I got vaccinated, then I got covid. What happened?

Mary Duenwald, a Bloomberg Opinion editor, tested positive for Covid-19 even after receiving both doses of the Pfizer Inc.-BioNTech SE vaccine. Here, she discusses her experience with Sam Fazeli, a pharmaceutical industry analyst for Bloomberg Intelligence. The conversation has been edited and condensed.

MD:  I was vaccinated for Covid recently  — I got my second Pfizer shot a few weeks ago. I was hoping that meant I wouldn't get Covid, yet I tested positive this week. How is that possible?

SF: Thank you, Mary, for sharing your story. The reality is that so-called sterilizing immunity, or protection that completely blocks a virus from infecting you, is rare. In fact, only one vaccine has been proven to provide that, and that is the smallpox shot. If you think back to the phase III trials of the Pfizer-BioNTech and Moderna Inc. vaccines — arguably the ones with the best efficacy so far —  they were 95% effective, meaning some vaccinated people still developed symptoms after the second dose, even if at a very low rate. Also, outside the environment of a clinical trial, efficacy will be slightly lower; real-world data is showing about 90% efficacy. It's also possible that you are infected with a new variant, such as the one that is circulating in New York, known as B.1.526. These variants are potentially better at getting past your antibody immunity.

MD: My symptoms are mild, as if this were just a cold. Can I assume the shot is still protecting me?
 
SF: Of course. You will never know how bad your symptoms would have been if you had not been vaccinated, but they could have been much worse. The vaccines have high efficacy against severe and critical disease — indeed their main purpose is to keep people out of the hospital and lower their risk of dying. Even the ones with apparently lower efficacy against mild and moderate disease, such as Johnson & Johnson's shot, show better efficacy against severe and critical illness.

MD: Does being vaccinated mean I am less likely to transmit the virus to someone else?

SF: Some data suggest that is the case, but it is early days. The antibodies generated by vaccination can stop a virus from actually infecting you or from allowing the infection to "take." Specifically, people who are vaccinated and then test positive have lower amounts of virus in their nasal passages and possibly even less in their lungs than if they were not inoculated. This means there is less virus in the air you breathe out. How long this lasts is not known, though. The data we have come from people soon after their vaccinations, when they have high levels of antibodies. As the months pass, these levels may fall to a level that they can't stop an infection in the absence of a vaccine booster or a natural infection like yours. We need to see the data on asymptomatic infections at six months and 12 months after the second dose of the vaccine to have a better idea.

MD: I was tested the same day I learned I had been in contact with someone with Covid, and again the next day, and both tests came back negative. The positive test came a week later. Does that mean I wasn't infected by that person?

SF: No – there is an incubation period that can last as long as two weeks. It's possible that people who are vaccinated have a longer incubation period as the virus is hampered by the immune system. The other interesting question is whether you need to quarantine as long as an unvaccinated person with an infection does. Might you be safe sooner because you've had the vaccine? You might, but we just don't know yet.

MD: What's the takeaway from my experience about how safe a vaccinated person should feel, and the kind of measures they still need to take, especially around the unvaccinated?

SF: Vaccinated people should feel safe until the data show otherwise. Currently, very few people are showing severe infections after being vaccinated, and that's a good thing. At the same time, as long as there are people out there who are not vaccinated, it is best to continue with masking and distancing — for their sake.

MD: In a similar vein, some on the Bloomberg Opinion team had questions about the risks involved with an elderly vaccinated person visiting unvaccinated relatives. Would that be dangerous?

SF: The best answer is to wait until everyone in your bubble is vaccinated. Then you can resume your normal behavior when together.

MD: After I get through this, will I be immune to future infections?

SF: It is possible that your infection has given you a second "booster," but I doubt anyone will ever be 100% immune to mild infections. What would be interesting to know is whether you are infected with one of the virus variants. If so, we could then look at your immune response to see if it has changed and can give you better immunity to this new variant the next time you encounter it. Sadly, we don't have a lab at Bloomberg to do this analysis.

Disclaimer: This article first appeared on Bloomberg, and is published by special syndication arrangement