WASHINGTON, United States -- The United States top health agency on Friday drastically revised its guidelines for masking to stop Covid-19 transmission, a decision that means most Americans won't be advised to wear them in indoor public spaces, including school children.

"We're in a stronger place today as a nation with more tools to protect ourselves and our communities from Covid-19," said Rochelle Walensky, director of the Centers for Disease Control and Prevention (CDC) in a call with reporters.

The changes involve the metrics used to determine whether people should mask up.

Under previous guidance, this was pegged to case rates -- with 95 percent of the country considered areas of high or substantial transmission, and thus covered by mask advisories.

The new metrics use caseloads but also include Covid hospitalizations and local hospital capacity, to create a new measure known as "Covid-19 community level."

Residents can look up on the CDC website whether their area is green, yellow or orange on a national map.

Now, more than 70 percent of the population live in areas that aren't advised to mask up according to the new framework, including schools in the green or yellow areas.

Calls to lift school mask mandates had multiplied in recent weeks, as concerns grew about their impact on children's social, emotional and educational development.

Walenksy said the new guidance was statistically rigorous, but could be reviewed "if or when new variants emerge, or the virus surges."

The revised guidelines don't however apply to transport systems. A federal rule on that will be reviewed in mid-March when it is set to expire.

The United States is coming out of its latest Covid wave, driven by the Omicron variant, which while highly transmissible, generally leads to less severe outcomes among people who were vaccinated or had a prior infection.

Some 65 percent of the US population are now fully vaccinated, and high caliber masks, such as N95s, which provide strong one-way protection to wearers, are now more widely available for people who require them, such as the immune compromised.

The administration of President Joe Biden has increased its rhetoric around normalization as the pandemic transitions to a new endemic phase.

The guidance is largely symbolic, since both Republican and Democrat-led states and cities have already taken the lead, either dropping their mandates or setting imminent timelines for doing so.