None of the authorized and recommended COVID-19 vaccines cause an immunized person to test positive on viral tests, which are used to see if you have a current infection. If a recently-immunized person tests positive, it means that they were infected just before, or possibly after, receiving the vaccine. No vaccine is 100% effective in preventing the virus, but they have been shown to substantially reduce the incidence of asymptomatic infection and have demonstrated a high level of efficacy in preventing symptomatic and severe disease.