Evaluating the safety profile of the CoronaVac in adult and older adult populations: A phase IV prospective observational study in Brazil

This Phase IV prospective observational study aimed to evaluate the frequency of solicited and unsolicited adverse reactions within seven days following the administration of each dose of CoronaVac (14-day interval) by age group (18–59 years and ≥60 years). Participants (n = 538; 487 adults and 51 older adults) were enrolled from three public health centers in São Paulo, Brazil from May 2021 to January 2022. The study involved a two-dose vaccination regimen administered 14 days apart. Solicited and unsolicited adverse reactions (ARs) were assessed within seven days after each dose, and medically attended adverse events following immunization (AEFI) were monitored for 42 days. Safety data were collected through participant diary cards, telephone follow-ups, and on-site visits. Among adults, the most frequently reported local AR after the first and second doses was pain (256 [52.6%] and 129 [29.5%], respectively), while the most common systemic AR was headache (158 [34.5%] and 51 [11.6%], respectively). Most local and systemic solicited ARs were of Grade 1 or 2 severity, with ARs being more prevalent in adults following the first dose. One serious adverse event related to the vaccine was reported in adults, with no fatalities. Nine adult participants experienced adverse events of special interest, including five cases of COVID-19. These findings support the overall safety profile of CoronaVac in adults and older adult individuals, with adverse events being generally mild and self-limited.