Anomalous observations on IPV and OPV vaccination.
Anomalies of vaccine performance are events or observations that go counter to the current paradigms on the properties of a vaccine. In the current paradigm, inactivated poliovirus vaccine (IPV) induces serum immunity to protect the individual from poliomyelitis, but not mucosal immunity to prevent subsequent wild poliovirus infection. Therefore, it is believed not to be suitable to interrupt virus transmission in polio-endemic tropical and developing countries, where poliovirus transmission is thought to be predominantly faecal-oral, unlike in countries with excellent sanitation, where it is predominantly respiratory. Oral poliovirus vaccine (OPV), because it is believed to mimic natural (wild) poliovirus infection and to induce serum and mucosal immunity, is considered ideal to interrupt virus transmission in countries with poor sanitation and endemic polio. Moreover, the vaccine viruses are shed in the stools and it is presumed to spread faeco-orally to unvaccinated children in the vicinity, thus increasing its effectiveness in the community. These paradigms on both vaccines may be seen in standard textbooks of virology and paediatrics. In reality, IPV induces very high mucosal immunity in a monkey model, lasting at least 12 months and providing complete protection against oral challenge with wild virus. In field trials, IPV has been found to have very high vaccine efficacy (VE) and to retard significantly wild-virus transmission in communities. OPV, requiring five to seven doses for individual protection, does not seem to induce effective mucosal immunity. Hence, 10-15 doses of OPV and near 100% vaccination coverage are necessary for interruption of virus transmission in countries with poor sanitation. In the monkey model, wild poliovirus infection did not offer any mucosal protection against a second infection, indicating that live virus infection is not the best way to induce mucosal immunity (unless repeated several times). The immunity induced by OPV has two arms, individual protection (by serum immunity) and mucosal protection from later infection by wild virus; these are dichotomous, as evidenced further by protected children participating in wild virus transmission in breakthrough outbreaks in communities well vaccinated with OPV. There is no evidence that polioviruses, wild or vaccine, spread faeco-orally to any meaningful extent. The median age of polio in India, in the pre-vaccine era, and even in recent years was 12-18 months. No other faeco-oral infection has such a low median age. Measles virus transmission is respiratory, and the median age of measles is over two years. Wild virus is shed faecally, but by the age when faecal microbes infect children, most of them are already immune to disease caused by polioviruses. As for vaccine viruses, even when heavy inocula are fed repeatedly, children may not always become infected. That exposure to the small amounts of virus through faecal contamination (unlike microbes that multiply in food) would infect them, is an unrealistic expectation. There are sufficient anomalies demanding our revision of the old paradigms. IPV is a better immunogen than OPV and is completely safe. During the interval between the cessation of wild virus transmission and the global stoppage of polio vaccination, it will be advantageous to use IPV, particularly combined with DPT vaccine, to provide individual protection, and herd effect to prevent spread in case of the introduction of wild polioviruses, whether unintentionally or otherwise, and to bolster the Expanded Programme on Immunisation.