Stricter COVID-19 measures, curfew arrive as health experts warn Quebec hospitals stretched thinMâsse says that, like with other vaccines, it takes about two weeks following the injection for the body to build an immune response, meaning the vaccine is of little use in preventing illness before then.“That’s why we do flu vaccination campaigns in October, when we expect the virus will arrive in Montreal in November or December,” he said.[ Sign up for our Health IQ newsletter for the latest coronavirus updates ]While he’s not surprised to see the outbreak continue to grow, Mâsse worries it could lead to the erroneous belief that the vaccine doesn’t work, or even that it’s harmful.Roxane Borges Da Silva, another Université de Montréal public.