There are two main types of immunity you can achieve with vaccines. One is so-called “effective” immunity, which can prevent a pathogen from causing serious disease, but can’t stop it from entering the body or making more copies of itself. The other is “sterilising immunity”, which can thwart infections entirely, and even prevent asymptomatic cases. The latter is the aspiration of all vaccine research, but surprisingly rarely achieved.
In the case of Covid-19, neutralising antibodies that recognise the virus bind to the spike protein on its surface, which it uses to enter cells. To achieve sterilising immunity, vaccines must stimulate enough of these antibodies to catch any virus particles entering the body and immediately disarm them.
What type of immunity do the Covid-19 vaccines provide?
“In a nutshell we don’t know, because they’re too new,” says [epidemiologist Keith] Neal.
There are some early hints that certain vaccines might be able to reduce transmission, even if they can’t eliminate it entirely. One way it might do this is by reducing the number of viral particles in people’s bodies. “It is quite likely that if the vaccines are making people less ill, they are producing less virus, and therefore will be less infectious, but that’s just a theory,” says Neal.