How Well COVID-19 Vaccines Protected Children Aged 3–5, 2022–23

On June 18, 2022, CDC's Advisory Committee on Immunization Practices recommended COVID-19 vaccines for the youngest children, based on safety, immune-response and limited efficacy data from trials:

Real-world effectiveness data were limited, so CDC used its Increasing Community Access to Testing (ICATT) program — free COVID-19 testing at pharmacies and community sites for people 3 and older — to estimate how well these original (monovalent) vaccines prevented symptomatic infection.

How the study worked

When children with at least one COVID-like symptom were tested, caregivers reported symptoms, past infection, health conditions and vaccination history. Researchers compared vaccination among children who tested positive with those who tested negative (a test-negative design), adjusting for age, gender, race and ethnicity, community vulnerability, region, pharmacy chain, local case rates and date.

Children were excluded if they were immunocompromised, had tested positive in the past 3 months, had mixed or non-mRNA vaccines, had a dose within 2 weeks of testing, or got a third dose from December 2022 on, when updated vaccines were recommended.

Results

• A full series of either vaccine protected against symptomatic infection for at least the first 4 months.
• The Moderna estimates suggest protection may wane by 3–4 months, as in older children and adults, though the confidence ranges overlapped. There weren't enough cases to check waning for Pfizer-BioNTech.
• One Moderna dose gave detectable short-term protection before the second dose. One Pfizer-BioNTech dose did not show significant protection, but two doses did — covering children waiting for their third. In the Pfizer-BioNTech trial, immune-response targets were met after dose 3 but not dose 2, so completing the series matters.
• Vaccination was uncommon: before exclusions, 18%, 17% and 26% of tested 3-, 4- and 5-year-olds had received at least one dose.

Context

The vaccination program aims mainly to prevent severe disease and hospitalization, and mRNA vaccines generally protect better against severe outcomes than against infection; severe-outcome data for this age group weren't yet available. By late 2022, 87% of U.S. children aged 6 months–4 years had signs of past infection, though caregivers reported a past infection for only about 20% of children here — so the estimates reflect protection in a largely previously infected population.

Updated (bivalent) vaccines, designed for Omicron and waning protection, were extended to children 6 months and older on December 9, 2022: a bivalent third dose for the Pfizer-BioNTech series, and a bivalent booster at least 2 months after a Moderna series. They added protection in adults, and similar benefits are expected in children.

Limits: the two vaccines' estimates aren't directly comparable; vaccinated and unvaccinated children may differ in risk or testing habits; there were no hospitalization data; vaccination was caregiver-reported; prior infection wasn't adjusted for; behaviors such as child care weren't recorded; and a mix of Omicron sublineages circulated. The estimates are preliminary.

Recommendation

Children should stay up to date with COVID-19 vaccines, including completing the primary series, and eligible children should get a bivalent dose.

Sources

Based on Fleming-Dutra KE, Ciesla AA, Roper LE, et al., "Preliminary Estimates of Effectiveness of Monovalent mRNA Vaccines in Preventing Symptomatic SARS-CoV-2 Infection Among Children Aged 3–5 Years — Increasing Community Access to Testing Program, United States, July 2022–February 2023," Morbidity and Mortality Weekly Report 72(7), CDC, as corrected; a work of the United States government in the public domain.