Participants and sample collection
Participants were recruited as part of the ‘BRACE COVID-19-specific’ (BCOS) sub-study of the Brazilian National Commission of Ethics in Research (CONEP): approval no. 4.572.593). BRACE trial participants in Brazil (recruited from BRACE trial sites in Mato Grosso do Sul, Rio de Janeiro and Amazonas) were eligible for inclusion if: (i) they consented to be contacted for future ethically approved projects; (ii) had a post-vaccination serum sample taken 28 (± 2 days) after the first or second dose of a COVID-19 vaccine.