COVID Quickly, Episode 18: Vaccines for Kids and the Limits of Natural Immunity


Tanya Lewis: Hi, and welcome to COVID, Quickly, a Scientific American podcast series.

Josh Fischman: This is your fast-track update on the COVID pandemic. We bring you up to speed on the science behind the most urgent questions about the virus and the disease. We demystify the research and help you understand what it really means.

Lewis: I’m Tanya Lewis.

Fischman: I’m Josh Fischman.

Lewis: And we’re Scientific American’s senior health editors. Today, we’re going to talk about kids vaccines—the low risks and the considerable benefits…

Fischman: … and why the shots protect people better than previous COVID infections do.

Fischman: The big news this week is that the FDA and CDC have made COVID vaccines available to kids ages 5 through 11. How are parents reacting, and what do we know about the vaccine’s safety in this age group? 

Lewis: Many parents are breathing a collective sigh of relief that they can now protect their young ones from COVID. They’re also thrilled that their kids can get back to doing activities they had to give up during the pandemic, like seeing their grandparents or playing with friends.

But other parents are reluctant to get their kids vaccinated, or still on the fence.

Here’s what we do know: While COVID is generally milder in children, more than a million kids were infected with the virus in September alone, and 41 died, according to CDC data. That’s 41 sets of parents who had to bury a child who died from a preventable illness. 

The Pfizer vaccine was tested in more than 1,500 kids in the 5-11 age group. They were given one third the dose adults get.

The vaccine was more than 90 percent effective at preventing symptomatic COVID, and the few infections that did occur in vaccinated kids were all mild. Vaccinated 5- to 11-year-olds also had a similar level of antibodies as teenagers who got the adult dose.

The side effects from the vaccine were similar to those in teenagers, but milder: they included fatigue, headache, fever, chills, and pain at the injection site.

And there were no cases of myocarditis or pericarditis—inflammation of the heart muscle or lining—which have occurred in a small percentage of teens and young adults who got the Pfizer or Moderna vaccines.

Myocarditis is more common in boys, but it’s so rare that it would be nearly impossible to have a big enough clinical trial to see the effects. Myocarditis can also be caused by infections, including COVID itself, where it is often more severe. 

While some young adults have been hospitalized with vaccine-related myocarditis, most cases resolve on their own, and there have been no deaths linked to the vaccine.

The FDA and CDC both concluded in separate meetings that the benefits of recommending the COVID vaccine for 5-11 year olds outweigh the risks. And we routinely vaccinate kids against diseases that hospitalize or kill far fewer children. Vaccinating kids also protects communities, and could get us closer to ending this pandemic.

Lewis: I keep hearing some people say they don’t need a COVID vaccine because they’ve already been infected. They say they are immune. Is that true? 

Fischman: It’s actually a reasonable thing to wonder. If you’ve been infected by the COVID-causing virus already, do you have enough natural immunity to prevent a second infection? The quick answer is that COVID vaccines work better.

Prior infection does give your immune system some resistance. However, when studies compare that to the immunity you gain from vaccines, it’s not as strong, or as long-lasting, or as good at keeping you out of the hospital. 

Now here’s the longer, more complicated answer. Infections with some other viruses do give most people lasting immunity. It happens with chicken pox; it happens with the mumps, for instance. But the COVID virus, SARS-CoV-2, behaves differently from these other viruses. Immunity to it doesn’t last as long. 

Yet this idea that you can maintain natural immunity has become part of the pushback against vaccine mandates. There’s even a vaguely-worded bill in Congress promoting it.

A new study does kind of a head-to-head comparison of natural immunity and vaccines. It looked at 7000 patients in the U.S., hospitalized with COVID-like breathing trouble. Some had been previously infected with the virus and weren’t vaccinated. Others had been, with either Pfizer or Moderna. 

When tested, the previously infected patients were more than five times more likely to be positive for the virus than were vaccinated patients. They had been reinfected. Many of the vaccinated people, in contrast, turned out to have some non-COVID illness. The study was led by Catherine Bozio of the CDC, if you want to look it up.

Another CDC study of a few hundred people in Kentucky, published in August, found that previously infected but unvaccinated people were twice as likely to get reinfected than were vaccinated folks.

There’s more. A study in the journal Science, from July, indicated that vaccination actually helps previously infected people. Vaccines boosted their antibodies against the virus by 1000-fold.

Lewis: So vaccine immunity is always better, then?

Fischman: Well, remember I said it’s complicated. A study of thousands of people in Israel found that previous infections prevented new infections more often than vaccines did. 

Some cautions, though. The researchers didn’t test all the unvaccinated people. Some may have had undetected mild or asymptomatic reinfections. There might have been enough of those to swing the numbers in favor of vaccination. 

Also that study hasn’t been peer reviewed. Finally, it found previous infection PLUS vaccination produced the best protection of all. 

Bottom line: The CDC just reviewed dozens of studies on this. It says that, broadly, both natural infection and vaccines keep risk low for about 6 months. But getting down to the individual level, there are wide swings in antibody levels after natural infection. Some high, some low. Antibodies after vaccination, though, are consistently higher and go after the virus harder. 

Given a virus that has killed 5 million people around the world, 750,000 in the U.S., and put 8300 young kids in this country in the hospital, the evidence says vaccines keep you much safer.

Lewis: Now you’re up to speed. Thanks for joining us.

Fischman: Come back in two weeks for the next episode of COVID, Quickly! And check out for updated and in-depth COVID news.

[The above text is a transcript of this podcast.]


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