The Moderna and Pfizer shots are hard to compare, so researchers and parents have lingering questions.
Source: Pfizer data
Efficacy against infection with the coronavirus SARS-CoV-2 was a bit harder to parse for each vaccine. Regulators allowed the vaccine makers to infer efficacy by demonstrating that the vaccines could elicit antibody levels similar to those that have been protective for teens and young adults, a concept known as immunobridging. That helped to speed up the trials.
But the companies did manage to collect some efficacy data. In the Moderna trial, 265 out of 5,476 kids contracted COVID-19, and the efficacy ranged from about 50% in infants and toddlers to less than 40% in children aged 2–5. The Pfizer vaccine seemed to do better, with an average efficacy of about 80% in children aged 6 months to 4 years. But these figures are based on a tiny number of cases — just seven infections in the placebo group and three in the vaccine group.
Wayne Marasco, a cancer immunologist at the Dana-Farber Cancer Institute in Boston, Massachusetts, brought up another concern that is relevant to both vaccines. He said that the first strain of a virus that a person is exposed to can bias their immune response to new variants of that virus for life — a phenomenon known as immune imprinting. That can be a problem for both children and adults.
Despite these worries, says Andy Pekosz, an immunologist at Johns Hopkins University in Baltimore, Maryland, “you’re still much better off getting a vaccine and getting that immunity, than really taking a risk and acquiring that immunity via infection”., and after a series of delays. The wait has been agonizing for some parents, and their frustration was palpable during the public-comment segment of the panel meeting.