How long can you spread Omicron?

The United States and the United Kingdom have slashed their recommended self-isolation periods for asymptomatic people — and more countries may soon follow suit, as the highly transmissible Omicron variant threatens to keep hospital staff and other key workers at home.

It comes amid record-setting case figures in both nations, and marks the first time since Omicron emerged that major countries have diverged from the World Health Organization’s recommended 10-day isolation period.

But most countries still follow the 10-day marker, while others, such as Germany, require up to 14 days in isolation. The disparities have led some to wonder exactly when, and how long, people are infectious with the Omicron variant.

The moves were made amid worries over the availability of key workers. “If you are asymptomatic and you are infected, we want to get people back to their jobs — particularly those with essential jobs — to keep our society running smoothly,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN this week.

But there is some emerging data behind the changes as well. The Centers for Disease Control and Prevention (CDC) said their decision 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.”

CDC shortens recommended Covid-19 isolation and quarantine time
An early CDC study, released on Tuesday, examined an Omicron cluster in Nebraska and found that the time between exposure and infection — known as the incubation period — may be around three days. That’s shorter than the Delta strain, which studies estimate has a four-day incubation period. 
A similar study of a Christmas party in Norway in which dozens became infected found comparable results.

“There is accumulating evidence, for vaccinated people, that if we are asymptomatic we are very unlikely to be infectious after about five to seven days,” Brown University’s Associate Dean of Public Health Dr. Megan Ranney told CNN on Tuesday. 

Emerging evidence that Omicron may be less severe than Delta likely played a role in the moves too.

But the new guidelines have still prompted some debate in the medical community, with experts yet to fully understand Omicron.

“For the unvaccinated, the data doesn’t really back up that you become non-infectious after five days,” Ranney said. “I’m quite worried about these new recommendations.”

She suggested having different guidance for unvaccinated people until more data comes in — which could also have the “added boost” of encouraging people to take up the vaccine if they haven’t already.

Erin Bromage, a biology professor at UMass Dartmouth, added on CNN Wednesday that there is “absolutely no data that I am aware of” to support the switch in guidance.

He added that people can still test positive on antigen tests up to seven or eight days after their initial test, even if they don’t have symptoms. Unlike the UK, where antigen tests are more plentiful, the US guidance is not dependent on getting a negative result.

Omicron is nonetheless tearing through workforces in several countries, and it’s likely more nations will shorten their isolation periods in the new year if the burden on hospitals grows. “With the sheer volume of new cases … one of the things we want to be careful of is that we don’t have so many people out,” Fauci said.


Three preprint papers released last week revealed some early good news about the severity of the Omicron variant.

The studies — one from England, another from Scotland and a third from South Africa — suggested that Omicron is associated with a lower risk of hospitalization than the Delta variant. The degree to which that risk is decreased ranged from between 40% to 80% across the studies.

That research included preliminary data, and the papers haven’t yet been published in a peer-reviewed journal. But they add to the growing evidence that the new strain, while highly transmissible, may be less severe.

Nonetheless, a lower risk of hospitalization could easily be offset by the higher number of concurrent infections that Omicron is causing in several countries. That’s why experts are urging caution — and encouraging anyone who hasn’t taken up the vaccine or booster to do so before Omicron takes hold.

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