How are we able to create vaccines against bacteria and why couldn't we do it for more strains i.e.,MRSA? My son spent some time in the hospital fighting a pne

How are we able to create vaccines against bacteria and why couldn't we do it for more strains i.e.,MRSA? My son spent some time in the hospital fighting a pneumonia. Now his doctors are looking into why his body didn't get immunity from the pneumococcal vaccine (Prevnar13). I was always under the impression only viruses could be immunized against.