To boost or not to boost — not a simple question

How long does the protective immunity provided by the COVID-19 vaccines last? Do we need to “boost” immunity for continued protection? Recently, these questions have thrown science and politics into a contentious situation. The Biden administration has strongly recommended booster shots for COVID-19 beginning this month, with the caveat that the Food and Drug Administration (FDA) would have to approve. As a result, significant public and political pressure has been put on the FDA to fast-track approval of the booster. The FDA traditionally proceeds slowly and cautiously with insistence on rigorous supporting data and usually does not respond well to pressure.

This conflict recently led to the resignation of the FDA’s director of the Center for Biologics Evaluation and Research (CBER), Dr. Marion Gruber, and her deputy director, Dr. Philip Krause. Both are capable scientists with years of experience evaluating vaccines. 

Unfortunately, the Biden administration appears not to have adhered to its plan to follow the science when making the recommendation for vaccine boosters. If vaccine-induced immunity is indeed waning, it would be prudent to ready the supply chain to administer millions of booster shots. However, if robust immunity remains sufficient to protect from severe disease, then this rush to boost wastes valuable resources and compromises the goal of ending the pandemic. This decision requires rigorous scientific evidence that protection afforded by the vaccine is indeed waning. 

In July, a large Israeli study reported that people vaccinated for the longest time were becoming more vulnerable to serious COVID-19 illness. The finding was interpreted to mean immunity waned as early as seven months after vaccination. Recently this report has been criticized primarily for overlooking key confounding differences between the two groups other than the time of vaccination — namely that the first Israelis to be vaccinated tended to be more affluent and, therefore, more likely to travel, increasing exposure risk to the delta variant. 

Nevertheless, this study fueled a debate about the need for vaccine booster shots, playing a significant role in the Biden administration’s booster recommendation. It is expected that boosters will be needed, particularly to enhance protection against more concerning variants. However, the scientific evidence for waning immunity in people under 50 with no underlying issues, like immune deficiency, is not clear. 

It is important to understand that effective vaccines protect from serious illness but often do not fully block infection. A fully COVID-19 vaccinated person can be infected in the same way as an unvaccinated person (the so-called breakthrough infections). This happens when virus particles infect cells of the upper respiratory tract and begin to replicate. However, what happens next between a vaccinated and an unvaccinated person can literally mean the difference between life and death. 

In vaccinated people, antibodies induced by the vaccine may capture some of the incoming viruses, reducing the number that gets into cells, and T cells quickly limit the infection in cells that were infected. The progression of the disease will be stopped, greatly diminishing the risk of severe disease, hospitalization or death. By contrast, unvaccinated individuals have no immune response waiting to slow the spread of infection to the lungs where serious disease occurs. One study done by the Centers for Disease Control and Prevention (CDC) shows that unvaccinated people are 29-times more likely than vaccinated people to be hospitalized with COVID-19.

During the first days of infection, vaccinated people infected with the delta variant may produce as much virus in their upper airway as unvaccinated people. However, virus levels rapidly decreased in vaccinated individuals, diminishing the probability that they could infect others. These studies prompted the CDC to recommend masking for vaccinated individuals. 

Maintaining effective immunity is key to controlling the pandemic. However, it is natural that immune responses decline, and antibody levels taper off over time, leaving only the most robust to provide protective memory of the vaccination. The COVID-19 vaccines are in their early days. It will require well-designed studies to determine when a booster is needed and when it will be most effective; the real-world data suggests we are not there yet.

The vaccines are still doing a remarkable job of reducing serious disease, hospitalization and death in vaccinated people who are not immunocompromised and do not have other underlying health issues, even against the threat of the delta variant. For this reason, health officials have advised the Biden administration to scale back the plans for boosters.

Besides the need for data to justify boosting the entire population now, we must also consider the ethics of using valuable vaccines for potentially unnecessary booster shots when many around the world do not have their first vaccination. Achieving worldwide vaccination is critical to ending the pandemic and stopping the emergence of dangerous variants. We can greatly reduce COVID-19 cases in this country by simply vaccinating more people. But we will never be free of the pandemic and new variants until the entire world is vaccinated.    

James Alwine is an emeritus professor and a virologist; he is a fellow of the American Academy for Microbiology and the American Association for the Advancement of Science. Lynn Enquist is an emeritus professor and a virologist; he is a fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Science, and the American Academy of Microbiology. Felicia Goodrum Sterling is a 2018 Public Voices fellow of the OpEd Project, a professor and a virologist.