How Long Is Contagious Period for Omicron, Delta?

How Long Is Contagious Period for Omicron, Delta?
  • UK health officials estimated the impact of different isolation periods on COVID-19 infectiousness.
  • Five days from symptoms or a positive test, 31% of people with COVID-19 were contagious, they said.
  • Testing negative on rapid tests on days 5 and 6 cut the risk of being infectious to 7%, they said.

It’s a critical question that has played on the minds of individuals with COVID-19 who want to see friends and relatives again: How long should I isolate to make sure I’m not contagious?

So far, there doesn’t seem to be a universally accepted answer.

In the US, government rules allow most people to end isolation after five days without having to take a test; that contrasts with the UK, where Brits with COVID-19 can exit on Day 6 but only after testing negative for the last two days of isolation and without a fever. Without a test, they have to wait 10 full days.

Policymakers have to strike the balance between limiting the advance of the pandemic and preventing the economy from grinding to a halt. President Joe Biden’s chief medical advisor, Dr. Anthony Fauci, said December 28 that US COVID-19 isolation guidelines were designed to “get people back to jobs.”

But some disease experts in both the UK and US have warned that isolation guidance doesn’t follow science and risks people inadvertently spreading the virus at work or school, especially if they haven’t taken a test.

Robert Wachter, a professor and chair of the department of medicine at the University of California, San Francisco said on Twitter on January 14, that his son’s rapid test was positive at Day 10. “If we hadn’t tested, he’d be on his third day of work,” he said.

The UK Health Security Agency, which informs UK policy, estimated that almost a third of people with COVID-19 were still contagious five days after symptom onset or a positive test. If people tested negative using a rapid test on Days 5 and 6 of isolation, 7% were infectious, according to the models. For comparison, the number of people contagious after 10 days was 5%, the data showed.

Gary McLean, a professor of molecular immunology at London Metropolitan University, told Insider that, generally, people with COVID-19 were “potentially infectious” from two days before symptoms and up to 10 days after. 

McLean said there was a good chance of not being infectious after following UK guidance, but it wasn’t foolproof. Immunity and the number of virus particles with which a person was infected can determine how long they remain infectious, he said. “Everyone’s a bit different,” he said. 

The estimates are also not specifically for Omicron infections.

Lawrence Young, a virologist and professor of molecular oncology at the University of Warwick, told Insider at the time of the data release that there wasn’t much data on the dynamics of an Omicron infection compared with other variants. This means we can’t predict exactly how infectious someone with Omicron is. 

Both Young and McLean agreed that the UKHSA data was the “best available” if you’re not sure whether you’re infectious, with the caveat that it wasn’t exact for every case.

This is the percentage of people who are contagious at certain points of time in isolation, according to the UKHSA data:

Five days since symptoms begin/positive test: 31%

  • Plus two negative rapid tests on consecutive days: 7%

Seven days since symptoms begin/positive test: 16%

  • Plus one negative rapid test: 9%
  • Plus two test negative rapid tests on consecutive days: 6% 

10 days since symptoms begin/positive test: 5%

14 days since symptoms begin/positive test: 1%

Other early data: 


A study of 21 people found vaccinated people hospitalized with Omicron had peak levels of infectious virus particles three to six days after diagnosis or symptom onset. No infectious virus particles were detected after 10 days, the study authors said.


A study from researchers at University of Exeter published in the International Journal of Infectious Diseases found 13% of 176 participants carried potentially infectious virus particles at Day 10. Some people retained these levels of virus for up to 68 days, they said. The study was carried out before Omicron became the most common variant.

Scientists from the London School of Hygiene and Tropical Medicine said that the number of days people were infectious in the community could be cut to almost zero by requiring at least two consecutive days of negative tests. This is regardless of the number of days after initially testing positive.

The researchers cautioned that the models used pre-Omicron data. The study hasn’t been published or formally scrutinized by other experts in a peer review.


Early research from the US suggested both Omicron and Delta infections last about 10 days and that people with Omicron have lower peak virus particles levels. The methods used don’t necessarily predict infectiousness, James Hay, the study lead from the Harvard T.H. Chan School of Public Health, cautioned on Twitter.