Bivalent COVID Shot Extends Benefit in Youngest Kids

The bivalent COVID-19 vaccines broadened the level of protection against emergency department (ED) or urgent care visits for kids 6 months to age 5, a CDC study found.

In children of this age who had completed their primary series, a single dose of either bivalent mRNA shot yielded a vaccine effectiveness of 80% (95% CI 42-96) against ED or urgent care visits compared with no vaccination, reported researchers led by Ruth Link-Gelles, PhD, of CDC's National Center for Immunization and Respiratory Diseases.

"However, few children had received a bivalent dose, so the estimate was imprecise," the researchers cautioned in Morbidity and Mortality Weekly Report. "In addition, the median interval since receipt of the bivalent dose was only 58 days, meaning there was little time for waning to be observed."

Still, the protection with Pfizer-BioNTech and Moderna's bivalent products surpassed that seen with the monovalent vaccines alone.

A single monovalent dose (an incomplete series) showed no protection, with vaccine effectiveness ranging from just 7% with the Pfizer vaccine to 23% with Moderna's shot; in both cases, the 95% confidence intervals included null values.

With a complete series (two doses for Moderna in kids 6 months to 5 years and three doses for Pfizer in kids 6 months to 4 years), effectiveness increased to 29% to 43% for the two products, respectively. But effectiveness waned, dropping from a high of 46% to 70% in the first 2 months following vaccination to 21% to 24% beyond that point.

"Of note, the predominantly circulating SARS-CoV-2 variants had evolved substantially from the strain included in the original monovalent COVID-19 vaccines by the time young children became eligible, highlighting the importance of receiving an updated vaccine," wrote Link-Gelles and colleagues.

But vaccine uptake in this population has been paltry, with the researchers noting that just 6.1% of children in this age group had completed their primary series vaccination by May 2023, nearly a year since the two vaccines were first authorized.

Their case-control study came from ED or urgent care visits at six sites across eight U.S. states in the VISION Network and included nearly 100,000 immunocompetent children ages 6 months to 5 years presenting with COVID-like illness from July 2022 to June 2023. All children had to have been tested for SARS-CoV-2 in the 2 weeks before a visit or within 3 days of the visit, and vaccine effectiveness was evaluated starting at 2 weeks after vaccination.

The Moderna monovalent analysis included 90,905 ED or urgent care visits among kids 6 months to 5 years of age (5.4% were case patients, the rest controls), whereas the Pfizer monovalent analysis include 81,077 ED or urgent care visits among kids 6 months to 4 years of age (5.7% case patients).

The bivalent analysis include an additional 96 patients (all control patients) who had received a dose of the Moderna product and 222 patients who received a dose of the updated Pfizer vaccine (including three case patients). For this analysis, only children with a complete primary series were included.

Overall, 6.6% of case patients had received at least one dose of a monovalent vaccine as compared with 12.1% of controls. When it came to the bivalent product, these rates were 0.4% and 0.06%, respectively.

A main limitation cited by the researchers involved the relatively low virus circulation during the study period, which combined with the few hospitalizations in this age group meant vaccine effectiveness against more serious outcomes could not be evaluated.

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