Quashing the polio virus for good may take a belt-and-suspenders approach, a new study suggests. One in which children get not only a dose of the cheap and easy-to-administer oral vaccine, but at least one dose of the inactivated vaccine - the one developed by Jonas Salk that is predominantly used in developed countries.
That finding, reported Thursday in Science Magazine, has prompted the World Health Organization to recommend that countries that exclusively administer oral polio vaccine to its infants and children ensure those children also get at least one dose of inactivated vaccine.
Aided by war, malnutrition, superstition and mistrust of the medical establishment, the polio virus continues to resist eradication, despite the determination of the World Health Organization to consign it to the medical history books. As of 2013, polio remains endemic only in Nigeria, Pakistan and Afghanistan. But polio won't go quietly: In recent years, polio outbreaks have occurred in countries where the disease had long been considered eradicated - in China and, more recently, in Syria.
Two excellent vaccines exist - the inactivated poliovirus vaccine developed by Salk in 1952, and the oral poliovirus vaccine developed by Albert Sabin and licensed for worldwide use a decade later. Given three doses of either vaccine, a person is almost certain to develop immunity to all three strains of polio virus.
But because vaccination campaigns are so easily disrupted by conflict, mass migrations and rumors of medical malfeasance, getting the highest level of polio immunity with the fewest vaccine doses delivered is key. So public health authorities and immunologists continue to squabble over which vaccine confers the highest levels of immunity and prevents the spread of the virus best.
That debate prompted a team of researchers from the World Health Organization to explore what combination of vaccine administration would best limit the spread of the virus and confer the greatest level of polio immunity in a population of youngsters.
The two vaccines have weaknesses and strengths:
The inactivated poliovirus vaccine must be injected, but is highly effective at preventing the progression of polio infection to paralysis - the most feared outcome of polio infection. At the same time, it has long been thought less effective than Sabin's oral vaccine at inducing "mucosal immunity" - effectively preventing the virus from replicating in the lining of its host's stomach and being passed into water in the stool.
In addition to being cheap to produce and easy to administer, the oral poliovirus vaccine is very good at inducing mucosal immunity, and therefore good at stemming the virus' spread. But that immunity doesn't last long, and so booster doses are required.
The researchers wondered whether, in a group of children who would all get the oral polio vaccine, an initial dose of inactivated polio vaccine would stem the spread of the virus by inducing a longer-lasting mucosal immunity. In a group of almost 1,000 northern India children, they found that it did. Compared to children who first got no vaccine or a dose of the oral vaccine first, those who got the inactivated polio vaccine first were less likely to "shed" the virus - thereby causing its spread - when four weeks later, they got the oral vaccine.
"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 wrote. "The answer to the vaccine controversy is apparent - both vaccines, IPV and OPV - should be used."