A year after mass vaccination against Covid-19 began, inactivated virus-based vaccines accounted for half of the doses administered in the world. Now, a large observational study carried out in Brazil and co-directed by Fiocruz and the Barcelona Institute for Global Health (ISGlobal), a center promoted by the “la Caixa” Foundation, observes that in people who initially received two doses of the inactivated CoronaVac vaccinea reinforcement of mRNA offers considerably higher protection against mild and severe Covid-19 caused by omicron, compared to a booster with the same vaccine. The research, published in the journal Nature Communications, has important implications for target reinforcement strategies in countries where the majority of the population received vaccines based on inactivated viruses.
Vaccination against Covid-19 has been shown to be very effective in protecting against severe illness and death. However, the efficacy of primary vaccination (that is, the two initial doses) was diminished with the arrival of new variants, especially the omicron, which justified the need to administer a booster dose.
Most studies on vaccine efficacy have focused on adenoviral and mRNA vaccines, despite the wide use of inactivated virus vaccines, especially in low- and middle-income countries. In fact, half of the doses administered globally until January 2022 correspond to inactivated virus vaccines. “Understanding the efficacy of the booster dose in populations vaccinated with inactivated virus is essential to guide future vaccination strategies in these countries,” says Otavio Ranzani, ISGlobal researcher and first author of the study.
Boost with the same vaccine or a different one?
In this study, Ranzani and colleagues evaluated the efficacy of boosting with either an inactivated virus vaccine (CoronaVac) or an mRNA vaccine (Pfizer) in Brazilian adults who initially received two doses of CoronaVac. The analysis, which included almost 1.4 million cases (with their respective control), was carried out between December 2021 and April 2022, a time when omicron BA.1 predominated, and was compared with a period in which dominated delta.
“The strength of our observational study is the large sample size and geographic coverage, encompassing each of the 5,570 Brazilian municipalities,” says Julio Croda, a researcher at Fiocruz and the Yale School of Public Health, and co-lead author. of the studio.
The results show that primary vaccination with two doses of CoronaVac vaccine offered almost no protection against symptomatic omicron disease and 40-50% protection against severe disease.
A booster dose with CoronaVac provided no additional protection against symptomatic disease, and only moderate protection against severe disease (74% and 40-50% in those older than 75 years). In addition, this protection seems to diminish after four months. In contrast, boosting with mRNA provided increased protection against the disease symptomatic and severe (56.8% and 86%, respectively) that seems to last more than four months.
“Our results have immediate implications for reinforcement delivery strategies in the context of the omicron variant,” says Ranzani. They show that, in individuals who initially received inactivated vaccines, heterologous boosting (in this case, with an mRNA vaccine) offers a significant increase in protection, even in the elderly.