The HPV vaccine protects against two strains of the infection and is most effective when given to women before they are exposed to the virus.

It had been shown previously that antibodies in cow's milk can protect infants against three strains of bacteria that cause infant diarrhea.

Prevnar, a vaccine for infants that protects against seven strains of bacteria, has been sold in the United States since 2000.

The vaccine helps protect the elderly against 23 strains of pneumococcal bacteria, the most common cause of bacterial pneumonia.

The vaccine, designed to protect against four strains of the N. meningitidis bacterium, is far from perfect.

The pneumococcal vaccine used in adults protects against 23 strains of Streptococcus pneumonia bacteria.

This year, as usual, the vaccine protects against three strains.

But some skeptics doubt that protecting against six strains, or serotypes, is enough.

And over the past five years, a vaccine that protects against 13 strains of pneumococcal bacteria, another common cause, has been added.

The pneumococcal vaccine protects against seven strains of S. pneumoniae - but there are many more of them out there.