The F.D.A. may authorize booster shots of vaccines different from the ones that Americans originally received. The science behind the move is promising.
The Food and Drug Administration seems likely to allow Americans to switch vaccines when choosing a Covid-19 booster shot. That authorization, which could come this week, is the latest development in a long-running debate over whether a mix-and-match strategy helps protect people from the coronavirus. Here are answers to some common questions about mixing and matching booster shots. Immunizations typically consist of two or more doses of the same vaccine. The Moderna vaccine, for example, is administered in two identical shots of mRNA, separated by four weeks. A double dose can create much more protection against a disease than a single shot. The first dose causes the immune system’s B cells to make antibodies against a pathogen. Other immune cells, called T cells, develop the ability to recognize and kill infected cells. The second shot amplifies that response. The B cells and T cells dedicated to fighting the virus multiply into much bigger numbers. They also develop more potent attackers against the enemy. In recent years, some vaccine researchers have experimented with a switch from one vaccine to another for the second dose. This strategy is technically known as a heterologous prime-boost. The pandemic spurred more research into this possibility. One of the first authorized heterologous prime-boost vaccines for any disease is the Sputnik V vaccine, developed last year by Russian researchers to prevent Covid-19. It uses two different adenoviruses to deliver coronavirus proteins, which the immune system then attacks. The first dose contains an adenovirus called Ad5, and the second contains another, called Ad26. Scientists have long suspected that heterologous prime-boosts sometimes work better than two identical doses. The designers of the Sputnik V vaccine were concerned that the first shot of Ad5 would create antibodies not just against the coronavirus proteins it delivered, but also against Ad5 itself. A second shot of Ad5 might be wiped out by people’s immune systems before it could boost protection against Covid-19. Studies of experimental H.I.V. vaccines also suggested that mixing vaccines could create a broader, more potent response than multiple doses of a single vaccine. Different types stimulate the immune system in different ways, and switching between two vaccines might give people the best of both worlds. The pandemic gave scientists new opportunities to test that idea. As the AstraZeneca vaccine was quickly rolled out in Europe, it became clear that younger recipients run a small but real risk of developing blood clots. Young people who had already received one dose of AstraZeneca were offered a second dose of Pfizer-BioNTech. The two vaccines are profoundly different. AstraZeneca’s formulation is based on a chimpanzee adenovirus. Pfizer and BioNTech make their vaccine with mRNA. When researchers looked at the immune response from this heterologous prime-boost, they found that it produced more antibodies than two shots of AstraZeneca alone.
