NEW YORK • Fully vaccinated Americans can be unmasked when exercising, dining and socialising outdoors in small groups, federal health officials said, and can gather indoors with other fully vaccinated people and family members without masks or social distancing.

But masks should remain on anywhere there is a large gathering, including at ballparks, malls or church, the US Centres for Disease Control and Prevention (CDC) said on Tuesday.

And it urged that vaccinated people be mindful around those with a higher risk for severe illness.

"Today is another day we can take a step back to the normalcy of before," Dr Rochelle Walensky, the CDC chief, said at a news briefing announcing the changes.

In a separate news conference at the White House, President Joe Biden urged Americans hesitant to be vaccinated against Covid-19 to reconsider, pointing to new US guidance that inoculated people can socialise outdoors without masks.

"Beginning today, gathering with a group of friends in a park, going on a picnic, as long as you're vaccinated and outdoors, you can do it without a mask," he said.

"For those who haven't gotten their vaccine yet, especially if you're younger, or thinking you don't need it," said the President, "this is another great reason to go get vaccinated now - now."

Dr Walensky cited a "really hopeful decline" of about 21 per cent in the seven-day average for Covid-19 cases.

The new recommendations, which represent one of the most significant relaxations of guidelines since the pandemic began, are complex and wide-ranging.

They come with almost 30 per cent of Americans fully inoculated and with increases starting to slow in the daily Covid-19 caseload.

At the same time, the guidelines send a message that getting vaccinated offers a clear route to a more normal lifestyle at a time when a high percentage of Americans remain vaccine-hesitant.

Under the new rules, fully vaccinated people can attend a small outdoor gathering maskless or dine at an outdoor restaurant with others, whether they are fully vaccinated or not.

They can also get together indoors with unvaccinated people of any age from one other household without masks or social distancing, according to the agency.

Additionally, inoculated Americans do not need to be isolated or get tested if they have been around someone with Covid-19, unless they have symptoms. If so, get tested and stay home, the agency urges.

"Over the past year, we have spent a lot of time telling Americans what they cannot do, what they should not do," Dr Walensky said. "Today, I'm going to tell you some of the things you can do if you are fully vaccinated."

But the guidance also includes some reservations. The CDC urges all Americans to wear masks when attending medium or large-sized gatherings, such as live performances or sporting events, and during visits to malls and full-capacity places of worship.

And the agency encourages people to be mindful when around those with an increased risk for severe illness from Covid-19, including people who have diabetes and some types of cancer, lung or kidney disease.

People are considered fully vaccinated two weeks after receiving the final vaccine dose of the two-dose vaccines made by Pfizer-BioNTech and Moderna, or a single dose of the Johnson & Johnson vaccine.

Sars-CoV-2, the virus that causes Covid-19, spreads largely through aerosolised viral particles. The size of a space, how many people are in it, length of exposure and air circulation all factor into how easily viral particles might spread. It has long been understood that indoor environments present the highest risk of virus transmission.

"You will still need to follow guidance at your workplace," the CDC said, adding that the fully vaccinated "should continue to follow guidance to protect themselves and others, including wearing a well-fitted mask, when indoors or in an outdoor setting or venue where masks are required".

About 141 million Americans have received at least one dose of vaccine, according to the Bloomberg Vaccine Tracker, but the pace of US shots has fallen under three million per day despite abundant supply.

BLOOMBERG