Ideally, vaccines would prevent infection entirely, inducing what’s known as “sterilizing immunity.” But early work on some of the vaccine candidates suggests they may not stop infection in the upper respiratory tract — and they may not stop an infected person from spreading virus by coughing or speaking.
A recently released study in which macaques were vaccinated with one vaccine candidate — this one being developed by Oxford University and AstraZeneca — showed the primates were protected from Covid-induced pneumonia. But the macaques still had high levels of virus replicating in their upper airways.