Effects of Vaccination and Previous Infection on Omicron Infections in Children

Danyu Lin, Yu Gu, Yangjianchen Xu, Donglin Zeng, Bradford Wheeler, Hayley Young, Shadia Khan Sunny, Zack Moore

Abstract:

On October 29, 2021, the Food and Drug Administration authorized the BNT162b2 (Pfizer-BioNTech) vaccine for emergency use in children 5 to 11 years of age, on the basis of an immuno-bridging study and a small efficacy study. Recent case-control studies have shown modest short- term effectiveness of the BNT162b2 vaccine for this age group during the early phase of the omicron era (from mid-December 2021 to mid- February 2022). Here, we report a large cohort study on the long-term protection of this vaccine and previous SARS-CoV-2 infection against infection, hospitalization and death in children aged 5 to 11 over 6 months of the omicron period.
The data sources and statistical methods for this study have been described elsewhere. Briefly, we extracted individual-level data on vaccination histories and clinical outcomes (SARS-CoV-2 infection confirmed by polymerase chain reaction and resulting hospitalization and death) up to June 3, 2022 by linking the North Carolina COVID-19 Surveillance System and Vaccine Management System, and we used 2020 census data to determine the total number of residents with each combination of demographic variables. There are a total of 881,255 children aged 5–11, with 143,775 SARS-CoV-2 infections; 205 of the infected children were known to be hospitalized, and 5 were known to have died (Table 1). A total of 215,937 children received at least one dose of the BNT162b2 vaccine between November 1, 2021 and June 3, 2022. We used Cox regression to formulate the time-varying effects of the BNT162b2 vaccine and prior SARS-CoV-2 infection on the risk of SARS-CoV-2 infection, while adjusting for the demographic variables listed in Table 1.
Figure 1 shows the effectiveness (i.e., 1–relative risk) of the BNT162b2 two-dose regimen and prior infection against SARS-CoV-2 infection. Vaccine effectiveness was higher among previously infected children than previously uninfected children, and it waned over time (Fig. 1A, Table S1). Among previously uninfected children, vaccine effectiveness reached 59.2% (95% CI, 57.2 to 61.0) at 5 weeks after dose 1 and declined to 7.5% (95% CI, 1.3 to 13.4) after 15 weeks. Among previously infected children, vaccine effectiveness reached 71.0% (95% CI, 61.1 to 78.4) at 5 weeks after dose 1 and declined to 33.4% (95% CI, 3.5 to 54.0) after 15 weeks. The immunity acquired via SARS-CoV-2 infection was higher than that acquired by vaccination, though it waned over time (Fig. 1B, Table S2). Infection with the delta or older variants had modest protection against infection with the omicron variant. The effectiveness of omicron infection in reducing the risk of reinfection with omicron was 90.1% (95% CI, 88.6 to 91.5) after 2 months and 63.5% (95% CI, 59.6 to 67.0) after 4 months.
There were only 11 known hospitalizations and no known deaths among the 186,056 recipients of the two-dose regimen. Vaccine effectiveness against hospitalization 3 weeks to 6 months after dose 1 was 96.3% (95% CI, 6.4 to 99.9). The effectiveness of prior infection against hospitalization from reinfection 2 weeks to 19 months later was 81.3% (95% CI, 41.7 to 94.0).
The Food and Drug Administration authorized the use of the BNT162b2 booster for children aged 5–11 on May 17, 2022. As of June 3, 2022, a total of 6,351 children had received the booster. There were 37 infections and no known hospitalizations or deaths after the booster.