In the battle to eradicate polio in areas where it remains a threat, 1 plus 1 could equal 0.
After decades of controversy over which polio vaccine to use, either Albert Sabin's live-attenuated oral poliovirus (OPV) or Jonas Salk's inactivated poliovirus (IPV), researchers now say it will take both to finally eradicate the crippling disease.
Hamid Jafari, MD, director of polio operations and research for the World Health Organization (WHO), Geneva, Switzerland, and colleagues conducted a randomized clinical trial in northern India (Moradabab) involving almost 1000 children aged 6 to 11 months, 5 years, or 10 years. Children who received IPV after multiple doses of OPV had stronger immunity compared with children who received an additional OPV dose, the authors report in an article published online August 21 in Science.
"The results that clearly demonstrate that IPV substantially boosts both mucosal and serological immunity in children previously vaccinated with OPV are historic and have major operational implications for the global polio eradication effort," Dr. Jafari, speaking from Turkey, said during a teleconference held August 20.
In the trial conducted between October and December 2011, the children were randomly assigned to 1 of 3 study groups. One group received IPV, a second group received bivalent OPV, and a third group received no additional vaccine. Four weeks later, all participants received a dose of bivalent OPV. Researchers measured poliovirus excretion on days 3, 7, and 14 of the trial. The groups did not differ significantly in age, sex, father's education level, or baseline seroprevalence levels.
The researchers found that
a single IPV dose boosts intestinal mucosal immunity against polioviruses in infants and children who previously received multiple doses of OPV;
the magnitude of the effect is substantially larger after IPV compared with an additional OPV dose; and
the oldest children showed the highest degree of waning intestinal mucosal immunity and had the highest prevalence of excretion after the additional OPV dose.
"Our study provides strong evidence that IPV boosts intestinal immunity among children with a history of multiple OPV doses more effectively than an additional OPV dose," the authors conclude.
Because of the remoteness of the study area, Dr. Jafari said in the teleconference, "The logistical organization to successfully complete the field work was phenomenal. For example, more than 3000 blood samples and more than 5000 fecal specimens were collected and tested."
He added that endemic polio is largely restricted to vulnerable populations in insecure, inaccessible, and war-torn areas. "What this study shows is that IPV should be used to accelerate the eradication of the virus in populations that have limited access to vaccination."
Further, he added, the study provides evidence showing that IPV should be administered to people who travel to and from those areas.
Waning Immunity Solution
"Because of its unique ability to induce mucosal immunity, needed to interrupt person-to-person spread of the virus, OPV is the appropriate vaccine through which to achieve global polio eradication," Roland Sutter, MD, WHO coordinator for research and product and a coauthor of the Science article, said during the teleconference, speaking from Geneva.
"However," he added, "there are 2 drawbacks associated with OPV use. The first is that mucosal immunity wanes after time, following immunization.... The second has implications for the period after eradication: OPV contains attenuated polioviruses. On extremely rare occasions, use of OPV can result in cases of [vaccine-associated] polio."
He continued, "The 2 vaccines used in conjunction with one another offer the most effective immunological profile for populations living in areas affected by poliovirus and those at highest risk of reinfection."
Endemic wild poliovirus transmission "is extremely geographically restricted," Bruce Aylward, MD, WHO assistant director-general for polio, emergencies, and country collaboration and a coauthor of the study, said during the teleconference, also speaking from Geneva.
"But in those areas, it persists tenaciously," he continued. "IPV will be a powerful additional tool in our arsenal to fight this disease in these remaining areas. Used strategically, and in conjunction with the appropriate oral polio vaccine, children will be able to be protected more efficiently and more rapidly against those specific strains that remain in circulation. As with any vaccine, however, it is only effective if children are reached. This will be critical, particularly in conflict-affected zones."
He said WHO and its partners would work with countries to "ensure IPV is introduced by 2015 into 26 countries currently using OPV only."
The researchers note in their article that the number of polio-endemic countries dropped from 125 in 1988 to 3 in 2013: Afghanistan, Pakistan, and Nigeria. Exportation of the virus from these countries still causes outbreaks in others, however.
This research was funded by Rotary International Polio Plus Program through a grant approved by the Polio Research Committee of WHO. The authors have disclosed no relevant financial relationships.
Science. Published online August 21, 2014. Abstract
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Cite this: Polio: 2 Vaccines Together May Be Key to Full Eradication - Medscape - Aug 21, 2014.