Nine vaccines have proved effective at protecting people from developing symptoms of Covid-19, the disease that can result from infection with the SARS-CoV-2 virus. Less is known about how well the various inoculations prevent people from getting an asymptomatic infection or passing the virus on to others. Preliminary evidence suggests that at least two of them do a reasonably good job protecting against any kind of infection and that one, the vaccine from Pfizer Inc.