(CNN) Shipments of the Covid-19 vaccine are arriving, with frontline health care workers getting immunized -- some for the cameras -- across the United States Monday.
While these developments mark a historic moment and hold much promise, that doesn't mean Americans can stop wearing masks anytime soon. CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor at George Washington University Milken Institute School of Public Health, explains why.
CNN: Does the vaccine protect people from getting Covid-19? If so, how come I still have to wear a mask?
Wen: This is a good question! It's important to be clear about what we know and what we don't know about what the vaccine does. What we know is that the Pfizer vaccine is very effective at preventing symptomatic illness and severe disease. That means the vaccine appears to prevent people from getting sick enough that they develop symptoms, and very importantly, it prevents people from becoming so severely ill that they end up in the hospital. This is really great news.
Here's what the studies don't yet show. They haven't looked at whether the vaccine prevents someone from carrying Covid-19 and spreading it to others. It's possible that someone could get the vaccine but could still be an asymptomatic carrier. They may not show symptoms, but they have the virus in their nasal passageway so that if they're speaking, breathing, sneezing and so on, they can still transmit it to others.
This is the main reason why we can't stop wearing masks right after we get the vaccine. The vaccine will protect you from getting ill and then ending up hospitalized. But it's possible that you could still carry the virus and be contagious to others. So those who get the vaccine should still be wearing masks and practicing physical distancing.
CNN: Does that mean we will need to wear masks in public from now on?
Wen: No, not forever, but for a while longer. It's estimated that about 70% of Americans must be vaccinated before we get to herd immunity through vaccination. That's the point where enough people have the immune protection that the virus won't spread any more.
This means about 230 million Americans must receive the vaccine. It will take time to produce this many vaccines — and remember the Pfizer and Moderna vaccines are two-dose vaccines, so you need double the number of doses as people. Then the vaccine must be distributed and actually given to people. If all goes well, the best estimates are that it will be late spring or early summer for most Americans to receive the vaccine. At that point, we could probably see one another without masks — but not before.
CNN: Can this time line be sped up?
Wen: Already, vaccine development has proceeded with incredible speed. The fastest that a vaccine was developed before this pandemic was four years. We now have an authorized vaccine within less than a year.
How quickly we reach herd immunity will depend on production, distribution and the willingness of the American people to take the vaccine. There is a concern that many Americans may not take the vaccine even if it's available. We need to have a thoughtful public education campaign that's tailored to different communities.
And we need everyone's help! When it's your turn, please take the vaccine. Help spread the word and convince your family and friends about the importance of the vaccine to saving lives and ending this pandemic.
CNN: What about people who can't get the vaccine? Do they need to keep wearing masks?
Wen: For now, everyone needs to keep wearing masks. There will be small numbers of individuals who cannot get vaccines. In the beginning, children won't be able to get vaccines because it hasn't yet been tested on children. It's also possible that there are some medical conditions that make it such that certain people can't get the vaccine, or that the vaccine is less effective for them. That's why the rest of us have to get vaccinated, to protect them. Herd immunity is also called community immunity: The community is getting vaccinated to protect everyone.
That's also why we keep wearing masks, too. We do it to protect ourselves, and to protect others. Remember that even after we get the vaccine, we can spread the virus to others. Also, the vaccine is very effective but it isn't 100%. The mask still protects you, too.
CNN: Would you still keep wearing a mask after you get the vaccine?
Wen: Yes. I'll do it to protect others, and to protect myself, too. Here's another way to think about the importance of mask wearing. The vaccine protects you if the virus reaches your nose and your mouth. Your body senses the virus, and instead of the virus attacking your body, your body's immune system kicks in and gets rid of the virus.
It's very important to prevent the virus from reaching your body in the first place. Wearing a mask does that. So does physical distancing. These are really important measures to prevent from getting coronavirus and transmitting it to others.
You will certainly see mask wearing among health care workers, who will be among the first groups to get the vaccine. The vaccine is one important layer of protection for us, but we will use these other measures to protect ourselves and those around us.
CNN: With Covid case numbers at an all-time high in the US, the vaccine couldn't come a moment too soon. With just a small percentage of the US population receiving the vaccine to start, however, it seems as if most people won't directly enjoy the benefits of this first rollout, right?
Wen: Absolutely. It's really astounding that we have a vaccine that looks to be 95% effective and very safe. We will be able to put an end to the pandemic. But this will take time. The initial allotment of vaccines will reach just over 1% of the population. It will take time to scale up to reach 70%.
We need for everyone to keep following the precautions we've been talking about all along: Wear a mask. Keep physical distancing. Avoid indoor gatherings. Wash your hands.
I'll add one more: Get the vaccine when it's your turn. We can get through this winter, and the spring and summer hold so much promise.