COVID Vaccine Less Effective in Younger Kids

A close up of a vial of Comirnaty approved for children held in blue rubber gloved fingers.

Effectiveness of the Pfizer COVID-19 vaccine (Comirnaty) against infection and hospitalization dropped among kids ages 5 to 11 a little over a month after vaccination, according to real-world data from New York during the Omicron wave.

From Dec. 13, 2021 to Jan. 30, 2022, vaccine effectiveness against infection within 2 weeks of full vaccination dropped from 65% (95% CI 62-68) to 12% (95% CI 8-16) by 28 to 34 days, reported Vajeera Dorabawila, PhD, of the New York State Department of Health, and colleagues.

Moreover, vaccine effectiveness against hospitalization dropped from 100%, albeit with a wide confidence interval (95% CI -189 to 100), to 48% (95% CI -12 to 75), they noted in a study published on the preprint server medRxiv.

Notably, vaccine effectiveness against hospitalization held up much better among kids ages 12 to 17, only dropping from 85% (95% CI 63-95) to 73% (95% CI 53-87). Effectiveness against infection in this age group fell from 76% (95% CI 71-81) to 56% (95% CI 43-63).

All effectiveness estimates among children ages 5 to 11 dropped below those for kids ages 12 to 17 by January 24, the authors noted, with a vaccine effectiveness against infection of 11% (95% CI -3 to 23) for age 11, and 67% (95% CI 62-71) for age 12.

Dorabawila and team examined data from three New York databases to assess the real-world effectiveness of the vaccine among children ages 5 to 17 during the Omicron wave. COVID cases were defined as positive nucleic acid amplification tests (NAAT) or antigen results.

Overall, 365,502 children ages 5 to 11 (23.4%) and 852,384 children ages 12 to 17 (62.4%) were fully vaccinated in New York state by the week of January 24-30. Mean age was about 8 among the younger age group, and about 15 for older kids. Median time since vaccination was 51 days for the younger group and 211 days for the older group.

The authors hypothesized that the lower dose among kids ages 5 to 11 accounted for the “markedly lower” vaccine effectiveness against infection. The younger group’s two-dose primary series consists of 10 μg apiece versus 30 μg apiece for the older group.

Interestingly, children who were 12 years old had the highest vaccine effectiveness of all ages, “potentially due to being small size relative to dose and more recent vaccination” by an average of 6 weeks versus adolescents ages 13 to 17.

“This gap suggests a threshold effect between the two [Pfizer] vaccine doses and need for study of numbers of doses, amount per dose, dose timing, and/or antigens targeted for children 5-11 years,” Dorabawila’s group wrote.

Home testing was not reported in this analysis, the authors acknowledged, though this would not affect hospitalizations. They also noted that there were “relatively few” hospital admissions for children during this period. Additionally, 12.5% of vaccinated kids ages 12 to 17 received a booster dose by the end of January, “likely adding protection to that group.”

“Should such findings be replicated in other settings, review of the dosing schedule for children 5-11 years appears prudent,” Dorabawila and colleagues concluded.


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