'Steady increase' in vaccines in past month, White House says

Just hours after the Centers for Disease Control and Prevention changed its policy on indoor mask use for vaccinated Americans, the agency sent out an alert to doctors, pressing them to step up vaccines.

The Health Alert Network Health Advisory reiterated that vaccine coverage needed to go up "to prevent surges in new infections that could increase COVID-19 related morbidity and mortality, overwhelm healthcare capacity, and widen existing COVID-19-related health disparities."

As of Tuesday, 66.6% of Americans above the age of 12 have received one vaccine dose, however the rates are lower in several southern states including Louisiana, Alabama and Mississippi. Those states, the alert noted, have seen a major jump in COVID-19 cases, hospitalizations and deaths.

"Unvaccinated persons account for the majority of new COVID-19 infections, hospitalizations, and deaths," the CDC said in its alert.

-ABC News' Anne Flaherty

The COVID-19 outbreak in Provincetown, Massachusetts, has now grown to at least 765 people, officials said Tuesday.

The number of cases from the popular tourist destination have been increasing since the Fourth of July weekend.

Of these cases, 469 individuals are Massachusetts residents, 199 of whom reside in Provincetown, Town Manager Alex Morse wrote in a Facebook post.

The remaining individuals who tested positive reside in other states, or jurisdictions, he said.

There have been no reported deaths linked to the cluster so far, however, three people linked to it have been hospitalized. One of the patients was vaccinated, according to town officials.

-ABC News' Arielle Mitropoulos

The Centers for Disease Control and Prevention on Tuesday reversed its mask guidance for vaccinated people due to the delta variant surge. 

Vaccinated Americans should now wear masks inside if they’re in places with substantial or high transmission, the CDC said.

CDC Director Rochelle Walensky said data show that on "rare occasions, some vaccinated people with the delta variant … may be contagious and spread the virus to others. This new science is worrisome and unfortunately warrants an update to our recommendations."

Walensky added, "This moment -- and most importantly -- the associated illness, suffering and death, could have been avoided with higher vaccination coverage in this country."

In May, the CDC said vaccinated Americans could stop wearing masks indoors.

The CDC also said Tuesday that masks should be worn in schools by all students, teachers, staff and visitors, even for those who are vaccinated. The CDC said students should return to full-time in-person learning this year with prevention strategies in place.

President Joe Biden in a statement called the CDC's new rules "another step on our journey to defeating this virus."

"While we have seen an increase in vaccinations in recent days, we still need to do better," he added.

-ABC News’ Anne Flaherty and Eric Strauss

New York City’s Hospital for Special Surgery is requiring its staff be vaccinated as of Sept. 15.

Only staff with a medical or religious reason will be exempt, the hospital said Tuesday.