Is the Pfizer or Moderna vaccine better for young children? It's hard to say

Parents of young children have had to wait longer than anyone to get their kids vaccinated against COVID-19. Now, they have a decision to make. 

Federal health officials have signed off on COVID-19 vaccines for young children, the final group to become eligible. But unlike with older children, who initially only could receive the Pfizer-BioNTech vaccine, there are two options from the start. 

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Is the Pfizer or Moderna vaccine superior? Many vaccine experts say it is too soon to tell. 

For adults, the two vaccines have more similarities than differences in regard to the way they are administered and the protection they offer. The two vaccines for young children have some differences that parents may want to take into consideration. 

The Pfizer vaccine is authorized for children 6 months to 4 years old. The Moderna vaccine is authorized for children 6 months to 5 years old. (The FDA also just authorized Moderna shots for kids ages 6 to 17.)

The benefits of both vaccines outweigh the risks for young children, experts from the FDA and the U.S. Centers for Disease Control and Prevention say. But they also note that there isn't enough data to determine if one of them is a better option. 

Dr. Cody Meissner, chief of pediatric infectious diseases at the Tufts Children's Hospital, told NBC News that more real-life data is needed before any comparison can be made between the effectiveness of the two vaccines.

"I would have a very hard time deciding which of the two vaccines to recommend. Statistics can play tricks on you when the numbers are small," he explained. "They both seem to stimulate a pretty good antibody response."

Vaccine efficacy

Dr. Paul Offit, a vaccine and infectious disease specialist at Children's Hospital of Philadelphia, told Nature that the Pfizer and Moderna vaccines "track side by side in terms of efficacy" among other age groups. But this is the first age group where they separate.

The dosage for the Pfizer vaccine is one-tenth of the amount used in the adult version. But at this low dosage, three doses are needed to produce an immune response strong enough to offer protection. The third shot must be given at least eight weeks after the second dose, which means the whole vaccination process takes 11 weeks. The first two doses are given three weeks apart. 

This was one of the concerns raised by the FDA advisory panel. The second dose appears to offer very little protection, so children who get the Pfizer vaccine will have to go another eight weeks unprotected, Dr. Amanda Cohn, a vaccines expert from the CDC, noted during the panel hearing.

Additionally, because the third dose was added later in clinical trials, there isn't enough data yet to determine the overall efficacy of the the three-dose series. The figures available – which are based on just seven COVID-19 cases among the study population – put the efficacy rating at 80%. But Dr. Doran Fink, the FDA's deputy director of vaccines and related products, called those figures "preliminary" and "imprecise" during the panel meeting. 

However, FDA officials say they are confident the three-dose regimen protects as well as the two-dose regimens that older age groups receive. This belief is based on studies that show the vaccines induce antibody levels similar to those deemed protective among teenagers and young adults. 

The Moderna vaccine for young children is only two doses administered four weeks apart. Each dose is about one-quarter of the adult version. Among children ages 6 to 23 months, the vaccine was 50.6% against symptomatic illness. For children ages 2 to 5 years old, the vaccine was 36.8% effective. It's possible that Moderna recipients may need a booster shot at some point. 

Adverse reactions

One of the concerns with the Moderna vaccine is that it causes more side effects than the smaller-dosed Pfizer vaccine, including fevers and fatigue. 

Among children ages 6 months to 2 years old, 14.6% of Moderna vaccine recipients had fevers – nearly double the 8.4% of children in the placebo group that reported fevers, STAT reported. Among Pfizer recipients in this age group, only 7.4% had fevers compared to 6.1% in the placebo group. 

Among children ages 2-4, 18.9% of Moderna recipients had fevers after the second dose – again, nearly double the 10.6% of children in the placebo group that reported fevers. In the Pfizer study, vaccine recipients in this age group reported fevers at about the same rate as those who received the placebo. 

Additionally, about 62% of children ages 3-5 in the Moderna study experienced fatigue. In the Pfizer trial, 45% of children ages 2-4 were fatigued afterward. 

In the Moderna study, about 1% of vaccine recipients also had underarm swollen lymph nodes in the same arm as the injection compared with 0.1% of those who received the placebo. Some Moderna vaccine recipients also experienced nausea and vomiting.

Myocarditis, type of heart inflammation associated with the vaccine among older age groups, does not appear to be a significant side effect with either vaccine for children under age 11, although more real-world studies are needed, the experts say.

Parental decisions

Even with two vaccines now available for young children, it is still unclear whether parents will choose to vaccinate their children against COVID-19. Because young children have had the lowest risk of becoming seriously ill with the coronavirus, some parents have remained skeptical that their children even need it.

In a survey by the Kaiser Family Foundation, only 18% of parents of children under 5 said they plan to get their kids vaccinated "right away." Another 38% said they would "wait and see" and more than 27% said they don't plan to get their kids vaccinated at all. And 11% said they would only do so if it was mandatory.

Even in this population, COVID-19 can cause hospitalization and death, especially since the omicron variant became the dominant form of the virus. According to the CDC, 485 children age 4 or under have died from COVID-19; thousands have been hospitalized.

The number of COVID-19 hospitalizations and deaths in children are much higher when compared with influenza-related deaths and hospitalizations, Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, told CNN.

"There still was, during the omicron wave, a relatively high rate of hospitalization during this period," he said. "That rate of hospitalization actually is quite troubling, and if we compare this to what we see in a terrible influenza season, it is worse."

The fact that the Pfizer vaccine for younger children must be given in three doses might put off some parents since it will be more of a logistical challenge.

"I have a lot of concern that many of these kids will not get the third dose, as we know the struggle to get people in for two," Jeannette Lee, a biostatistician at the University of Arkansas for Medical Sciences, told Nature. She added that a lot of people haven't come in for the booster doses, either.

In November, children ages 5-11 became eligible for vaccination. However, just 29% of these children are fully vaccinated, according to the CDC. 

Dr. Jeffrey S. Gerber, who oversaw the CHOP portion of the Moderna vaccine trial, reassured parents during a media briefing that these authorizations are based on quality data.

"Although these vaccines – all COVID-19 vaccines – have come to market quickly, no corners have been cut," he said. "The process has just occurred in a more compressed time frame. The companies and the federal government have been laser focused on these vaccines."

Dr. Lori Handy, medical director of infection prevention and control at CHOP, said all eligible children – even those who already have had COVID-19 – should get vaccinated.

Though both types of immunity are important, the vaccine provides a more specific immunity, Handy said. But children who recently had COVID-19 should hold off a few months before being vaccinated. 

And what about children under 6 months who are not eligible for any COVID-19 vaccine? 

Handy said the best way to protect them is for the mother to have been vaccinated before or during pregnancy so she can transfer her antibodies to her child, and for everyone around the child to be vaccinated to prevent the risk of infection.