Booster shots, or additional shots of COVID-19 vaccinations given about two months after vaccination with J&J and about six months after vaccination with Moderna or Pfizer, are now available to all adults in the United States.

Hospital epidemiologist Michael Klompas, MD, MPH, recently spoke with the Brigham Clinical and Research News about the science behind booster shots and what the next phase of the pandemic may look like.

Can you talk about the risk of breakthrough COVID-19 infections?

MK: We know breakthrough infections in fully vaccinated people can occur and can sometimes be serious. But  there’s new evidence demonstrating that even though fully vaccinated people remain at risk for SARS-CoV-2 infection, they enjoy substantial protection from every aspect of SARS-CoV-2:  they are less likely to be get infected, less likely to be silent carriers, less likely to transmit to others, and if they do get infected, they’ll most likely experience a milder illness and faster recovery. They’re less likely to be hospitalized, become critically ill or die from an infection. Big picture: Vaccines are not perfect, but they provide excellent protection against both mild and severe of disease.

Can you speak about the evidence around boosters and why they might be an important tool?

MK: We have two lines of data that support the use of boosters. One is the increasing number of studies that indicate that with time, the effectiveness of the basic vaccination series wanes, especially against mild infections, but also to a degree against more severe infections. This is particularly the case for more vulnerable people. So, there’s a need to pump up the immune system to get a person back to their original, powerful vaccine response. Second, data from countries that have been using boosters for a couple of months have shown that introducing boosters into the population reduces hospitalization and death rates, returning one’s protection back to the high level we enjoyed in the first few months after vaccination.

Have scientists researched whether waning immunity might be impacted by the rise of the Delta variant?

MK: Yes, this question has been researched. It appears vaccines are maybe slightly less effective against Delta compared to prior strains of the virus. There’s not a huge difference, though, the much greater threat to people is waning immunity over time.

Is the COVID booster going to become an annual vaccine like the flu shot? Is there a clear picture of what might lie ahead?

MK: I don’t know! Some very smart people like Dr. Anthony Fauci suggest that the third shot will give us long-lasting protection and we may not need to get additional boosters. On the other hand, the pharmaceutical companies Moderna and Pfizer have been saying annual boosters may become the norm. Time will tell. Presumably, we’ll learn more about the effectiveness of boosters from countries like Israel, who quickly rolled out initial vaccines and boosted much of their population over the summer. Stay tuned.

What might the pandemic look like through winter 2021 or spring 2022 in the United States?

MK: A lot of variables are at play in the United States. On the one hand, we know rates of infection tend to increase in the winter because people spend more time packed together in poorly ventilated indoor spaces, which increases the risk of transmission. We also know a large segment of society is no longer taking any kind of protective measures — distancing, masking and the like. And we know that many people are now more than 6 months out since their last vaccine so are more susceptible once again to infection.  On the other hand, we’re privileged to live in Massachusetts, which has one of the highest vaccination rates in the nation. Boosters are easily available.  And kids aged five and up are now eligible for the vaccine. When you marry together all these harmful and helpful factors, I don’t know exactly where we’ll land. Among the vaccinated, I anticipate we’ll see the largest increase in disease among those who received their initial dosages a long time ago and have not received a booster. But the unvaccinated carry the greatest risk of severe infection and hospitalization. Finally, we are all watching the news about the latest variant of concern, Omicron, very closely to see how that will affect the picture.

Can you touch on the global aspect of the pandemic? Can we predict what may be in store for us based on trends in other countries?

MK: A few countries preceded us with their Delta waves: India, the United Kingdom and other parts of Europe. We’ve seen different patterns play out in different countries. India had a massive spike of cases back in the early spring, which came down to a very low level after that. The United Kingdom by contrast saw a peak in the late spring and early summer but they didn’t quite come down to their prior baseline level. Their case counts decreased to a moderate level, and as they loosened restrictions, they saw another increase in cases. Which of those patterns might the U.S. follow? I think we’re already starting to see more of a U.K. pattern; our case counts have gone down from August and September, hit a plateau and are now increasing in many states across the country. I think this rise will continue.

How might recent advances in treatment affect the pandemic’s trajectory?

MK: It’s important to note that the significance of a case today compared to a case a year ago is quite different. This is partly because different people are getting infected, partly because vaccination rates are decent and partly because better treatments like monoclonal antibodies, remdesivir, steroids, and other anti-inflammatories are now available.  In addition, the antiviral pills produced by Merck and Pfizer that appear to prevent hospitalization and death will soon be available. For people who do get hospitalized, we now have a much better understanding of the virus’s mechanisms and additional drugs are available to ensure maximal results in treatment.  We’re already seeing that similar numbers of cases in society translate into many fewer hospitalizations and deaths versus a year ago. So, comparing cases from now and the past is like comparing apples to oranges.


Klompas recently wrote an editorial in JAMA about breakthrough infections that you can consult for further discussion on this issue.