One year after mass vaccination against COVID-19 was launched, inactivated virus vaccines accounted for half of the doses administered worldwide. Now, a large observational study performed in Brazil and co-led by Fiocruz and the Barcelona Institute for Global Health (ISGlobal), shows that in people who initially received two doses of the inactivated CoronaVac vaccine, an mRNA boost offers considerably better protection against mild and severe COVID-19 than a boost with the same vaccine. These findings, published in Nature Communications, have important implications for guiding boosting strategies in countries where most of the population received inactivated virus vaccines.