MSNBC anchor Andrea Mitchell asked Murthy about “breakthrough cases,” specifically the possibility of a fully vaccinated person being infected with COVID-19 and spreading the virus to unvaccinated individuals, such as young children or immunocompromised adults.

“So the people that we’re most concerned about are the unvaccinated,” Murthy responded to the question from the cable TV host. “For example, if you happen to have a lot of interaction with folks who are unvaccinated, let’s say you’re a parent like me who has young children at home who are not vaccinated, that’s a circumstance where we’re being extra cautious and wearing that mask even if you’re fully vaccinated, wearing it outside, when you’re in indoor public locations, is an extra step to protecting those at home.”

“I want to emphasize though that if you are vaccinated the likelihood of having a breakthrough infection is still low because, again, the vaccines are working to help prevent infection, particularly serious infection,” Murthy said Wednesday. “But in the unusual event that a breakthrough does happen, we know transmission can take place.”

“So that’s why, especially when you have a lot of virus circulating in a community, it’s important to take that extra step, go that extra mile, wear that mask in indoor settings, outside the house, so you don’t contribute to transmission,” the surgeon general said.

Mitchell brought up the Centers for Disease Control and Prevention’s new guidance that vaccinated people need to wear masks indoors in high transmission areas, and asked Murthy if the new recommendation would hurt efforts to get more Americans vaccinated.

Murthy said vaccines “save lives, they reduce severe disease extraordinarily successfully, and they reduce the likelihood that you will get sick and transmit the virus to others.”

Murthy noted the new CDC guidance doesn’t “erase extraordinary progress we made and this extra layer of protection with masks will help us reduce this threat of Delta, which again is an extraordinarily contagious version.”

Major cities and some states have already adopted the CDC’s guidance and reimplemented face mask mandates, even for Americans who are vaccinated.

During the interview, Murthy was asked about the possibility of COVID-19 booster shots.

“We are following cohorts or groups of individuals in various settings, nursing homes, medical professionals and others to see if there is a decline in immunity and an increase in breakthrough rates,” Murthy stated. “If there is, as soon as we see that, we will make a recommendation on boosters and the good news is that we will have the supply available to provide that to the public, but at this point, based on the collective data the recommendation is not to have boosters implemented, so far.”