Vaccine effectiveness of CoronaVac against symptomatic and severe COVID-19 among children in Brazil during the Omicron period (preprint)

Compared to previous variants, the SARS-CoV-2 B.1.1.529 (Omicron) variant has relatively high infection rates, including among children. Even though severe COVID-19 in children is rare, this group is susceptible to the multisystem inflammatory syndrome in Children (MIS-C), long-COVID and downstream effects of COVID-19, including social isolation and education disruption. There is evidence that vaccination with an mRNA vaccine offers protection against infection and severe forms of COVID-19 for children. However, data on the effectiveness of inactivated virus vaccine, the most used platform worldwide, is scarce during the Omicron period. In Brazil, children between 6 to 11 years are eligible to receive the CoronaVac vaccine. Using a national linked database from January 21, 2022, up to April 19, 2022, during the Omicron dominant period in Brazil, we conducted a test-negative design with 194,258 tests to assess CoronaVac effectiveness against infection and severe (hospitalisation or death) outcomes among children aged 6 to 11 years. The estimated VE for symptomatic infection was 35.0% (95% CI 27.7–41.5) at 0–13 days and 41.5% (95% CI: 34.4–47.7) at ≥ 14 days post-second dose. For severe outcomes (hospitalisation or death) VE was 69.2% (95% CI: 11.7–93.6) at 0–13 days and 63.5% (95% CI: 5.8–90.0). Two doses of CoronaVac in children during the Omicron period showed low levels of protection against symptomatic infection, and modest levels against severe illness.

Protection conferred by vaccine plus previous infection (hybrid immunity) with vaccines of three different platforms during the Omicron variant period in Brazil (preprint)

Hybrid immunity (infection plus vaccination) provided high protection against infection and severe disease in the periods of delta and gamma variants of concern. However, the protection of hybrid immunity in the Omicron era remains unknown. We performed a test-negative study using Brazilian national databases between January 01 and March 22, 2022, a period of predominant circulation of the Omicron variant in Brazil. Hybrid immunity offered low protection against infection, with rapid waning, compared to unvaccinated with or without previous infection. For severe illness (hospitalisation or death), the protection, although already high for unvaccinated pre-infected increased regardless of the type of vaccine (Ad26.COV2.S, BNT162b2, ChAdOx-1 or CoronaVac). In conclusion, during the Omicron-dominant period in Brazil, hybrid immunity offered high protection against severe illness and low protection against infection.