Abstract
BACKGROUND: In response to the coronavirus disease 2019 (Covid-19) pandemic, Brazil authorised the Astra Zeneca/Fiocruz vaccine in January 2021. As the Delta variant emerged in May 2021, interval between vaccine doses was adjusted. By September 2021, the Brazilian National Immunisation Program recommended a booster dose for individuals over 70, and later expanded the recommendation to all adults. OBJECTIVES: Assess the equivalence of IgG antibody response against the Covid-19 S protein before and approximately 28 days after the third dose of a Covid-19 recombinant vaccine. Two groups received initial two doses with intervals of eight and 12 weeks. METHODS: This is a phase IV clinical study, uncontrolled, non-randomised. The study proposes calculating the ratio of geometric means titres (GMT) 28 days after the third dose, with a target ratio of confidence interval (CI) between 0.77 and 1.3. FINDINGS: In the primary endpoint, there was no equivalence between the eight- and 12-week intervals with a slight variation favouring the eight-week group. Post-third dose, both groups showed increases titres at 28 days, three months, six months and 12 months. Both groups responded similarly to Delta and Omicron BA.1, with a more significant increase for Delta. MAIN CONCLUSIONS: The study showed strong and consistent immune response in all age groups receiving the Covid-19 recombinant vaccine. Third dose elicited an increase in GMT by at least three times aligned with Ministry of Health strategies emphasising Bio-Manguinhos crucial role in pandemic control in the country.