See, no. You got it wrong. It DOESN'T prevent infection - NOTHING DOES, it's a virus, it's too small. It helps FIGHT infection in the future.
Look at this photo. It is a very famous photo. Both these children caught polio on the same day. The one on the left is unvaccinated. The one on the right was vaccinated at birth. This is widely considered a successful vaccination story.
BOTH CHILDREN ARE SICK. But the vaccinated one is already on his way to recovery. The unvaccinated one is, as you can see, a much more serious case.
Vaccines NEVER prevented infection entirely. Ever. But reducing the rate of infection, by reducing the window of transmissibility, and reducing the effects and fatality of the virus, ABSOLUTELY ARE the desired functions of a successful vaccine.
And for God's sakes, this is basic stuff here. Never mind your claim about your medical degree, this is stuff that gets taught on Sesame Street to toddlers, for Christ's sakes.