Even after receiving Covid-19 jabs, people should wear face masks in areas where the virus is spreading, the WHO said Friday, after the US decided the vaccinated do not need masks.

"Vaccines are life-saving but on their own, they are not enough," the World Health Organization told AFP in an email.

The comment followed a decision by the US Centers for Disease Control and Prevention on Thursday to lift mask-wearing guidance for people who are fully vaccinated against Covid-19.

Almost 60 percent of US adults now have one or both doses, while cases are falling fast, down to a seven-day-average of 38,000, or 11 per 100,000.

The WHO refrained from commenting specifically on the US situation, but experts highlighted that the decision to remove Covid restrictions, including mask recommendations, should rely on more than just the vaccination rate.

"It's about how much virus is circulating," WHO Covid-19 lead Maria Van Kerkhove told reporters.

"It's about the amount of vaccines and vaccinations that are rolling out, it's about the variants... that are circulating."

The vaccines in use against Covid-19 have been shown to be highly effective in preventing serious illness and death, and there is also increasing evidence that they provide high protection against infection and transmission of the virus.

But WHO chief scientist Soumya Swaminathan stressed they are "not 100 percent effective against preventing infection".

"You can have asymptomatic or mild illness or even moderate symptoms even after being vaccinated," she said, warning that "vaccination alone is not a guarantee against infection or against being able to transmit that infection to others".

It may be rare, but could still occur, she said.

"That's why we need the other protective measures like the mask wearing, and the distancing and so on until countries get to the level at which a large number of people are protected and virus circulation and the transmission goes to very low levels."

So far, she warned, "very few countries are at the point now where they can drop these measures by individuals and by governments."

WHO emergencies director Michael Ryan agreed.

Relaxing measures and taking away mask mandates, he said, "should only be done in the context of considering both the intensity and transmission in your area, and the level of vaccination coverage."

"Even in situations where you have high vaccine coverage, if you've got a lot of transmission, then you wouldn't take your mask off."