Comparing the Pfizer and Moderna Covid vaccines for young children 

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You have a child under age 5. You’ve been waiting anxiously to get said child vaccinated against Covid. Now, finally, this is an option available to you.

But there are two vaccines. Which do you choose?

We can’t tell you which is best for your child. But we can tell you that in this age group, the Pfizer-BioNTech and the Moderna vaccines probably differ more than they do in any other age group on the vaccination spectrum.

Both these vaccines are made using the messenger RNA platform. In adults, their initial efficacy estimates were virtually identical — 95% for Pfizer’s vaccine, 94% for Moderna’s. They were issued emergency use authorizations by the Food and Drug Administration within a week of each other in the United States in December 2020.

But vaccine doses given to adults are not appropriate for young children. And the two companies went separate ways in their decisions about how much antigen to put into their vaccines for babies, toddlers, and preschool-aged children.

We’ve laid out what’s known about the differences in the vaccines here, in the hopes it will help you make your decision.

Target population

The Pfizer-BioNTech vaccine is authorized for children 6 months to 4 years of age. The Moderna vaccine is authorized for children 6 months to 5 years of age.

Dosing differences

The two manufacturers took very different approaches to dose sizes when they developed their regimens for the youngest children to be vaccinated. There are benefits and potential drawbacks for each of them as a result. It remains to be seen which choice was the smartest.

Pfizer leaned hard into tolerability by choosing an especially small dose, figuring that if a vaccine was too reactogenic — caused too many side effects that made kids miserable — parents might not bring their children back for additional doses or might eschew the vaccine altogether based on word-of-mouth reports from other parents.

The company’s vaccine contains just 3 micrograms of antigen. That’s one-tenth of the amount of vaccine in its adult dose — which itself contains a little under one-third of the amount of antigen that Moderna’s adult dose contains.

But two doses of the Pfizer vaccine, given 21 days apart, didn’t trigger a strong enough immune response in young children, and the company announced in mid-December it was adding a third dose to its regimen. The third shot will be given at least eight weeks after the second dose, meaning the interval from the start of vaccination to completion of the series will be 11 weeks.

The third dose is critical, experts on an FDA advisory panel heard last week. Without it, children are likely not protected. After the second dose, there appears to be “very little or no effectiveness,” said panel member Amanda Cohn, a vaccines expert from the Centers for Disease Control and Prevention, who noted the need for a third dose means “these kids would not be protected at all for an additional eight weeks.’’

Moderna, on the other hand, developed a two-dose vaccine that contains 25 micrograms in each shot. That’s one-quarter of the amount an adult dose contains. The shots are given four weeks apart, after which a child would be termed fully vaccinated or up to date on their Covid vaccines. (That doesn’t mean boosters won’t be needed; more on that later.)

The bigger dose paid off. Moderna cleared the FDA’s bar for approval with just two doses, even in the time of Omicron, a variant that is sufficiently different from the original strain in the vaccine that two doses wasn’t enough to prevent some breakthrough infections in some adults who received the Moderna vaccine.

The FDA requirement was that the manufacturers show their vaccines induced in young children similar levels of antibodies as they did in young adults.

The benefits of the Moderna vaccine: Only two trips to the doctor, clinic, or pharmacy completes the primary series. (A booster or boosters will probably be needed later; again, more on that later.) One fewer dose to have to cajole a toddler into getting. Quicker protection.

The benefits of the Pfizer vaccine: Milder reactions.

The drawback of the Moderna vaccine: These bigger doses of vaccine sound like they may be harder on kids, in terms of transient side effects, than the Pfizer vaccine. (More on side effects below.)

The drawbacks of the Pfizer vaccine: It will take nearly three months to get a child fully vaccinated. Getting this vaccine requires three medical visits, and three vaccinations in an age group that typically hates getting needles. Failing to get the third shot means a child probably wouldn’t be protected.

Vaccine efficacy

The vaccines were authorized for this age group based on the comparisons of the immune responses, measured by antibody levels, of children who received them with vaccine recipients in previous age groups. This is a tried and true method, used for flu shots.

For the two-dose Moderna vaccine, it’s also possible to measure the efficacy of the vaccine by comparing the number of times volunteers in the 6,400-person clinical trial started feeling sick and tested positive for Covid.

Among kids who were six to 23 months old, the Moderna vaccine was 50.6% effective. That means there were about 7 cases of Covid for every hundred children that received placebo, compared to about 3.5 for every 100 who got the vaccine. In kids aged 2 to 5, the vaccine efficacy was 36.8%. That determination is based on 180 Covid cases.

But for the three-dose Pfizer vaccine, there have only been 10 Covid cases spread across the vaccine and placebo groups, meaning the FDA and CDC don’t feel comfortable estimating the vaccine’s efficacy.

This was a result of the fact that the Pfizer shot was not effective enough as a two-dose vaccine to warrant authorization. The company, consulting with the FDA, decided to test a third dose, but there has simply not been enough time for cases of Covid to occur. FDA officials say they are confident the three-dose regimen protects as well as two doses in other age groups.

The tolerability factor 

Direct comparisons between different clinical trials are risky, because different researchers don’t always take measurements the same way — not even for something as straightforward as a fever. But side effects do appear more pronounced with the Moderna shot.

Take the youngest kids studied, between six months and two years. In the Pfizer study, 7.4% of vaccine recipients had fevers compared to 6.1% of those who received placebo. In the Moderna study, those figures were 14.6% and 8.4%.

Among 2- to 4-year-olds, Pfizer saw fevers in 4.9% of vaccine recipients and 5.2% of placebo recipients. Among 2- to 3-year-olds, Moderna saw fevers in 18.9% of vaccine recipients after dose two compared to 10.6% of placebo recipients.

Other factors aside from the vaccine may play a role in these apparent differences. Moderna mentioned that other illnesses were circulating, which may explain why fevers were higher in the placebo group. But rates of other side effects were higher too.

Both vaccines may cause irritability, decreased appetite, fever, tenderness, redness, and pain at the injection site. In studies of the Moderna vaccine, researchers also saw underarm swollen lymph nodes in the same arm as the injection, which occured in about 1% of vaccine recipients compared to about 0.1% of those who received placebo, as well as nausea and vomiting, which occurred in 9.8% of the vaccine recipients between the ages of 2 and 5  after the second dose versus 4.8% of those who received placebo.

In adolescent males, both mRNA vaccines have been linked to an extremely rare condition called myocarditis, an inflammation of the heart. (Covid infection can cause myocarditis too.) The CDC and FDA say patients usually make a full recovery. But this condition, which is more common in teens even without the vaccine, so far does not seem elevated by the shots in children under 11. However, the trials in young children would be too small to detect such a rare condition.

Prospects for the need for boosters 

As mentioned above, in adults two doses of vaccine weren’t enough to forestall infection in the era of Omicron. The FDA expert panel thought it was likely that little kids who get the Moderna vaccine will need another dose at some point.

But that door isn’t closed for Pfizer either. “In terms of what the efficacy is after a third dose and whether an additional dose beyond that would be needed is going to require a little bit more data to sort out,” Doran Fink, FDA’s acting deputy director of the Office of Vaccines Research and Review, said during the June 15 meeting of the agency’s vaccines advisory committee.

Bottom line

Over time, as the vaccines are used in greater numbers of children, we’ll get real-world evidence on how protective these two vaccines are. At present, though, parents are going to need to do some personal calculus, weighing two doses against three doses, the time to protection, and the side-effect profiles of the two products.

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