The United States has halved the recommended isolation time for people with asymptomatic Covid infections from 10 to five days.
The Centers for Disease Control (CDC) said that this must be followed by five days of wearing a mask around others.
While the new recommendations are not mandatory, they are closely followed by many businesses and policymakers across the US.
It comes as a surge in Covid cases causes travel chaos across the country.
President Joe Biden has pledged to deal with a shortage of tests as Omicron threatens to overwhelm hospitals.
The CDC said the change is "motivated by science" demonstrating that most transmission happens early in the course of illness.
It has also updated recommendations for people exposed to Covid who are either unvaccinated or yet to get a booster jab.
For these groups, the CDC said it "now recommends quarantine for five days followed by strict mask use for an additional five days" after exposure.
People who have received their booster shot do not need to quarantine following exposure but should wear a mask for 10 days after the exposure, the CDC said.
In the press release announcing the update, CDC director Rochelle Walensky said the changes "ensure people can safely continue their daily lives."
"Prevention is our best option: get vaccinated, get boosted, wear a mask in public indoor settings in areas of substantial and high community transmission, and take a test before you gather."
Omicron is now the dominant strain in the US, which has recorded more than 200,000 daily Covid infections in the last two days.
President Biden has warned that hospitals could be "overrun" but that people do not need to panic as the country is better equipped to deal with the latest Covid surge.
He also acknowledged that more needs to be done to make testing readily available.
"Seeing how tough it was for some folks to get a test this weekend shows that we have more work to do," he said as he joined a call with the administration's Covid response team and state governors. "It's clearly not enough."
He said that new steps will include improving at-home test manufacturing and making it easier to use Google to locate nearby test centres.
Different states and local authorities in the US are introducing new measures in a bid to combat rising cases. New vaccine requirements have come into force in New York City, including a vaccine mandate for private-sector workers in the city.