The coronavirus outbreak that originated in Wuhan, China, has killed at least 2,250 people and infected more than 76,000.

Research from a group of Chinese scientists suggests the virus' incubation period could be as long as 24 days.

Another case study found that an asymptomatic patient's incubation period was 19 days.

The CDC has instituted a 14-day federal quarantine for the US citizens evacuated from Wuhan. But these recent studies raise doubts about whether that's long enough.

Visit Business Insider's homepage for more stories.

The novel coronavirus that originated in Wuhan, China, in December has killed at least 2,250 people, infected more than 76,000 people, and spread to 29 other countries.

As researchers race to learn more about the virus, a crucial question lingers: How much time passes between when a patient gets infected and when their coronavirus test comes back positive?

"Every day there are new estimates coming out for the incubation period," Lauren Ancel Meyers, a professor of integrative biology at the University of Texas at Austin, told Business Insider.

According to the Centers for Disease Control and Prevention (CDC), the coronavirus' incubation period is believed to be two to 14 days.

But a study of six family members infected with the virus, published Friday in the Journal of the American Medical Association (JAMA), revealed 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 That study has not yet been peer-reviewed, however.

Uncertainty about the incubation period, Meyers said, "is particularly relevant when you're thinking about how long you actually have to monitor people to make sure that they're not infected."

Quarantining efforts around the world

The US and many other countries have formulated their quarantine policies based on a 14-day incubation period.

"It's widely accepted that there's a 14-day rule of thumb," Stephen Morse, an epidemiologist at Columbia University, told Business Insider. "That's how long you have to wait to go back to your daily life."

The UK, Australia, France, India, Italy, and Canada have also said that citizens repatriated from Wuhan will be quarantined for 14 days.

About 800 US citizens evacuated from Wuhan have brought to military bases in nine states and put under a mandatory two-week quarantine. More than 200 of those evacuees were released this week, and another 195 left on February 10.

But if these new studies are correct, 14 days may not be sufficient.

Is a 14-day quarantine long enough?

A security official stands guard as an ambulance arrives with a patient at Princess Margaret Hospital in Hong Kong on January 22. Anthony Kwan/Getty Images

The patient found to have a 19-day incubation period was a woman who lived in Wuhan but traveled to Anyang on January 10. There, she interacted with five family members; by January 26, all five tested positive for the coronavirus. The Wuhan woman had passed it to them without showing any symptoms, researchers determined.. She positive for the virus herself on January 28, but she never showed any symptoms at all.

The study published earlier in February found that the median incubation period for all 1,099 cases examined was three days, though the full range of incubation periods ranged from zero to 24 days.

Still, CDC officials have said they're confident that the US evacuees' two-week isolation is long enough.

"The incubation period is obviously really important for us as we look to make sure that we're releasing these people safely from quarantine," Nancy Messonnier, the director of the CDC's National Center for Immunization and Respiratory Diseases, said on February 12.

"We still think that for today, for now, 14 days is the right interval to use," she added.

Dozens of US citizens who were evacuated from Wuhan throw their face masks after their quarantine ended in California on Tuesday. Riverside County Department of Public Health

Michael Ryan, the executive director of the World Health Organization's health-emergencies program, said last week that the WHO was also "not considering changing anything" about its quarantine recommendations.

Pinpointing the incubation period is difficult and important

Identifying a virus' incubation period is critical for modeling 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," Morse said.

Morse added that there were a few reasons to be skeptical of the 24-day estimate: "Was this one patient or a significant number that experienced a 24-day incubation period?" he said.

That study didn't specify how many patients had incubation periods longer than two weeks.

Medical staff members who took over a large temporary hospital built in an exhibition center in Wuhan. Feature China/Barcroft Media via Getty Images

What's more, Morse said, the 24-day figure could have been based on inexact data.

"These are largely patients who have been found through hospitalization," he said. "So we don't know how long they've had the virus based on when they went to hospital. We have to rely on their memory of when they were exposed, which may or may not be accurate."

Morse also questioned the lower end of the incubation-period range in that study.

"Zero days makes me suspect — does that mean patients are developing symptoms the same day as contact?" he said. "One to two days I can accept, but less than 24 hours seems extreme."

Ryan offered another possible explanation for longer-than-expected incubation periods: "A very long incubation period can reflect a double exposure," he said.