In children and adolescents, the Pfizer-BioNTech vaccine was only mildly effective against symptomatic and asymptomatic COVID-19 infections. Broken down by variant, Omicron infections were more likely to occur and more likely to be asymptomatic.
COVID-19 variants like Delta, Omicron, and the “Deltacron” subvariant have proven especially adept at causing breakthrough infections in fully vaccinated individuals. However, the vaccines still offer lasting protection against severe disease, hospitalization, and death.
The Pfizer-BioNTech vaccine was approved for children months after adults were eligible to receive COVID-19 vaccination. In combination with the US Centers for Disease Control and Prevention (CDC), investigators from the University of Arizona Health Sciences explored how effective COVID-19 vaccination was in preventing infection and severe disease in children and adolescents.
The study, published in the CDC’s Morbidity and Mortality Weekly Report (MMWR), was conducted from July 25, 2021-February 12, 2022, during which Delta and then Omicron was the dominant COVID-19 variant.
The real-world data was collected from the CDC’s PROTECT study, which tested participants 6 months-17 years of age every week in Arizona, Texas, Florida, and Utah. Legal guardians provided information about the participants’ health, demographics, vaccination history, and prior COVID-19 infection history. The infants, children, and adolescents submitted a weekly survey and nasal swab for PCR testing and genome sequencing.
The study cohort included 1364 participants, 77% (n=1052) of whom were children 5-11 years old and 23% (n=312) of whom were adolescents 12-15 years old. Overall, 76% of participants were from Arizona, 52% were female, 75% were White, 34% were Hispanic, and 10% had 1 or more chronic medical conditions.
Participants who received 1 or more doses of the Pfizer-BioNTech vaccine (the only vaccine authorized for children and adolescents) reported wearing a mask during 84% of school hours and 70% of community hours. Unvaccinated children and adolescents reported a mark for 60% of school hours and 48% of community hours. More COVID-19 cases occurred in participants who wore a mask less.
A total of 381 COVID-19 infections were recorded among children 5-11 years old, and there were 127 infections among adolescents 12-15 years old. 93% (n=352) of the children’s infections and 76% (n=97) of the adolescents’ infections were Omicron.
Among the children 5-11 years old, 65% (n=682) received 2 vaccine doses, 7% (n=69) received 1 dose, and 29% (n=301) were unvaccinated. Among the 12-15-year-old adolescents, 68% (n=212) received 2 vaccine doses, 5% (n=15) received 1 vaccine, and 27% (n=85) were unvaccinated.
There were 186 breakthrough infections among the vaccinated participants, 37.6% of which were asymptomatic. Of the 252 COVID-19 infections among unvaccinated participants, 44% (n=112) were asymptomatic. Unvaccinated participants were more likely to be asymptomatic during Omicron (49%) than during Delta (60%). Overall, 51% of Omicron infections were asymptomatic and 34% of Delta infections were asymptomatic.
Vaccination with 2 doses of the Pfizer-BioNTech vaccine reduced the risk of Omicron infection by 31% in children 5-11 years old and by 59% in adolescents 12-15 years. This was a significant drop from the Delta period, during which protection against infection was 87% in adolescents (children were not yet authorized to receive the vaccine).
The study authors concluded that 2 doses of the Pfizer-BioNTech COVID-19 vaccine were effective in preventing symptomatic and asymptomatic infections in participants 5-15 years old. Vaccine efficacy was highest against Delta in the adolescent cohort (12-15 years) and lowest against Omicron in the child participants (5-11 years).