How worried should vaccinated people be about Covid-19 breakthrough infections?

Coronavirus infections are on the rise again in the United States. While more than 99% of deaths are among those unvaccinated, anecdotal reports abound of breakthrough infections, or cases of fully vaccinated people who still test positive for Covid-19 — including several New York Yankee baseball players.

How worried should vaccinated people be of contracting Covid-19? If you’re vaccinated, are you still able to transmit coronavirus to others, such as young children too young to get the vaccine themselves? Does the more transmissible Delta variant change the equation, and what precautions should vaccinated people still be taking?

To give us some guidance during these uncertain times, we turned to CNN Medical Analyst Dr. Leana Wen. Wen is an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. She’s also author of a new book out next week, “Lifelines: A Doctor’s Journey in the Fight for Public Health.”

CNN: Can vaccinated people become infected with Covid-19?

Dr. Leana Wen: Yes, they can. Here’s what the Covid-19 vaccines do. First, and most importantly, they protect you very well against severe disease. That’s key. This is a disease that has taken the lives of over 600,000 Americans and millions of people around the world. If you get the vaccine, you know that you are very unlikely to become severely ill to the point of needing to be hospitalized or to succumb to the disease. According to federal health officials, 99.5% of deaths from Covid-19 are now among the unvaccinated. That is a real testament to the power of the vaccines.

The vaccines also protect against becoming ill from Covid-19, but this protection is not 100%. With the Delta variant, the vaccines may be even less effective against mild disease — though still effective against severe disease.

That means breakthrough infections — or infections in people who are fully vaccinated — can and do happen.

CNN: Does it matter if you’re in a community with a lot of infection? Are you more likely to get a breakthrough infection?

Wen: Yes, and that’s why it matters what’s going on around you even if you are fully vaccinated. Risk is additive. The vaccine protects you well, but if you are constantly exposed to people who are carrying coronavirus, at some point you could have a breakthrough infection.

FILE - In this Thursday, March 11, 2021 file photo, desks are arranged in a classroom at an elementary school in Nesquehoning, Pa. In the fall of 2021, vaccinated teachers and students should no longer wear masks inside school buildings and no one need bother with them outside, the Centers for Disease Control and Prevention said Friday, July 9, 2021, in relaxing its COVID-19 guidelines. (AP Photo/Matt Slocum, File)

Related article What the new CDC guidance for schools means for children

I’ve used the analogy before of a raincoat. The vaccine is an excellent raincoat. If you’re in the occasional drizzle, you’ll probably be fine. But if you’re going from thunderstorm to thunderstorm, at some point, you could get wet.

This is also why we have to see vaccination as not just an individual choice. Even if you’re vaccinated yourself, it matters if others around you are vaccinated, too.

CNN: Do we know how common breakthrough infections are or whether people who are vaccinated but get infected are able to transmit to others?

Wen: These are really important questions, and unfortunately, we don’t know the answers. In May, the US Centers for Disease Control and Prevention decided to stop tracking mild breakthrough infections. They are only reporting cases of breakthrough infections severe enough to result in hospitalization and death. Many of us in public health have argued that we need to know the data around mild breakthrough infections, too. It’s important to know how often these are occurring and among whom. For example, are they more common in older individuals and those with compromised immune systems, suggesting that these individuals may need a booster shot sooner? Do they increase in frequency after a certain point following vaccination, and are they more common with one vaccine versus another? Do people with breakthrough infections develop long-haul Covid?

Another key question is whether people with breakthrough infections are able to transmit Covid-19 and infect others. Earlier studies found that vaccination also substantially reduces the amount of virus someone exposed to Covid-19 would carry. That even if they test positive or develop mild symptoms, they are unlikely to infect others. However, these studies were done before the Delta variant became the dominant form of the virus in the US – the CDC has reported that 83% of cases in the US are now due to Delta.

One way to have a healthy conversation about the Covid-19 vaccine is to accompany your loved one on a doctor's visit to discuss the pros and cons.

Related article How to speak to someone who's hesitant to get vaccinated

Other research finds that the unvaccinated who are infected with the Delta variant carry 1,000 times the amount of virus than people with the original variant. That calls into question what happens if someone is vaccinated but infected with the Delta variant. We just don’t know, at this point, how likely it is for a vaccinated person with a breakthrough infection due to the Delta variant to be contagious to others.

CNN: Do the unknowns around the Delta variant change your recommendations for vaccinated people interacting with others?

Wen: I would use an abundance of caution until we have more data. A vaccinated person around other fully vaccinated people is probably pretty safe and would not need precautions like masking and distancing. On the other hand, a vaccinated person who is exposed constantly to unvaccinated people, in crowded, indoor settings where no one is wearing masks, could become infected themselves. And even if they don’t have symptoms, there is definitely the possibility that they could carry the virus and infect others.

Until we know more about whether those vaccinated but contract the Delta variant could transmit it to others, I would urge people to be cautious if they live at home with unvaccinated or immunocompromised family members. They should consider wearing masks in indoor spaces like grocery stores and trying to avoid high-risk settings like crowded bars where others around them are unmasked and have unknown vaccination status.

CNN: To be clear, you would still recommend the vaccines?

Wen: Absolutely. The most important thing the Covid-19 vaccines do is to protect against hospitalization and death. They have proven to be remarkably effective at this, even with the Delta variant. Breakthrough infections are to be expected, and the more infections occur among the unvaccinated, the more infections there will be among the vaccinated.

The key to stopping the pandemic is for us to reach a high enough level of immunity that the virus has nowhere else to go. We can get there — but all of us have to do our part and get vaccinated. Vaccination protects the individual, and it protects others around us, too.