Now that both the Pfizer-BioNTech and Moderna COVID-19 vaccines have been approved for children 6 months and older, you may be wondering which vaccine to get for your child. The short answer is that you should get whichever one is available. Both vaccines are safe and effective, and the most important thing is getting your child vaccinated against COVID-19 as soon as possible.

Differences between the vaccines

If you have access to both the Pfizer-BioNTech and Moderna vaccines, there are some differences between the two that you may want to consider.

  • The Pfizer-BioNTech vaccine is 3 micrograms, which is one-tenth of the adult dose. It is given in three doses. The first two doses are given at least 21 days apart and the third dose given at least eight weeks after the second dose.
  • The Moderna vaccine is 25 micrograms, which is one-quarter of the adult dose. It is given in two doses separated by at least 28 days.

An individual is considered fully protected two weeks after completing the vaccine series. Because the Pfizer-BioNTech vaccine requires three doses, it takes nearly three months for a child to be considered fully vaccinated, and some children may not be fully protected by the time the school year starts. If your child is starting daycare, pre-k or kindergarten in the fall, then the Moderna vaccine may be preferable because its two-dose series allows children to achieve immunity in just over one month.

Vaccine safety profiles

Pfizer enrolled 4,526 children in their 4 and under clinical trials. The vaccine was well tolerated and there were no new safety issues; most side effects were mild or moderate.

Moderna enrolled 6,388 children under 6 years old in their clinical trials. Most side effects were mild or moderate and mostly reported after the second dose.

With both vaccines, there were no serious safety concerns.

How effective are the vaccines?

The Pfizer-BioNTech vaccine had had an efficacy of about 76 percent in children ages 6 to 23 months and about 82 percent in children ages 2 to 4 years based on clinical trial results.

The Moderna vaccine had an efficacy of about 51 percent in children between ages 6 and 23 months, and about 37 percent of children between ages 2 and 5 years based on clinical trial results.

It is important to know that these numbers are likely to change with each new circulating variant and new vaccine boosters are being tested to address different variants.

The bottom line

Over time, as more children are vaccinated, we’ll get more real-world data for how protective the Moderna and Pfizer-BioNTech vaccines are. Right now, though, families will need to figure out what works best for them, weighing the number of doses, time to protection and potential side effects of the two vaccines.


Sarah Schaffer DeRoo Sarah Schaffer DeRoo, MD, is a board certified pediatrician at Children's National Hospital.

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Posts from Sarah Schaffer DeRoo, MD

1 reply
  1. Kristen Johnson says:

    I always enjoy these newsletters and love the presentation of information that will help parents make health decisions for their children. I do think it is important to note, however, that interpreting the results of the Pfizer and Moderna under 5 COVID vaccines is a bit more nuanced that just comparing their percent efficacy. If one just looks at a single number to assess efficacy, it would seem like the Pfizer vaccine is the clearly superior choice for vaccination. However, it is really important to bear in mind confidence intervals around a measurement of efficacy! How confident are we that the Pfizer vaccine was 76% effective at preventing symptomatic disease in children under 5? Well, the confidence interval around that percentage runs from -369% to 99.6% – in other words, we are 95% confident that the true efficacy lay somewhere between -369% (actually increasing one’s risk of COVID) and 99.6% (almost 100% effective). When looking at effectiveness of a vaccine with relatively few participants, and relatively few cases in either arm of the trial, it is hard to have certainty in the outcome. Compare this to the Moderna vaccine results, where efficacy was 50.6% with a 95% confidence interval of 21.4%-68.6%. That means one can be 95% confident that the true effectiveness of the vaccine lay between 21.4%-68.6% – in other words, we can be highly confident that it provided some protection against symptomatic disease.

    Obviously, these are complex concepts if one isn’t used to thinking about uncertainty in data. But I think it is really important that we try to get comfortable discussing more nuances in health data, otherwise we might be perplexed by health care and government recommendations. (I recently had a patient ask my why would one consider getting the Moderna vaccine when Pfizer was clearly more effective? That is the kind of conclusion that one could draw if just comparing single percentages of efficacy without considering the confidence intervals around them.). The author of this article appropriately notes that the efficacy estimates will change as more data emerges, which is always a good point and emphasizes that knowledge, particularly in medicine, is a constantly evolving target.

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