Many who are vaccinated also have asymptomatic infections and recover in a shorter span. A study by the CDC recently established that 27% of the total breakthrough cases (positive RT-PCR tests) reported to them was asymptomatic, many didn’t recall being sick enough, less than 10% needed to be preventively hospitalized and a little less than 2% accounted for mortality.
Case studies also show that vaccinated people who catch COVID-19 may sometimes have a lesser than usual viral load in their respiratory tracts, and feel a fraction of the ‘milder’ symptoms of COVID-19. Hence, the severity of disease with full immunization is likely to be avoided to a certain extent, and offer good pandemic control.
It should be noted that these factors are only accounted for only amongst people who have been fully immunized- i.e. at least 14-20 days have passed post their second dose. A single dose or not enough immunity can still increase the risk of infection and related complications.