What type of protection do polio vaccines offer?
When a child receives OPV, the vaccine virus enters the child's mouth and gut and replicates. The child then mounts immune responses in three places: (1) antibody response in the blood that protects against the virus invading the nervous system and causing paralysis, (2) immune response in the mouth which prevents shedding of virus in oral secretions and spread from those secretions and (3) intestinal immunity (also called gut or mucosal immunity), which prevents shedding of the virus in the stool. Thus, children vaccinated with OPV who come into contact with wild poliovirus are less likely to excrete poliovirus in their oral fluids or stool than unvaccinated persons. The predominant mode of transmission in the developing world is thought to be fecal-oral. Virus is shed in the feces and, in poor sanitary conditions and with suboptimal hygiene measures, can infect other persons if transmitted by dirty hands or contaminated food and water. Therefore, strong intestinal immunity prevents transmission.
IPV is an inactivated vaccine (killed virus) that stimulates a very good humoral response (antibodies in the blood) in children after only 1 or 2 doses. IPV also prevents children from excreting virus in their mouths as effectively as OPV and hence to the extent that polioviruses are transmitted through oral secretions, IPV is very effective at blocking that type of transmission. However, IPV alone does not induce the same level of intestinal immunity as OPV. Thus, while individuals vaccinated with IPV alone are protected against paralysis, they may excrete the virus and allow it to spread.
The combination of IPV with bOPV provides the advantages of both vaccines: strong intestinal immunity and antibody protection against the two serotypes in bOPV, types 1 and 3. This combination gives both the child and the child's community the best protection.