@BillRM,
I've no idea of the efficiency, but I'd have thought it would be better than 50%.
Vaccines can also reduce severity of an illness even when they don't completely prevent someone from getting sick.
Plus, every person that a vaccine prevents from getting sick is also someone who doesn't transmit the illness on to others. Vaccines can prevent illness this way even for people who are not vaccinated.
So overall vaccines do quite a lot to reduce the severity of influenza.