Inactivated vaccines (such as inactivated polio vaccine or killed pertussis vaccine) and
purified protein vaccines (such as diphtheria and tetanus toxoids, Hib vaccine, and
hepatitis B vaccine) do not have any living germs in them. These vaccines stimulate the
immune system without causing any infection.
Live, attenuated vaccines (such as measles, mumps and rubella vaccines) do infect cells
and multiply in the body. The vaccine viruses have been sufficiently weakened, or
attenuated, in the laboratory that they stimulate immunity without causing a full-blown
infection. They do not spread from a vaccinated child to another person.
Oral polio vaccine (OPV), however, not only infects the intestinal tract, but is also excreted
in the feces. The vaccine strains can spread from person to person. Such spread is usually
helpful, though, because those who get the polio vaccine virus this way may also become
immunized. OPV is a very safe and effective vaccine. There is, however, an incredibly
small risk of getting paralytic polio from the vaccine. The most recent estimate of the risk
of vaccine-associated paralytic polio after the first dose of OPV (given to infants) is 1 out
of every 1.3 million doses. The risk is much smaller for susceptible persons exposed to a
vaccinated child.
To avoid the very small risk of disease associated with the oral vaccine, all Canadian
provinces except Manitoba have switched to the inactivated polio vaccine (IPV).
Switching to IPV should maintain the remarkable accomplishment of eradication of
paralytic polio in the Western Hemisphere.