By June 2023, COVID-19 had caused more than 2 million cases, more than 20,000 hospitalizations and more than 400 deaths among U.S. children aged 6 months to 4 years. The original monovalent mRNA vaccines were recommended for young children in June 2022, based on safety, immunobridging and limited efficacy data from trials; on December 9, 2022, CDC expanded recommendations for updated bivalent vaccines to everyone 6 months and older. Because trial data were limited, real-world effectiveness data were needed.
The study
- Who: children aged 6 months–4 years (Pfizer-BioNTech, a 3-dose primary series) and 6 months–5 years (Moderna, a 2-dose series) with a COVID-19–like illness who visited an emergency department or urgent care (ED/UC) at six sites in eight states, from July 4, 2022, to June 17, 2023.
- Method: a test-negative design — comparing vaccination among children who tested positive for SARS-CoV-2 (cases) with those who tested negative (controls), adjusting for age, sex, race and ethnicity, region and calendar time.
- Size: the Moderna analysis included 90,905 visits (4,934 positive); the Pfizer-BioNTech analysis, 81,077 (4,642 positive).
Few children were vaccinated. Among all children who tested positive, 6.6% had received at least one monovalent dose and 0.06% a bivalent dose; among those who tested negative, 12.1% and 0.4%.
Results: vaccine effectiveness against ED/UC visits
| Vaccine and doses | Time since last dose | Effectiveness |
|---|---|---|
| Moderna, 1 dose | ≥14 days (median 64) | 23% — not significant |
| Moderna, 2 doses (complete) | ≥14 days (median 100) | 29% |
| 14–59 days | 46% | |
| ≥60 days | 21% — not significant | |
| Pfizer-BioNTech, 1 dose | ≥14 days (median 58) | 7% — not significant |
| Pfizer-BioNTech, 2 doses | ≥14 days | 37% |
| 14–59 days | 46% | |
| ≥60 days | 27% — not significant | |
| Pfizer-BioNTech, 3 doses (complete) | ≥14 days (median 75) | 43% |
| 14–59 days | 70% | |
| ≥60 days | 24% — not significant | |
| ≥1 bivalent dose after a complete primary series, either maker | ≥14 days (median 58) | 80% |
"Not significant" means the 95% confidence interval included zero.
What it means
- A complete primary series of either vaccine helped protect young children against COVID-19 ED and urgent care visits — though protection waned, as it does in older children and adults.
- A single dose did not protect. Children who hadn't finished their series had often gone longer than recommended since their first dose — a median of 64 days after Moderna dose 1 (vs. 4–8 weeks recommended) and 58 days after Pfizer-BioNTech dose 1 (vs. 3–8 weeks). Nationally, as of May 2023 — nearly a year after vaccines were recommended — only about 10% of children aged 2–4 had received a dose and 6.1% had finished the series.
- A bivalent dose gave 80% protection, but few children had one, so the estimate is imprecise, and with a median of only 58 days since the dose, there was little time to see waning.
- By the time young children became eligible, circulating variants had drifted far from the strain in the original vaccines — which is why updated vaccines matter.
- An earlier study using pharmacy testing data found similar patterns but higher effectiveness in children aged 3–5, perhaps because of differences in who gets tested at pharmacies versus EDs, study periods, subvariants and other circulating viruses such as RSV and flu.
All children should stay up to date with recommended COVID-19 vaccines and start vaccination as soon as they're eligible.
Limitations
- Moderna and Pfizer-BioNTech estimates aren't directly comparable: eligibility dates, infection rates and subvariants differed.
- Vaccination coverage was low, and vaccinated children may differ from unvaccinated ones in risk or care-seeking.
- Low coverage, relatively low virus circulation and few hospitalizations meant severe outcomes couldn't be assessed — though preventing them is the program's main goal — and bivalent effectiveness couldn't be estimated by manufacturer.
- Previous infection couldn't be accounted for. By July–August 2022, 66%, 74% and 83% of children aged 6–11 months, 12–23 months and 2–4 years had evidence of past infection, so these results show vaccination's added benefit in a largely already-infected population.
- Data from eight states may not represent the whole country.
Sources
Based on Link-Gelles R, Ciesla AA, Rowley EAK, et al., "Effectiveness of Monovalent and Bivalent mRNA Vaccines in Preventing COVID-19–Associated Emergency Department and Urgent Care Encounters Among Children Aged 6 Months–5 Years — VISION Network, United States, July 2022–June 2023," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The imported copy was cut off in two places; the missing text and the effectiveness table were taken from the same report's full text in PubMed Central (PMC10441825).
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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