It has a 95% efficacy rate. A vaccinated person can still catch the virus, but won't have a severe case or he may be asymptomatic. There is a possibility that one can transmit the virus to people who are not vaccinated. This is why I still wear a mask, and why I avoid large crowds. I am the only person that lives in my house that is vaccinated, I don't want to risk giving it to members of my family.