US health officials have halved the recommended isolation time for people with asymptomatic Covid-19 from 10 to five days, amid a surge in cases.
The Centers for Disease Control (CDC) said this must be followed by five days of wearing a mask around others.
It said most transmissions happen in the two days before and three days after symptoms develop.
The measure is expected to alleviate disruptions caused by staff shortages in many areas because of infections.
The sharp rise in cases has been fuelled by the Omicron variant of the virus, which is now the dominant strain in the US. Early studies suggest Omicron is milder but more contagious than other variants.
But the growing number of people becoming infected and having to isolate has put pressure on several industries, including air travel, with thousands of flights cancelled during the Christmas holiday.
The change, which the CDC said was "motivated by science", was announced as more than 200,000 daily infections were confirmed in the last two days in the country.
CDC director Dr Rochelle Walensky said the reviewed guidance "balances what we know about the spread of the virus and the protection provided by vaccination and booster doses".
There was no recommendation for people to take rapid at-home tests before the end of their isolation periods, a measure recommended by many experts as it helps to show whether people continue to spread the virus.
In England, people are now required to isolate for seven days, instead of 10, with two negative lateral flow tests needed to end their self-isolation - the first no earlier than day six of isolation and a second 24 hours later.
Prof Sir John Bell of Oxford University told BBC Radio 4's Today programme: "My view is that lateral flow tests are quite a good way of marking who's infectious and who isn't."