Fully vaccinated people do not need to wear masks outdoors and can avoid wearing them indoors in most places, according to updated guidance from the US Centers for Disease Control and Prevention (CDC).

The CDC also said fully vaccinated people will not need to physically distance in most places. The agency also hopes the guidance will prod more Americans to get vaccinated.

"Big news from the CDC: If you're fully vaccinated, you do not need to wear a mask – indoors or outdoors, in most settings," the White House tweeted with a picture that said: "Fully Vaccinated People CAN STOP Wearing Masks."

President Joe Biden told Americans that the lifting of the rule on indoor mask wearing was "a great day" in the fight against Covid-19.

"I think it's a great milestone, a great day," he said in a nationally televised address at the White House.

CDC Director Rochelle Walensky defended the easing of guidance just two weeks after its most recent update, citing a sharp reduction in cases, expansion of vaccines to younger people and vaccine efficacy against coronavirus variants.

"We followed the science here," Dr Walensky said adding, "a coalescence of more science that has emerged just in the last week."

The CDC had faced criticism, even from public health officials, that it has been too cautious in its guidance. Critics have said people need to see more benefit of getting vaccinated in terms of returning to normal activities.

"In the past couple of weeks, we have seen additional data to show these vaccines work in the real world, they stand up to the variants, and vaccinated people are less likely to transmit the virus," the agency said in a news release.

It added, "We needed to take the time to review the full body of evidence to get this right, and that's how we came to this decision."

The revised guidance is a major step toward returning to pre-pandemic life, but the agency still recommends vaccinated people wear masks on planes and trains, and at airports, transit hubs, mass transit and in places like hospitals and doctor's offices.

The US government last month extended mask requirements across transportation networks until 13 September. Dr Walensky said the CDC plans to soon issue updated guidance for transit.

Masks became a political issues in the United States with then-president Donald Trump resisting mandating masks while President Joe Biden embraced masks and mandated them for transit hubs.

Some US states issued aggressive mask mandates while others declined or dropped them months ago.

The CDC said fully vaccinated people should still wear masks where required by federal, state, local, tribal or territorial laws, and abide by such rules and regulations, including from local businesses and workplace guidance.

In late April, the CDC said fully vaccinated people can safely engage in outdoor activities like walking and hiking without wearing masks, but recommended continuing to use face coverings in public spaces where they are required.

Dr Walensky said today immune-compromised individuals should consult doctors before shedding masks and emphasised people who have not been vaccinated should continue to wear them.

She added that vaccinated people who have Covid-19 symptoms should put masks back on.