Originally Published: 13 APR 22 10:58 ET

By Katia Hetter, CNN

(CNN) -- As the United States navigates a third year of the Covid-19 pandemic, cases are rising modestly in the Northeast, Philadelphia has reinstituted an indoor mask mandate and some colleges are requiring masks during the final days of the spring semester.

People in the US won't face a shutdown like the one currently in place in Shanghai, where no one is allowed to leave their residential compounds, Covid testing is mandatory and food supplies are running low. It's unlikely most Americans will see local governments bring back even moderate pandemic restrictions. That leaves it mostly to individuals to protect themselves.

After three years, most people know how to do that: vaccines, masks, physical distancing, handwashing and -- let's not forget -- ventilation.

But as we head into offices, schools and public spaces, there are some things we can control -- like masking -- and others that are not in our control, such as whether our offices have improved ventilation.

How should people weigh which precautions they want to follow? Should indoor mask mandates come back? Are there settings where people should consider masking outdoors? What if you have to go back to the office, and others aren't masking? And should we skip large indoor events again?

To help us with these questions, I spoke with CNN Medical Analyst Dr. Leana Wen, an emergency physician and professor of health policy and management at the George Washington University Milken Institute School of Public Health. She is also author of "Lifelines: A Doctor's Journey in the Fight for Public Health."

The following conversation has been lightly edited for length and clarity.

CNN: Can you give us a quick refresher -- what protective measures are effective against this very contagious BA.2 variant?

Dr. Leana Wen: BA.2 is a subvariant of the Omicron variant. It appears to be even more contagious than Omicron. Like previous variants, it's spread through direct contact and is airborne. Someone coughing and sneezing will exhale droplets that can transmit the virus. The virus can also be carried on microscopic aerosols that are expelled by breathing. In addition, surfaces may play a greater role in Omicron transmission. Someone who coughed onto their hand and then touched a door could seed virus there and transmit it to you if you touch that door and then your nose or mouth.

The protective measures we talked about early in the pandemic still work against Omicron. Masks work very well, though here I would urge that people wear an N95 or equivalent mask, such as a KN95 or KF94. A simple cloth mask is much less effective, especially against a variant as contagious as BA.2.

Ventilation is very important. The risk of outdoor transmission is exponentially lower than indoor transmission, and a well-ventilated, less crowded space also has much lower risk than people packed together in an unventilated space. Surfaces play less of a role than aerosol transmission, but it's still good practice to wash your hands well or to use hand sanitizer if you shake people's hands and touch frequently used surfaces. That will reduce not only this coronavirus transmission but other respiratory and gastrointestinal pathogens, too.

We also have more tools at our disposal than we did in 2020. Namely, we have vaccines and boosters, which protect very well against severe illness and also reduce the risk of Covid-19 infection. Testing prior to getting together with others tells people if they are infectious and need to avoid the gathering. That's another key preventive measure that we now have access to.

CNN: What should people consider when deciding what precautions they want to bring back, like canceling restaurant reservations or masking up indoors?

Wen: I'd consider three things. First, what is the Covid-19 risk in your community? I've talked before about how the new guidelines from the US Centers for Disease Control and Prevention can be used here. If your community is in an orange, or "high risk," area by the CDC's metrics, you should mask indoors. If it's in green or yellow -- or "low" or "medium" risk -- you could choose not to, depending on the other two factors.

The second thing to think about is, what is your medical situation and the circumstances of others in your household? If you are generally healthy and fully vaccinated and boosted, your chance of severe illness from Covid-19 is very low. That's different if you are immunocompromised or live with someone who is elderly with multiple medical conditions. You would want to have a higher threshold for caution depending on your family's medical situation.

Third, how important is it to you to continue to avoid Covid-19? Of course, none of us want to get infected by the coronavirus, and no one should be trying to get it. But there are some people who want to avoid it at all costs and others who accept that if they engage in travel, resume parties, dine in restaurants and engage in other pre-pandemic activities, they will have some level of risk. That's a personal decision that will differ from person to person.

CNN: Does it make sense to mask in some situations but not others?

Wen: Absolutely. Risk is additive. You can bring back some things that are lower risk or higher value but still take some precautions. For example, if you work in an office that has pretty good ventilation and is well-spaced, and requires proof of vaccination, you may consider not masking in that setting. But you might still decide to mask in crowded, indoor places like grocery stores and train stations. Again, I'd urge that if you are wearing a mask, wear an N95 or equivalent mask, because it will be your best chance of protecting against the very contagious BA.2 variant.

CNN: What about outdoors? Is BA.2 so contagious that it could transmit there?

Wen: The risk of outdoor transmission is very low to the point that it's virtually nonexistent. Outdoor social gatherings are very safe, and certainly lower risk than the equivalent setting indoors. I do not think that people need to mask outdoors, but there may be situations where someone is extremely vulnerable -- for example, a cancer patient on chemotherapy -- and wants to be extra careful. In that case, they should definitely feel free to wear a mask when in outdoor and more crowded settings.

CNN: What if you have to go back to the office, and others aren't masking?

Wen: You need to decide what's best for you, with the understanding that one-way masking with an N95 or equivalent works very well -- that is, even if others around you aren't masking, you are still well-protected if you wear a well-fitting, high-quality mask at all times.

It also depends on the specifics of your workplace. Let's say you work in a well-ventilated office, everyone around you is vaccinated, and you are well-distanced from the next person. You could decide to remove your mask when sitting at your desk, but put it on again to go into a crowded elevator or a conference room where you're sitting shoulder-to-shoulder with other people. You could decide to go to an outdoor lunch with your colleagues, but skip the happy hour in a packed bar. These are all reasonable decisions depending on how you think about your own risk.

CNN: Should people avoid indoor gatherings, knowing that outbreaks can happen?

Wen: Not necessarily. It again depends on what's going on in your community, what your medical situation is, and how much you want to keep avoiding Covid-19. For me, my area, in Baltimore City, is in a green or "low-risk" area for Covid-19 transmission, according to the CDC. My husband and I are vaccinated and boosted. We have two little kids who are too young to be vaccinated, but we also recognize that it's going to be very difficult to keep them from getting the coronavirus given how transmissible this new variant is.

As such, I am attending large events, including indoor conferences and meetings. I'd prefer that these events require proof of vaccination and same-day negative test result. If they don't, they are less safe, and I certainly understand if others want to avoid them for the time being—just as I understand if others will want to keep attending them. This, to me, is no different from a decision to go back to the gym, resume travel or dine indoors in restaurants. Some will think those activities are worth the risk. Others will not.