Combined use of polio vaccines boosts immunity

Administering a single dose of an injectable polio vaccine to those who have already been given an oral polio vaccine may boost their immunity, according to a clinical trial carried out on nearly 1,000 infants and children in India.
Researchers found that giving the Salk inactivated poliovirus vaccine (IPV), which is administered by injection, to individuals who had already been given the Sabin live-attenuated oral poliovirus vaccine (OPV) may improve their immunity to the poliovirus.
"IPV should be used to accelerate the eradication of the virus in populations that have limited access to vaccination," said Dr Hamid Jafari, World Health Organisation's director for polio operations and research, lead author of the report.

"The study has also provided the evidence for use of IPV among travellers to limit further international spread of the virus," Jafari said.
Since the development of the polio vaccine in the 1950s, efforts to eradicate the poliovirus, which can cause life-long paralysis, relied mainly on OPV rather than IPV.
"This is because OPV has a superior ability to induce mucosal immunity, is easy to administer requiring no needles, and is substantially cheaper," said Caroline Ash, a senior editor at the journal Science in which the study was published.

However, in parts of the globe where polio is proving difficult to eradicate, the choice between OPV and IPV continues to be widely debated by scientists.

This is because mucosal immunity (the protection offered by the linings that separate the body's sterile interior from the outside world) diminishes rapidly after OPV treatment, meaning several doses of this vaccine are required.
Moreover, OPV's use can still lead to polio virus being shed in the stools, which creates risk of viral spreading.
Researchers tested whether use of both vaccines would improve mucosal immunity. Although administering IPV after OPV is known to close certain immunity gaps, its specific effect on intestinal mucosal immunity is less well-known.
The international research team conducted a large, community-based, randomised clinical trial in nearly 1,000 infants and children in Uttar Pradesh in 2011.

All subjects were given one vaccine or the other. After four weeks, all subjects, regardless of the initial dose type, got a dose of OPV.
In children who were given IPV, viral shedding was reduced, meaning that these children were much less infectious to others - a key in halting the spread of the virus.
Researchers also found that a single dose of IPV induced intestinal mucosal immunity more effectively in children than an additional OPV dose.
"The answer is now very clear," said Dr Roland Sutter, WHO's coordinator for research and product development, polio operations and research.
"Both vaccines complement one another and should be used to interrupt the final chains of transmission to attain a polio-free world in the most rapid and effective way possible," Sutter said.