the CDC says.

But Bill Hanage, an associate professor of epidemiology at the Harvard School of Public Health, said the answer to the question “is an unambiguous yes” – he thinks people can transmit the virus before they know they’re infectious.

That type of cryptic, or undetected, transmission affects the spread of the disease, he told CNN.

A recent study of the coronavirus’ genome in the US showed that this cryptic transmission could have been happening for the past seven weeks within Washington state, where 23 people have died and nearly 180 have been infected.

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