2 factors not included in the CDC guidelines can help you decide when it's really safe to go maskless

• Fully vaccinated people can ditch their masks at any indoor or outdoor gathering, per the CDC.
• But public-health experts caution that this advice might not apply to all communities.
• If cases in your local area start to rise or few people have been vaccinated there, consider a mask.

When the Centers for Disease Control and Prevention announced last week that fully vaccinated people could ditch their masks, many Americans - and some public-health experts - wondered whether the recommendations were too broad.

"We're missing a critical step, and that step is fully vaccinated people around other fully vaccinated people in large or small groups," Leana Wen, the former health commissioner of Baltimore, told Insider. "That should be safe, but now we're going from 'you can only dine indoors with a few people' to 'everybody can take off their masks because we're not checking their vaccination status.'"

The CDC's recent guidance says that fully vaccinated Americans can take off their masks indoors and outdoors at gatherings of any size - even with unvaccinated people present. The exceptions to that rule include healthcare settings, homeless shelters, public transportation, and airports. Private companies such as stores and airlines are also be free to enforce mask rules as they see fit.

But for people who want a more tailored way to gauge their personal risk, experts highlight two circumstances in which masks are still a good idea for vaccinated people. The first is if new daily cases start to rise at all in your local community. The second is a low level of vaccination in your area - one significantly below the current national average of 37%.

But even in the absence of these red flags, experts noted, the CDC is not mandating that anyone to go maskless.

"It doesn't mean that you have to take your mask off," Anne Rimoin, an epidemiology professor at the University of California, Los Angeles, told Insider. "It means that you can take your mask off."

She added: "If you are still feeling vulnerable, if you're somebody who has a compromised immune system, if you are in an area when you're still higher transmission, then the conservative thing to do would be to continue to wear a mask in situations where you're going to be around people where you don't know what their vaccination status is."

Any increase in cases could be a reason to mask up

The more real-life data researchers get about the US's authorized vaccines, the more the shots seem to work well against variants and reduce transmission. So Rimoin said the new CDC guidelines simply underscore "how confident we are in these vaccines and the ability of these vaccines to really protect people."

President Joe Biden announced on Monday that US coronavirus cases are declining in all 50 states for the first time since the pandemic started. It's another indicator that vaccines are stopping the virus from spreading.

But it also means that any increase in cases in your local area should be concerning at this point, according to Lisa Lee, an epidemiologist at Virginia Tech.

"If we get an uptick, we're going to have to consider what is happening because we have a lot of people vaccinated and we should not be seeing more infections," Lee told Insider. "We should continue to see less."

If cases start rising in your community, she said, that's a good reason to consider putting a mask on - especially when indoors around other people.

"In a place like rural Colorado that happens to be a hotspot, people have to think carefully," Lee said. "Even vaccinated folks need to really be careful and think about whether that's a risk they're willing to take."

Is your area's vaccination rate below the national average?

Around 37% of the total US population is fully vaccinated so far. Vaccinated people living in communities that are significantly below that threshold might consider keeping their masks on for now, Lee said.

"When we say the United States has nearly 50% of people fully vaccinated now, that's great, but that doesn't mean anything for a particular place," Lee said, referring to the roughly 47% of US adults who are fully vaccinated. "We really do need to think carefully about our own situation and our own community."

It's safer to remove your mask, for instance, in McKinley County, New Mexico, or Essex County, New York, where more than half the population is fully vaccinated, than in Warren County, Virginia, or Hancock County, Georgia, where just 3% of residents are fully vaccinated.

Those who want to exercise maximum caution could wait to remove their masks until at least 75% of their community is vaccinated, Lee added. That's likely the threshold at which an area has reached herd immunity - the point beyond which the virus can't easily pass from person to person.

Reaching herd immunity would also make it far less likely that the US would suddenly see a new, more contagious or deadly variant emerge. (The virus can't mutate if it isn't spreading.)

"We want to get out of this cycle of chasing the variants with vaccines," Lee said. "A way we're going to get out ahead of it is by ensuring that we get 75% to 85% of the population vaccinated. So we've got to work on that. We cannot give up."

Until then, though, the threat of new variants means there's always a chance the CDC might need to reinstate mask guidelines.

"We have to watch for variants that might evade the efficacy of the vaccine," Lee said. "If we see those, we're going to have to be flexible enough to agree that, OK, maybe it is time for a few weeks or months of masks again."