A South Korean study involving children aged 5 to 11 years estimates the vaccine effectiveness (VE) of two doses of the monovalent (single-strain) Pfizer/BioNTech COVID-19 vaccine against Omicron variant infection to be 58%, 50%, and 41% at 15, 31, and 61 days, respectively, with 100% protection against critical illness for up to 90 days.
Researchers from the Korea Disease Control and Prevention Agency published the study yesterday in JAMA Pediatrics. The team analyzed national COVID-19 surveillance data from 3,062,281 children aged 5 to 11 years from Mar 31 to Aug 6, 2022, when Omicron accounted for all sequenced cases in Korea.
Substantial protection against severe illness
By study end, 29,473 children (1.0%) had received two monovalent vaccine doses, while 3,016,913 were unvaccinated. Among the unvaccinated, 616,835 (182.6 per 100,000) tested positive for COVID-19, and 14 of those cases were critical (0.01/100,000). Among two-dose recipients, 1,867 (119.5/100,000) tested positive, but no cases were critical.
Estimated VE against infection after two vaccine doses was 57.6% (95% confidence interval [CI], 51.6% to 62.8%) at 15 to 30 days, 46.9% (95% CI, 43.7% to 49.9%) at 31 to 60 days, and 41.2% (95% CI, 34.3% to 47.4%) at 61 to 90 days. VE against critical infection was 100% (95% CI, 100% to 100%) at 90 days.
The authors noted that the study was conducted during a rapid rise in COVID-19 infections in Korean children. They also cautioned that the results may have been affected by the small number of critical infections.
"Other unmeasured bias might have occurred, including risk and behavioral differences between vaccinated and unvaccinated children," they wrote. "Further studies are needed about bivalent [two-strain] vaccine effectiveness because the present data are from a program using a monovalent vaccine."
The findings, the researchers said, reinforce previous research data showing that COVID-19 vaccination helps protect children against infection and serious illness. "COVID-19 vaccination, even with moderate effectiveness against all infection, can substantially prevent the risk of serious consequences of SARS-CoV-2 infection among children," they concluded.