In a recent study published in The New England Journal of Medicine, researchers evaluated the effectiveness of the BNT162b2 vaccine against the new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) Omicron in five to 11-year-old children. They used the incidence rate ratio of all reported SARS-CoV-2 infections and coronavirus disease 2019 (COVID-19)–related hospitalizations among partially and fully vaccinated children to determine vaccine effectiveness (VE) of the BNT162b2 vaccine.
Only a few studies have evaluated the effectiveness of the messenger ribonucleic acid (mRNA)-based SARS-CoV-2 vaccines, including BNT162b2, in preventing Omicron infections in five to 11-year-old children.
About the study
In the present study, researchers analyzed data of 255,936 children in the age group of five to 11 years between January 21, 2022, and April 8, 2022, when Omicron caused over 99% of sequenced SARS-CoV-2 infections in Singapore.
They assessed all reported SARS-CoV-2 infections, reverse transcriptase-polymerase-chain-reaction (RT-PCR) confirmed SARS-CoV-2 infections, and COVID-19–related hospitalizations among unvaccinated children and those partially or fully vaccinated with the BNT162b2 vaccine. The study outcomes covered the date of hospitalization and the notification of a positive test result to a parent or guardian, regardless of the type of testing method, RT-PCR testing, or rapid antigen testing (RAT). Further, the team estimated the VE of both one- and two doses of the BNT162b2 vaccination from the incidence rate ratio of all three study outcomes, and they defined VE as one minus the incidence rate ratio.
The study data also covered each participating child's socioeconomic status based on housing type, age, gender, and ethnicity (e.g., Indian, Chinese, other). In Singapore, the Ministry of Health has mandated RAT for symptomatic children (e.g., shortness of breath, prolonged fever for five or more days) and those in close contact with infected persons, plus an additional RT-PCR test. The researchers analyzed the occurrence of such incidences as they serve as a proxy for increased disease severity.
To assess the effect of the BNT1262b vaccine on disease severity, simultaneously preventing the exclusion of patients in whom infection developed during the study period but who were hospitalized after the study period, the team also assessed the incidence of COVID-19-related hospitalization up to and including April 1, 2022, seven days before the study end date.
The vaccination status (unvaccinated, partially, or fully vaccinated) and vaccination date helped the researchers calculate the contribution of person-time risks. They considered a child partially vaccinated starting the day after he/she received the first dose up to six days after the second dose administration. Likewise, they considered a child fully vaccinated after seven days of receiving the second vaccine dose. Including the data of seven days or more after each vaccination allowed vaccine-induced antibody levels to increase for accurate VE estimation.
The team compared the incidence rate ratios of outcomes between three study groups using the Poisson regression method in which the unvaccinated group served as the reference group. The researchers also performed two sensitivity analyses stratifying the study cohorts according to the number of days after vaccination in the first analysis and the age groups and time periods in the second analysis. For VE estimates in the second sensitivity analysis, the team used the geographic region as an additional covariate.
The final study population comprised 255,936 children, of which 67.7% of five-to-11-year-olds were fully vaccinated, with a median interval of 24 days between two doses, 12% received only one BNT1262b dose, and 20.3% remained unvaccinated. Although vaccination rates were similar for boys and girls, compared to other ethnicities, Indian, Chinese, and Malay children had higher vaccination rates. The vaccination rate was also higher among children in older age groups and those living in three-to-five-room public housing units.
Of 53,429 confirmed SARS-CoV-2 infections, 10% were RT-PCR-confirmed infections across 17.9 million person-days at risk. Over 16.4 million person-days at risk of hospitalization, only 288 were hospitalized. Among five children who received supplemental oxygen during hospitalization, one was unvaccinated, and two and two were partially and fully vaccinated, respectively.
The incidence rates of all reported infections, RT-PCR-confirmed infections, and hospitalizations in the partially and fully vaccinated groups were 2997.3, 391.2 and 19.1, and 2770.3, 111.8, and 6.6 per million person-days, respectively. Based on incidence rate ratios, VE against the three study parameters in partially vaccinated as compared with unvaccinated children was 13.6%, 24.3%, and 42.3%, respectively. Relatively, fully vaccinated children had much higher VEs, 36.8% against all reported infections, 65.3% against RT-PCR-confirmed SARS-CoV-2 infections, and 82.7% against hospitalization.
The estimated VE of BNT1262b against COVID-19 was higher than the SARS-CoV-2 Delta VOC, close to 91% in previous studies conducted in several countries, including the United States, Finland, Poland, and Spain in June 2021. In the current study, the VE of two doses of the BNT162b2 vaccine against all SARS-CoV-2 infections was barely 36.8% against Omicron among five to 11-year-old children. However, VE reached 82.7% against hospitalization in the short term after vaccination among these children. Overall, BNT162b2 conferred moderate protection against infection and severe illness among five to 11-year-old children. The protection against hospitalization was higher with two doses of BNT162b2 than with one (82.7% vs. 42.3%).