When exactly should you wear a mask? Here’s what the experts say

After reporting a rise in new COVID-19 cases, Cambridge is urging residents — whether they are vaccinated or not — to wear masks and practice social distancing.Matthew J. Lee/Globe staff

With COVID-19 cases beginning to rise again in Massachusetts, you may be wondering whether it’s still safe to go shopping, catch a movie, or do any other indoor activity without a mask.

If you’re fully vaccinated, you already have the best defense against falling ill with COVID-19, experts say. But wearing a mask could still go a long way to protect others and inhibit the spread of the virus.

Although breakthrough infections are happening, and were expected, it’s unvaccinated people who make up more than 97 percent of those hospitalized with COVID-19 across the country.

In Massachusetts, 62.6 percent of the population is fully vaccinated, among the highest rates in the country. Though new COVID-19 cases are spiking, with the seven-day average nearly tripling since July 5, the hospitalization rate has increased by a modest 17 percent.

You don’t have to panic if you’re fully vaccinated, but there are some circumstances where wearing a mask as an extra layer of protection is still prudent. Here’s what to consider.

Louder for the people in the back: Vaccines work

“If you’re vaccinated, I have full confidence in these vaccines that you will be fine,” said Dr. Sabrina Assoumou, an infectious disease physician at Boston Medical Center and assistant professor at Boston University School of Medicine. The vaccines also work against all variants identified in the United States, she said.

“The thing that would worry me and make me think that we need to change any of our policies is if we start seeing a lot of people who are fully vaccinated get hospitalized or die from COVID,” Assoumou said. “But we’re not seeing that.

“You want people to reap the benefits of [getting vaccinated],” Assoumou said. “We’re moving forward; we’re not back in April 2020.”

Still, here’s when you should probably wear a mask

The US Centers for Disease Control and Prevention still recommends all unvaccinated people, including children ages 2 to 12, wear face coverings indoors.

Dr. Anthony Fauci said over the weekend that the Biden administration is considering reissuing stronger mask-wearing guidelines.

Regardless of your vaccination status, masks are a good idea in crowded indoor settings, Assoumou said.

For fully vaccinated folks in less crowded indoor spaces, Assoumou said, there are a few factors to consider, including whether you or those around you are vulnerable or immunocompromised, what the vaccination and COVID-19 rates are in your community, and your own personal comfort level.

It’s probably still safe to gather unmasked with a small group of vaccinated friends, said Dr. David Hooper, chief of the Infection Control Unit at Massachusetts General Hospital. But being in spaces with a large number of people whose vaccination status you don’t know, such as at a wedding, can be riskier, particularly if you’ve got an immunocompromised family member at home.

Experts still don’t fully know the extent to which asymptomatic vaccinated people can spread the virus to others, Hooper said.

“So, I think your choice to wear a mask in those mixed settings is less about protecting you from illness, and more about protecting others in the chance that you might catch the virus and spread it.”

The risk varies based on where you are

The recent surge in cases has led officials in some Massachusetts towns to reemphasize masking.

An early July outbreak in Provincetown infected at least 256 people, many of whom were vaccinated. Though officials said the vaccinated individuals experienced mostly mild symptoms, the town over the weekend approved a new indoor mask mandate and a measure clearing the way for stricter capacity limits on local businesses.

Cambridge and Nantucket have issued similar advisories while in Boston, Acting Mayor Kim Janey said all Boston Public Schools students will be required to wear masks in the fall.

Hooper said tailoring local advisories based on each community’s circumstances makes sense, given disparities in the proportion of COVID tests returning positive across the country and the state.

He said if he lived in areas with low vaccination rates and rising cases, such as parts of Florida and Missouri, wearing a mask would be a no-brainer.

“Here [in Massachusetts] it’s a little less certain because the prevalence is still quite low,” Hooper said. “But certainly, if people feel more comfortable and are wanting to wear masks … then nobody’s going to say no.”

Last year’s guidance still applies: Outdoors is better

The risk of contracting the virus drops dramatically outdoors because of ventilation, Assoumou said.

Given the option, it’s always best to meet friends and family outdoors, especially now that it’s summer, and to choose outdoor patios at restaurants when possible, she said.

Unless there is an extremely large crowd outside, masking is not necessary, Assoumou said.

“Just space yourself and you’ll be fine,” she said. “It’s been a long year and I think that it’s still important to emphasize to people what are alternatives that are safer.”