Coronavirus: The dangers of weak vaccines Mutated coronavirus strains could escape a weak immune response and make the pathogen more aggressive. That way, even people who have already recovered could get infected again — and the vaccines would need updating. For all intents and purposes, the fight against SARS-CoV-2 in the Brazilian city of Manaus should be won already. By August 2020, three quarters of the population in the capital of the state of Amazonas had already been infected with the novel coronavirus — enough for the area to build up solid herd immunity. But then last December, COVID-cases were suddenly spiking again. Now hospitals in Manaus are running out of oxygen to keep corona-patients alive.