COVID-19 vaccines partially effective against Omicron BA.2 infection in children and adolescents in HK

According to a retrospective, territory-wide, case-based study, the mRNA-based BNT162b2 vaccine and inactivated CZ02 vaccine were partially effective against SARS-CoV-2 infection in children and adolescents during the Omicron BA.2 wave in Hong Kong.

Previous research demonstrated vaccine effectiveness (VE) of BNT162b2 and CZ02 against symptomatic infection in children and adolescents when SARS-CoV-2 variants, such as Delta and Omicron BA.1, were widely circulating worldwide. [JAMA 2022;327:2210-2219; medRxiv 2022;doi:10.1101/2022.03.24.22272854; Nat Commun 2022;13:4756] “In the time of the Omicron BA.2 wave, however, VE of BNT162b2 and CZ02 in children and adolescents remains unclear,” the researchers wrote.

Using vaccine coverage statistics and infection surveillance data from the Department of Health between 1 January 2022 and 19 April 2022, the researchers analyzed VE of BNT162b2 and CZ02 against SARS-CoV-2 infection among children and adolescents at 3–18 years of age during the Omicron BA.2 outbreak in Hong Kong. SARS-CoV-2 infection cases involving heterologous prime-boost vaccines or COVID-19 vaccines other than BNT162b2 and CZ02 were excluded. [Commun Med (Lond) 2023;3:3]

The calculated daily incidence rates of infection were stratified by age, gender and vaccination status. After adjusting for gender and calendar day of the study, the incidence rate ratios (IRRs) against the unvaccinated reference group were estimated by negative binomial regression. VE was estimated as (1-IRR) x 100 percent.

In the population aged 3–18 years (n=953,400; boys, 51.3 percent), 79.4 percent had received ≥1 dose of COVID-19 vaccine, with vaccination rates of 64.0 percent in children aged 3–11 years (n=506,100; boys, 51.5 percent) and 96.9 percent in adolescents aged 12–18 years (n=447,300; boys, 51.2 percent). In the vaccinated group, 252,918 children (78.0 percent) had received ≥1 dose of CZ02, while 372,004 adolescents (85.9 percent) had received ≥1 dose of BNT162b2.

Throughout the study period, there were 100,274 cases of SARS-CoV-2 infection (10.6 percent), 137 of which (0.14 percent) occurred in children and adolescents who had received heterologous prime-boost vaccines or vaccines other than BNT162b2 and CZ02.

After exclusion, VE of two doses of CZ02 against SARS-CoV-2 infection was 55.0 percent (95 percent confidence interval [CI], 38.2–67.2) in adolescents and 40.8 percent (95 percent CI, 12.8–59.5) in children aged 3–11 years.

VE of two doses of BNT162b2 against SARS-CoV-2 infection in adolescents was 54.9 percent (95 percent CI, 38.9–66.8). Among children aged 5–11 years, VE of one dose of BNT162b2 was 33.0 percent (95 percent CI, 3.0–53.3), while VE of two doses of BNT162b2 was unestimable.

“We hypothesize similar VE against [SARS-CoV-2] infection in children aged 5–11 years and adolescents,” the researchers noted. “In previous studies, point estimates of VE against [SARS-CoV-2] infection and mild disease were 45–65 percent in children who had received two doses of BNT162b2.” [JAMA 2022;327:2210-2219; N Engl J Med 2022;387:525-532; Morbidity Mortality Weekly Rep 2022:71;352-358; N Engl J Med 2022:387;227-236]

“Our findings reveal partly preserved VE against SARS-CoV-2 infection [with BNT162b2 and CZ02] in children [and adolescents] during the BA.2 wave in Hong Kong,” the researchers concluded. “More studies should evaluate VE with different vaccine platforms among children and adolescents in various settings, including [a pandemic with] newly emerging variants of SARS-CoV-2.”