COVID-19 vaccination has been shown to be very effective in protecting against serious illness and death. However, the efficacy of immunization number one (i. e. , the initial two doses) decreased with the arrival of new variants of concern, adding omicron, justifying a booster dose.
Most vaccine effectiveness studies have focused on mRNA and adenoviral vaccines, inactivated virus vaccines have been widely used, especially in low- and middle-income countries. In fact, in January 2022, they are part of the doses administered worldwide. “Reinforcements in populations that have received inactivated vaccines are essential to advise long-term vaccination methods in these countries,” says Otavio Ranzani, ISGlobal researcher and first author of the study.
Booster with the vaccine or another?
In this study, Ranzani and colleagues evaluated the efficacy of booster with an inactivated virus vaccine (CoronaVac) or with an mRNA vaccine (Pfizer) in Brazilian adults who first received two doses of CoronaVac. The analysis, which included nearly 1. 4 million pairs of cases and controls, was conducted between December 2021 and April 2022, an era in which omicron BA. 1 predominated, and compared to an era in which delta predominated.
“The strength of our observational study lies in the length of the giant pattern and geographic coverage, which covers each of Brazil’s 5570 municipalities,” says Julio Croda, a researcher at Fiocruz and the Yale School of Public Health, and co-director of the study.
The effects show that the number one vaccination with two doses of the CoronaVac vaccine provided almost no coverage against symptomatic diseases through omicron, and 40-50% coverage against severe diseases. Stimulation with CoronaVac did not confer additional coverage against symptomatic diseases and further moderated coverage against serious diseases (74% and up to 40-50% for other people over 75 years of age). In addition, this coverage fades in the next 4 months. In contrast, an mRNA booster provided superior coverage compared to symptomatic and severe disease (56. 8% and 86%, respectively) and gave the impression of lasting at least 4 months.
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