BNT162b2 Vaccine Effectiveness Against the SARS-CoV-2 Omicron Variant in Children Aged 5 to 11 Years

Abstract

This cohort study examines data for all children aged 5 to 11 years in South Korea to gauge the effectiveness of the BNT162b2 vaccine when the Omicron variant was the dominant SARS-CoV-2 infection in the country.

A rapid increase in COVID-19 cases due to the SARS-CoV-2 Omicron variant among children has raised concerns.¹ We estimated the effectiveness associated with the BNT162b2 messenger RNA vaccine (Pfizer BioNTech) against SARS-CoV-2 infection and critical infection among children aged 5 to 11 years during an Omicron-dominant period in South Korea.

Methods

The study cohort included all children aged 5 to 11 years residing in South Korea on March 31, 2022 (N = 3 062 281). Data for children who had laboratory-confirmed SARS-CoV-2 infection and critical infection (intensive care unit admission or death) were censored on the date of infection or critical infection. This study was based on surveillance data reported to the Korea Disease Control and Prevention Agency (eFigure in Supplement 1). We extracted anonymized data for the study period of March 31 through August 6, 2022, when the Omicron variant accounted for 100% of sequenced cases of SARS-CoV-2 infection in Korea. In Korea, children are required to be tested when they have symptoms (fever or respiratory symptoms) or have contact with an infected person. The vaccine has been offered to children aged 5 to 11 years since March 31, 2022. A time-dependent Cox proportional hazards regression model was used, and hazard ratios (HRs) with 95% CIs from an adjusted model with covariates (sex, age, days elapsed since vaccination, and region) were calculated to estimate vaccine effectiveness as (1 − HR) × 100. For a sensitivity analysis, we added case-control sampling nested within the study cohort, with 1:4 matched controls without a positive SARS-CoV-2 test result in the same week as the case randomly selected.

This study was conducted as a mandated public health investigation under legal authority and thus did not require ethics review. We followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

Results

By the end of the study period, 29 473 children aged 5 to 11 years (1.0%) were vaccinated with 2 doses, and 3 016 913 (98.5%) were unvaccinated (Table). A total of 616 835 infections (182.6 per 100 000) and 14 critical infections (0.01 per 100 000) occurred in unvaccinated children, while 1867 infections (119.5 per 100 000) and 0 critical infections occurred in children vaccinated with 2 doses. The estimated effectiveness against all infections after 2 doses of BNT162b2 in children was 57.6% (95% CI, 51.6%-62.8%) at 15 to 30 days, followed by 46.9% (95% CI, 43.7%-49.9%) at 31 to 60 days, and 41.2% (95% CI, 34.3%-47.4%) at 61 to 90 days (Figure). The effectiveness against critical infection remained 100% (95% CI, 100%-100%) until 90 days of observation.

Table. Characteristics of Children Aged 5 to 11 Years by COVID-19 Vaccination Status, March to August 2022.

Characteristic	Unvaccinated, No. (%)	Vaccinated, No. (%)
		1 Dose	2 Doses
Total	3 016 913	15 895	29 473
Sex
Male	1 547 102 (51.3)	8200 (51.6)	14 883 (50.5)
Female	1 469 811 (48.7)	7695 (48.4)	14 590 (49.5)
Age group, y
5-7	1 210 752 (40.1)	2999 (18.9)	5189 (17.6)
8-11	1 806 161 (59.9)	12 896 (81.1)	24 284 (82.4)
Geographic region
Metropolitan area	1 505 229 (49.9)	7351 (46.2)	14 424 (48.9)
Nonmetropolitan area	1 511 684 (50.1)	8544 (53.8)	15 049 (51.1)
Previous history of COVID-19
None	1 576 666 (52.3)	12 410 (78.1)	23 420 (79.5)
Present	1 440 247 (47.7)	3485 (21.9)	6053 (20.5)
Outcomea
All infection	616 835	6559	1867
Critical infection	14	0	0

^a Observed person-days and rates per 100 000 as follows: 337 869 716 person-days for unvaccinated, all infection: 182.6/100 000; 2 609 542 person-days for 1-dose vaccinated, all infection: 251.3/100 000; 1 561 815 person-days for 2-dose vaccinated, all infection: 119.5/100 000; 267 893 578 person-days for unvaccinated, critical infection: 0.01/100 000; 2 335 766 person-days for 1-dose vaccinated, critical infection: 0/100 000; and 802 370 person-days for 2-dose vaccinated, critical infection: 0/100 000.

Figure. Adjusted Vaccine Effectiveness (VE) Against All SARS-CoV-2 Infection and Critical Infection.

Discussion

Our findings indicate that vaccine effectiveness against all infection caused by the SARS-CoV-2 Omicron variant was 41.2% at 61 to 90 days after the second-dose vaccination in children in Korea, which is similar to Israeli data.² Previous studies from Italy and the United States reported that, in the setting of Omicron, the effectiveness against SARS-CoV-2 infection was 30% to 40%.^3,4 Nonetheless, our data also suggested that the effectiveness against critical infection remained high throughout the observed period, which is consistent with data from Singapore, with effectiveness against hospitalization of 82.7%.⁵ The observation period for this analysis coincided with a high incidence of SARS-CoV-2 infection in children in Korea.

The estimated effectiveness against critical infection should be interpreted with caution given the small number of critical cases. Other unmeasured bias might have occurred, including risk and behavioral differences between vaccinated and unvaccinated children. Further studies are needed about bivalent vaccine effectiveness because the present data are from a program using a monovalent vaccine.

The vaccine effectiveness described in this report for the BNT162b2 vaccine against the Omicron variant reinforces previous findings that the vaccination provides a preventive benefit in children. 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.

Supplement 1.

eFigure. Overview of Korea COVID-19 Vaccine Effectiveness Study (K-COVE)

Supplement 2.

Data sharing statement