This week, the FDA authorized Pfizer and Moderna to start administering vaccine boosters that have been reengineered to specifically target the Omicron BA.4 and BA.5 variants that caused this summer’s COVID-19 wave. The CDC is expected to issue its recommendation shortly, and boosters could be rolled out as early as next week.
The vaccines are what’s called bivalent, meaning they contain mRNA snippets of both the original virus strain and the Omicron variants (the spike protein the vaccine targets is identical for BA.4 and BA.5). As a result, your immune system is trained on both versions of the virus and will create antibodies specific to each one.
The reason this update is necessary is because the Omicron strains, including the initial BA.1 variant that descended upon the U.S. last winter, are very different from earlier versions of SARS-CoV-2. Specifically, the spike protein that the virus uses to sneak into cells–and crucially that antibodies recognize to defend against the virus–has mutated dramatically. As a result, the antibodies the immune system produces via vaccination or infection with prior variants barely recognize the Omicron spike protein.
“It is important that we have updated vaccines for COVID-19, just like we update influenza vaccines. And the reason I think it’s particularly important right now is because Omicron is so different from the original strain,” says Anna Durbin, a professor in international health at Johns Hopkins University. “It doesn’t make sense to me to give the same vaccine over and over when we know that that sequence is not a good match.”
Compounding the spike protein problem is the fact that antibody levels wane over time. If you got a booster or were infected more than a few months ago, your total number of antibodies has likely declined. A vaccine specific to Omicron will both raise your overall antibody levels and improve how well those antibodies can attach to the mutated spike proteins. Researchers think this will provide better protection against symptomatic infections than if the original vaccine strain was used alone. Because the vaccine is bivalent, if the virus mutates again to look more like its initial form, antibodies for that version of the spike protein will be boosted, too.
While there isn’t clinical data yet for the BA.4/BA.5 vaccines specifically, there is evidence from research conducted earlier this year on a vaccine targeting BA.1. Data from Pfizer showed that a BA.1 specific booster is just as safe and offered better protection against Omicron than the original vaccine.
“I don’t have any concerns about safety,” says Florian Krammer, a professor of vaccinology at the Mount Sinai Icahn School of Medicine. “In terms of efficacy and how well this works, we have to wait and see. But of course, having a more specific response to what is circulating should provide better protection against both symptomatic and asymptomatic disease.”
People who were infected with Omicron will produce some variant-specific antibodies, but it’s still critical that everyone get boosted. For one thing, the initial Omicron strains that circulated last winter are different enough from BA.5 that those initial antibodies won’t provide very much protection. That’s why the U.S. decided to pursue a BA.5-specific vaccine instead of using the BA.1 vaccine that was developed earlier.
The other main reason to get boosted–even if you were infected with BA.5–is that antibody responses to Omicron appear not to be as strong as to earlier versions of the virus. That may explain why some people have gotten infected with Omicron multiple times this year.
“Omicron is doing a lot of things differently than other variants, and one of them is that the immune response from infection alone isn’t as strong as what we saw with other variants,” says Andrew Pekosz, a professor in molecular microbiology and immunology at Johns Hopkins.
Getting a booster targeting Omicron on top of an earlier Omicron infection will likely provide superior protection because your immune system will already be primed to recognize and defend against that version of the virus. However, if you just had Omicron recently, wait at least a few months–Durbin says three, Pekosz says six–before getting boosted.
“To get those responses to be stronger for a longer period of time, you have to give your body a little bit of a break and let your immune system come back to normal. Then stimulate it again a few months down the line,” Pekosz explains. “That’s one of the best ways to generate what we call memory [immune] responses, which are those responses that will kick in later when you get exposed to the virus.”
It’s important to remember that even with the new vaccine, you may have protection against infection only for a few months, but the real purpose, Durbin says, is “to protect against severity of illness, particularly hospitalization.” You can schedule an appointment for a booster here.
Dana G. Smith is award-winning health and science writer with a PhD in psychology.