[way]

Vaccine need to detect the virus in the body first , and then the antibodies white blood cells will start being produced inorder to attack the virus. So during the period of window the virus has yet to be detected by the antibodies to when all the virus has been eliminated, an infected person still can pass the virus to somebody else. No vaccine can block the virus getting into the body. Vaccine only trigger the alarm to alert the white blood cells that there are intruders.