Kid covid vaccines: What the heck just happened?

The long covid winter just got longer for U.S. parents with kids under 5 years old.

“At this time, it makes sense for us to wait until we have the data from the evaluation of a third dose before taking action,” said Peter Marks, the FDA’s top vaccine regulator, during a call with reporters.

The announcement is disappointing for anyone who thought young children might soon be eligible for the protection that covid vaccines provide — but the delay isn’t a sign that the vaccine is unsafe or doesn’t work. Here’s what just happened, and why.

What did the FDA announce?

The FDA pressed pause on authorizing the Pfizer and BioNTech vaccine for young kids by canceling an advisory committee meeting scheduled for Feb. 15. The agency’s independent advisers — who include medical doctors and public health experts — would have reviewed data from the companies on the efficacy of their two-dose vaccine regimen. They also would have heard from FDA staff about the agency’s assessment of the Pfizer-BioNTech trial results.

The FDA isn’t bound to follow the recommendations of its advisory committees, but it usually does. An advisory panel vote is normally the last step before the agency makes its final decision on a vaccine.

“The data that we saw made us realize that we needed to see data from a third dose in the ongoing trial in order to make a determination that we could proceed with doing an authorization,” Marks said.

The recent surge of omicron infections in kids means that Pfizer and BioNTech will get that data faster than expected, he added — though he declined to give a timeline for authorization. Pfizer and BioNTech have said they expect to have three-dose data by early April.

Wait, didn’t we already delay this at least once?

Yes.

Faced with that disappointing data, the companies said they would modify the trial to test whether giving a third dose eight weeks after the second dose would produce better results. That set back their time frame for seeking authorization from the FDA from January to sometime in the first half of 2022.

How is this different from adults needing a third shot as a booster?

Adults were the first age group eligible for covid shots. The FDA authorized Pfizer’s and Moderna’s covid vaccines for adults in December 2020 as a two-dose regimen. Clinical trials showed that approach was roughly 95 percent effective for each vaccine.

Almost a year later, after new variants emerged and data from the U.S., Israel and other countries suggested the protection of those initial two doses was wearing off over time, the FDA advised adults to get a third shot as a booster. What Pfizer and the FDA are saying now about shots for 2- to 4-year-olds is that two initial doses alone don’t provide strong enough protection. They’re betting that age group will need three doses to start; boosters might still be necessary in the future.

Why won’t the FDA just authorize the vaccine for kids under 2?

Vaccines are normally tested first on adults, and if they are proved safe and effective, the next step is to conduct a trial in teens, with younger children coming last.

The approach is called “age de-escalation.” It’s based on a simple biological fact: Children’s immune systems work differently than those of adults or teens, and their brains and bodies are still developing. So clinical trials start with the least vulnerable group, adults, and move to younger ages only if a vaccine proves workable for older people. That rationale probably won’t satisfy frustrated parents of kids 6 months to 2 years, but it’s a time-honored approach in medicine that is based on the idea that safety is paramount. Does the FDA’s latest action mean the vaccine isn’t safe for kids?

Does the FDA’s latest action mean the vaccine isn’t safe for kids?

No.

The message so far from Pfizer, BioNTech and the FDA has been that the vaccine appears safe, based on existing data. The delay stems from concerns that two doses aren’t effective enough for preschool-aged kids.

Why might three doses work better than two?

When designing vaccine trials, figuring out what dose size will provide good protection without too many side effects is usually an educated guess. Doing it under crisis conditions in a global pandemic ups the ante.

People 12 and up receiving the Pfizer-BioNTech vaccine get two shots of 30 micrograms each. That’s too much for 5- to 11-year-olds, who get only 10 micrograms a shot.

Researchers first tried two shots of an even smaller dose — 3 micrograms — for the littlest kids. For whatever reason, that wasn’t enough to stimulate the protective immune response seen in older kids and adults.

When will we get answers?

Later this spring, most likely.

What about Moderna?

Moderna’s covid vaccine is FDA-approved for people 18 and older. The company has been testing its shot in teens and children, and said this month that it expects to have data on kids ages 2 to 5 in March. Exactly when the company might seek FDA authorization isn’t clear.