Your proof of Covid vaccination is about to become a lot more important.

On Tuesday, New York City Mayor Bill de Blasio announced that people will need to show proof of Covid vaccination to enter indoor restaurants, gyms and performance venues starting on Aug. 16. The move comes a day after two high-profile businesses issued similar nationwide policies: On Monday, Equinox Group, the parent company of SoulCycle and Equinox fitness clubs, said that members, riders and staff must show proof of vaccination to enter facilities beginning next month.

In New York City, de Blasio said, the mandate is intended to encourage more people to get vaccinated. "It is time for people to see vaccinations as literally necessary to living a good, full and healthy life," he said. "If you're unvaccinated, unfortunately, you will not be able to participate in many things."

According to Dr. Jennifer Lighter, an epidemiologist at NYU Langone Health, this is "a great step forward" because it makes indoor spaces safer while incentivizing unvaccinated people to get on board.

But are vaccinations alone enough to keep you safe, or are there still risks to being indoors with other fully vaccinated people?

The answer depends on a few factors, including your community's rate of Covid transmission and your own personal risk profile. And at this stage in the pandemic, as the more transmissible delta variant spreads, vaccinated people still need to be careful.

Here's what you need to know:

Masks help — even surrounded by vaccinated people

If you're indoors with a group of people whose vaccination status you don't know, you can easily minimize your own personal risk by wearing a mask — regardless of your vaccination status. The Centers for Disease Control and Prevention currently recommends that everyone wear masks indoors in places where there's "high" or "substantial" community transmission. You can check your area's level of transmission by using the CDC's data tracker map, which analyzes data by county.

The agency also emphasizes that people who are immunocompromised (or live with someone who is) or otherwise at risk of severe illness from Covid should wear masks indoors, regardless of local transmission rates.

Why? To avoid breakthrough infections, says Ravina Kullar, a Los Angeles-based infectious disease expert and member of the Infectious Diseases Society of America. New data from the CDC shows that people who are fully vaccinated can get infected by the more transmissible delta variant of Covid and spread it to other people. While these breakthrough cases are rare and tend to be asymptomatic or mild, they can contribute to the further spread of Covid — and put people who are unable to get vaccinated at risk.

As long as Covid is still spreading, communities risk breakthrough infections, according to the CDC.

Outdoor activities are still safer than indoor ones

Outdoor activities are still safer than indoor ones, even if you're wearing a mask indoors.

"If you're indoors — whether it's a restaurant, a gym or a concert — you're going to be more prone to acquiring the virus, whether you're vaccinated or not, just from that unventilated setting," Kullar says.

You can, of course, mitigate your risk of transmitting Covid indoors by avoiding large groups of people and ensuring that the room has adequate ventilation, according to Dr. Ezekiel J. Emanuel, vice provost for global initiatives and co-director of the Health Transformation Institute at the University of Pennsylvania. The CDC suggests opening windows and using fans to improve a room's air flow.

Large groups of people increase the chances of transmission, both indoors and outdoors, for vaccinated and unvaccinated people alike. Kullar suggests planning ahead to steer clear of crowds: If you're going to the gym, for instance, go at off-peak times and avoid crowded group classes.

Relying on the honor system isn't foolproof

New York City will not start fully enforcing its vaccine mandate until mid-September. A major variable will be how individual businesses implement these policies.

One major risk with requiring proof of vaccination, experts say: forgeries.

"When your safety measures rely on the honesty of individuals, it's a little tough to count on," Mercedes Carnethon, vice chair of preventive medicine at Northwestern University Feinberg School of Medicine, told CNBC Make It in early August. "I think the vast majority of people will try to do the right thing, but some will not."

There's a chance that people who were already skeptical about vaccination will dig their heels in further in the face of a mandate, Kullar says. But recent evidence shows that mandates can motivate people to get vaccinated: Last month, the day after France announced a vaccine mandate for restaurants, stores, planes and trains, more than 1 million people across that country signed up to get vaccinated.

Plus, faking vaccination documents can come with severe consequences. Unauthorized use of an official government agency's seal — like that of the CDC or Department of Health and Human Services — is a federal crime that could land you up to five years in prison, plus a fine. And in June, New York's state Senate and Assembly passed a bill to criminalize possession of falsified Covid vaccination documents. The bill will be delivered to New York Gov. Andrew Cuomo to sign it into law.

All the more reason to get vaccinated for Covid if you haven't already.

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