How Long Are You Contagious With COVID? More Than 10 Days for Some, Small Study Suggests

A new study has found that some people infected with COVID may be contagious for longer than 10 days.

It comes as the Centers for Disease Control and Prevention has shortened its minimum recommended COVID isolation period from 10 days to five, provided people are asymptomatic or have not had a fever for 24 hours.

The research, by scientists at the University of Exeter in the U.K., found that 13 percent of COVID patients in a 176-person cohort retained "clinically relevant" levels of potentially active viruses for even longer than 10 days.

If this is the case, it could pose a risk of transmission even after people have finished their isolation period, the researchers said. However, the findings have been described as a "theoretical risk."

The Exeter scientists tested people for COVID with what they called a "newly adapted test," stating that conventional PCR tests cannot specify if the virus is still active or the person is infectious.

The test used in the Exeter study, however, only gives a positive result if the virus is active, according to a press release from the university. It looks for something called subgenomic RNA—described as a proxy for active viruses.

Although a positive result on this test does not necessarily indicate the presence of a virus capable of replicating, the Exeter researchers say their results suggest that "in some cases, the infectious period may extend beyond the 10-day quarantine period currently imposed."

The study, "Persistence of clinically relevant levels of SARS-CoV2 envelope gene subgenomic RNAs in non-immunocompromised individuals," was published in the International Journal of Infectious Diseases on December 7.

What Have Experts Said About the Exeter Study?

Dr. Stanley Perlman, professor of microbiology, immunology and pediatrics at the University of Iowa, told Newsweek that subgenomic RNA is detected only when the virus is replicating "but does not mean that infectious virus is present."

He added that the study's suggestion that some people may have been infectious for longer than 13 days "would be significant" if true, but said: "In the absence of measurements of infectious virus, these data are hard to interpret."

This point was echoed by Ian Jones, professor of virology at the University of Reading in the U.K.

Jones told Newsweek: "The weakness of the paper is that there is no actual proof that this signal is from infectious viruses. You would need to isolate the virus for that.

"It's a theoretical risk which I doubt will be as high as they suggest. In the time of Omicron, or Omicron Jr. down the line, when more or less everyone will get it eventually, I can't see it changes much."

Greg Towers is professor of infection and immunity at University College London. He told Newsweek that the significance of the study was context-dependent.

"We're desperately trying to get back to normal and everyone is trying to sort out what level of ongoing infection we are prepared to tolerate," he said.

"For example, should people now come out of isolation earlier even if there's a small chance they could still be infectious—for example, five days instead of 10 days? Different governments are coming to different decisions regarding this. Of course, better tests would help make this choice."

Lorna Harries, professor of molecular genetics at the University of Exeter and co-author of the study, told Newsweek: "SgRNAs are a proxy for infectious potential, not a definitive measure. No test is perfect and this remains a better option for a series of archival samples than conventional PCR for viral genomes that can persist regardless of infective potential.

"Our data are however reasonably concordant with the UK Health and Security Agency's own modelling predictions of the number of people who remain infectious, particularly at 5 days, when they estimate 31 percent may remain infectious."

How Long Are People Contagious For?

Most evidence suggests that COVID viruses capable of replicating and spreading cannot usually be recovered from people with mild-to-moderate cases beyond 10 days of symptom onset, the study states.

This was reflected in the CDC's isolation recommendations which, until recently, said people should self-isolate for 10 days after symptom onset and may then leave isolation as long as they have not had a fever for 24 hours and other symptoms are improving.

"For most patients with COVID-19, efforts to isolate live virus from upper respiratory tract specimens have been unsuccessful when specimens are collected more than 10 days after illness onset," the CDC stated, adding that this may be longer in people with severe cases.

Perlman told Newsweek: "Most studies looking for infectious virus do not find it after day nine and then only sometimes."

On December 27, the CDC updated its isolation recommendations to state: "People with COVID-19 should isolate for 5 days and if they are asymptomatic or their symptoms are resolving (without fever for 24 hours), follow that by 5 days of wearing a mask when around others to minimize the risk of infecting people they encounter."

The health agency said the change was "motivated by science demonstrating that the majority of SARS-CoV-2 transmission occurs early in the course of illness, generally in the 1-2 days prior to onset of symptoms and the 2-3 days after."

Update, 11:30 a.m. ET, 1/18/2022: This article has been updated to include a comment by professor Lorna Harries.

Person holding COVID test
A stock photo shows a health worker holding a COVID swab. A study has suggested some people may be infectious past ten days of symptoms. Diy13/Getty