New research appears to have resolved a polio vaccine debate raging for more than 50 years: Is it more appropriate to use the oral, attenuated vaccine developed by Albert Sabin, MD, or the inactivated, injectable vaccine that Jonas Salk, MD, introduced?
The answer, based on a study published online today in Science, is that using both is best.
“This study has really revolutionized our understanding of inactivated polio vaccine and how it can fit within the global eradication program to help get this finished as quickly as possible,” coauthor Bruce Aylward, MD, MPH, the World Health Organization’s (WHO) assistant director-general of polio, emergencies and country collaboration, said during a press briefing.
Sabin’s oral formulation became the vaccine of choice in global eradication efforts because it induces superior mucosal immunity, is easy to administer, and costs less. Local immunity at mucosal surfaces such as those lining the gastrointestinal, respiratory, and urogenital tracts is important because they’re major entryways for viruses and other pathogens to enter the body.
However, the oral vaccine also has drawbacks. Mucosal immunity wanes over time, so multiple doses are necessary. Revaccination can be difficult or impossible in conflict zones such as Afghanistan, Nigeria, and Pakistan, where poliovirus remains endemic.
Also, the virus used to make the oral vaccine is weakened but still alive. It can be shed in feces, which increases local, national, and international transmission risks. On very rare occasions it can cause polio in the person who is vaccinated.
Scientists already knew that the inactivated vaccine prevented paralytic polio disease by spurring antibody production. But they weren’t certain about its role in mucosal immunity. So an international research team led by Hamid Jafari, MD, director for polio operations and research at WHO, enrolled nearly 1000 children in a clinical study to determine whether the inactivated vaccine boosts mucosal immunity. The trial took place in the Uttar Pradesh state of northern India, a longtime stronghold for the virus.
Children in the trial already had received multiple doses of oral vaccine as part of immunization programs. For the trial, they randomly received 1 dose of inactivated or oral vaccine, or no vaccine. Four weeks later, all the children received a 1-dose challenge with oral vaccine. At 3 follow-up periods during the next 2 weeks, the investigators measured poliovirus excretion in their stool—a sign of being infectious.
Compared with the control group, children who received the inactivated vaccine excreted anywhere from 39% to 76% less virus in their stool, depending on poliovirus type and children’s age. Older children excreted more virus than younger children. Only the older children who received oral vaccine excreted significantly less virus than controls.
“Giving inactivated polio vaccine makes people very, very less infectious,” Jafari said during the press briefing.
Since 1988, global eradication efforts have reduced polio cases by 99%. But Jafari said that effort now is at a crossroads. Endemic polio is confined to unstable regions that often are inaccessible to outsiders. “Yet the virus in these areas persists with incredible tenacity and threatens the increasingly vulnerable populations in polio-free countries with a weak or conflict-affected health system,” he noted.
“Inactivated polio vaccine will be a powerful additional tool in our arsenal to fight this disease in these remaining areas,” Aylward added.
The Global Polio Eradication Initiative’s strategic “endgame” plan calls for the introduction of inactivated polio vaccine by the end of 2015 in countries now using only the oral vaccine. “Both vaccines complement one another and should be used to interrupt the final chains of transmission to obtain a polio-free world in the most rapid and effective way possible,” coauthor Roland Sutter, MD, the WHO’s coordinator for research and product development, polio operations and research, said during the briefing.