Vaccine coverage and effectiveness against laboratory-confirmed symptomatic and severe Covid-19 in indigenous people in Brazil: a cohort study (preprint)

Background Indigenous people have historically suffered devastating impacts from epidemics and continue to have lower access to healthcare and be especially vulnerable to respiratory infectious. We estimated the coverage and effectiveness of Covid-19 vaccines against laboratory-confirmed Covid-19 cases among indigenous people in Brazil.Methods We linked nationwide Covid-19 vaccination data with flu-like surveillance records and studied a cohort of vaccinated indigenous people aged ≥ 5 years between 18th Jan 2021 and 1st Mar 2022. We considered individuals unexposed from the date they received the first dose of vaccine until the 13th day of vaccination, partially vaccinated from the 14th day after the first dose until the 13th day after receiving the second dose, and fully vaccinated onwards. We estimated the Covid-19 vaccination coverage and used Poisson regression to calculate the relative risks (RR) and vaccine effectiveness (VE) of CoronaVac, ChAdOx1, and BNT162b2 against Covid-19 laboratory-confirmed cases incidence, mortality, hospitalisation, and hospital-progression to Intensive Care Unit (ICU) or death. VE was estimated as (1-RR)*100, comparing unexposed to partially or fully vaccinated.Results By 1st Mar 2022, 48·7% (35·0–62·3) of eligible indigenous people vs 74·8% (57·9–91·8) overall Brazilians had been fully vaccinated for Covid-19. VE for the three Covid-19 vaccines combined was 53% (95%CI:44–60%) for symptomatic cases, 53% (95%CI:-56-86%) for mortality and 41% (95%CI:-35-75%) for hospitalisation. Among hospitalised patients, VE was 87% (95%CI:27–98%) for progression to ICU and 96% (95%CI: 90–99%) for death.Conclusions Lower coverage but similar Covid-19 VE among indigenous people than overall Brazilians suggest the need to expand access, timely vaccination, and urgently offer booster doses to achieve a great level of protection among this group.

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.