As many Americans celebrated Independence Day they also thought they were celebrating independence from the pandemic.

But, a recent COVID outbreak in Massachusetts following the holiday is forcing the CDC to rethink masking. 74 percent of the people infected in that outbreak were vaccinated, and doctors say the Delta variant is to blame.

"Vaccination does not fully break the chain of transmission," said Dr. Megan Ranney, professor of emergency medicine at Brown University.

The findings are in a leaked document from the CDC. The findings show not only can vaccinated people get the Delta variant, they can spread it too, which is why the CDC is changing course on masking, asking everyone to mask up in public.

States may soon follow.

"We were the first state in the country to mandate masks, so we've done it before and we'll do it again if we need to, we'll do it again," New Jersey Gov. Phil Murphy said.

But, there is some good news. Doctors say the vaccine does protect people from serious illness.

"People who are getting severely ill are those unvaccinated. 99.5% of deaths are in unvaccinated individuals," said Dr. Richina Bicette, medical director of Baylor St. Luke's McNair Campus.

The message - vaccines are still your best shot.

"The results are telling us, if you're not vaccinated, you have a really poor outcome," said critical care physician Dr. Christopher Thomas.