This case-control study estimates the effectiveness of prior SARS-CoV-2 BA.1 or BA.2 infection and booster vaccination against Omicron BA.5 subvariant infection.

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.

No abstract

National cohort data collected during the coronavirus disease 2019 (COVID-19) delta and omicron periods in Korea revealed a lower risk of severe infection in recipients of three doses of the COVID-19 vaccine (adjusted odds ratio [aOR], 0.05–0.08). The risk of death was reduced during the omicron period compared to the delta period (aOR, 0.75; 95% confidence interval, 0.67–0.84).

B ooster doses of mRNA vaccines have been shown to reduce the risk for severe SARS-CoV-2 infection (1,2); however, protection wanes a few months after vaccination, particularly in high-risk populations (3,4). A second booster dose at least 4 months after the first booster dose of mRNA vaccine was found to increase immunity against COVID-19; thus, the second booster has been introduced in some countries (5).Other studies have postulated that additional doses of the COVID-19 vaccine can enhance cellular and humoral immunity against the Omicron variant (6,7), and some studies have already shown the effectiveness of second boosters in preventing COVID-19 infections (5,8,9). However, population-based studies are needed to assess the effect of the second booster vaccine on COVID-19 in high-risk groups.During February-April 2022, a second booster of mRNA vaccine was recommended for immunocompromised persons and long-term care facility (LTCF) residents in South Korea. We used a nationwide population registry to estimate the effect of a second booster dose of mRNA vaccine on risk for laboratoryconfirmed SARS-CoV-2 infection, critical infection, and death in immunocompromised persons and LTCF residents. MethodsIn South Korea, COVID-19 is a notifiable disease; all laboratory-confirmed cases are reported to the Korea Disease Control and Prevention Agency (KDCA). COVID-19 vaccination records, including the date of vaccination and type of vaccine, are also collected and maintained by the KDCA. All suspected COVID-19 case-patients (anyone with a history of close contact with a COVID-19 patient) or SARS-CoV-2-infected persons, regardless of symptoms, were mandated to be tested by PCR or rapid antigen test during the observation period. By linking the vaccination registry and the surveillance database, we created a largelinked database through unique resident registration number. The observation period was February-May 2022, when 100% of SARS-CoV-2 detected in South Korea was identified as an Omicron variant (BA1.1, BA2, and BA2.3 subvariants) (10).