Why the new COVID vaccine boosters are safe and likely very effective

In case you haven’t heard, there’s now a new set of vaccine booster shots that protect against the latest variant of Covid-19, BA.5. This new variant is highly infectious, and the original vaccine doesn’t protect people against it as well as it protected against earlier variants.

Now, before someone takes that last sentence out of context, let me emphasize that the original Covid-19 vaccines are still highly effective at preventing severe disease and hospitalization. Anyone who isn’t vaccinated would be well-advised to get one of those, if that’s their only option.

But the new vaccines protect against two different strains of Covid-19: the original strand and the latest Omicron variant, BA.5. BA.5 just emerged in August, and it’s spreading extremely fast–it now accounts for 88% of the cases in the U.S. The FDA authorized these “bivalent” boosters on August 31.

My main point today is to explain how this happened so quickly, and why it didn’t involve many months of clinical trials the way that the original vaccines did. Some people have expressed suspicion about the speed with which these boosters appeared, but those suspicions are entirely unfounded.

First of all, we do have data showing that the new vaccine boosters are safe: a study showing these data for the Moderna booster just appeared in the New England Journal of Medicine. That study also showed that people generated robust antibodies against the BA.1, BA.4, and BA.5 Omicron variants after getting the new booster.

Second, to explain why that’s all the data we need, let me explain something about the flu vaccine. Many people are well aware that we have a new influenza vaccine every year–but what they might not know is that each year’s vaccine contains a different strain of the flu virus from the previous year. (Actually, it has up to four different strains, and any or all of them could be new.)

And yet we don’t run large, months-long clinical trials of the flu vaccine each year to measure it’s efficacy. Why not? Well, the basic designs of the flu vaccines (there’s more than one) have been tested in large clinical trials, and we know they’re safe. Replacing the vaccine strain with a new one doesn’t affect the safety of the vaccine, as decades of experience with the flu vaccine have now demonstrated. And because flu mutates quickly, we need to change the vaccine strain every year if we want the vaccine to work.

So that’s what we do: we change the flu vaccine strain to match whatever seems to be circulating, but everything else about the vaccine design remains the same.

This is exactly what scientists have done with the new Covid-19 booster shots from Pfizer/BioNTech and Moderna: they simply added a new strain, in this case to match the latest Omicron variant.

Both of the new boosters are mRNA vaccines, which means the vaccine contains a tiny bit of genetic code (mRNA) with instructions on how to make the “spike” protein from SARS-CoV-2, the Covid-19 virus. When you get the vaccine, your own cells follow those instructions to make a limited number of copies of the spike protein–but nothing else. The virus itself can’t be created without many other genes, and the spike protein alone can’t form a virus.

But your immune system recognizes that an invader is present–that spike protein–and it generates antibodies against it. After that, your immune system is primed to recognize and defeat Covid-19, if it ever infects you.

So the new boosters contain mRNA from the BA.5 spike protein as well as the original spike protein, and now you get protection from both old and new strains. Other than that, it’s the same vaccine as before.

The vaccine makers have produced safety data showing that the booster is just as safe as the original vaccine (see that NEJM article I mentioned above). But the FDA didn’t require them to prove that the booster really works better against BA.5, because that would require months of trials, and by that time the virus could mutate again. So while it’s possible that the boosters won’t help against BA.5, it’s very, very likely that they will.

One of the big advantages of mRNA vaccines is precisely this: we can swap out the mRNA easily, allowing us to respond quickly to mutations in circulating viruses. That’s a good thing, and it might be our only path to controlling Covid-19.

The bottom line is that the new boosters should be very safe, and they will likely provide much better protection against the current strains of the Covid virus. I’ve already gotten my booster, and I hope everyone else will too.

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