The trial of the injection to deliver inactivated polio vaccine in children who had already been immunized with the oral poliovirus vaccine has been published in The Lancet and was funded by the Bill & Melinda Gates Foundation in support of The Global Polio Eradication Initiative.
Because polio remains endemic in Afghanistan, Nigeria and Pakistan, other countries are at risk of imported disease, leading to wider cases of crippling poliomyelitis - the virus invades the nervous system and can cause irreversible paralysis "in a matter of hours," says the initiative. Such imported cases, say the researchers, have led to large outbreaks in Africa, Asia and Europe.
Lead author of the paper, Dr. Jacob John of the Christian Medical College in Vellore, India, says:
"Adding a supplementary inactivated polio vaccine dose to children already vaccinated with oral poliovirus vaccine may hasten polio eradication by boosting herd immunity in endemic regions, act as a booster to prevent international spread by travelers, and minimize the risk of polio outbreaks due to imported wildtype or vaccine-derived polioviruses."
"Most vaccination campaigns use multiple doses of oral vaccine that provide some gut immunity, although this wanes over time," Dr. John explains.
Because the booster dose tested in the trial "is injected into the arm, rather than taken orally, it has been assumed it does not provide much protection in the gut" and would be less effective than the oral vaccine, he continues.
"In the 1960s, there was extensive rivalry between the scientists who developed the two vaccines, with oral poliovirus vaccine eventually becoming the most popular."
Dr. John concludes:
"However, we found that where the children already had a level of immunity due to oral poliovirus vaccine, the injected vaccine actually boosted their gut immunity."
Immunization programs, 'take note'
A leading polio vaccine expert writing a commentary on the study in the same journal issue supports the calls for vaccination programs to consider the booster.
"If all goes well with current efforts at polio eradication, countries will soon achieve and maintain the goal of stopping transmission of all wild polioviruses contemporaneously," writes Prof. Kimberly Thompson of the University of Central Florida's College of Medicine in Orlando.
Because the booster dose "is injected into the arm, rather than taken orally, it has been assumed it does not provide much protection in the gut" and would be less effective than the oral vaccine.
"Leaders of immunization programs might find these results helpful as they consider the role of inactivated polio vaccine in their present and future immunization programs."
The oral poliovirus vaccine (OPV) used in current programs is, say the study investigators, "highly effective, easy to administer, and relatively inexpensive" - but its ability to generate a strong intestinal immunity to infection "wanes as early as a year after vaccination."
"Thus, vaccinated children and adults can still be infected and shed wild poliovirus, contributing to the spread of the disease," the researchers say.
Picking out the specific results of the effect of the additional injected vaccine, as measured by virus survival in stool samples, Prof. Thompson says that adding a dose of inactivated polio vaccine (IPV) does increase protective serum antibodies in children from India aged 1-4 years.
In the group that received the extra dose through IPV, fewer children shed poliovirus in their stool samples - between 8% and 12% (depending on virus serotype), compared with virus shedding being found in up to a quarter of the children who had not been given the IPV booster.
Decision time for global vaccine programs
While agreeing with the researchers' findings that the booster they trialled "might help," Prof. Thompson adds a note of caution, saying countries will need to "consider the injection's impacts."
"Giving an extra dose of IPV or OPV to already OPV-vaccinated children with waned immunity will provide some incremental individual benefit," she says. "[However] the effects on overall population immunity, and cost-effectiveness of an extra dose, remain uncertain."
Senior author of the study, Prof. Grassly, from Imperial College London in the UK, is more enthusiastic about the potential contribution to stopping polio, highlighting the choice between an extra dose of the injected form versus an extra dose of the oral version:
"The substantial benefit of using IPV rather than further doses of OPV to boost intestinal immunity in children within the typical age range for mass vaccination supports its use as part of the global eradication program."
Polio threat to south America and Europe
While the World Health Organization (WHO) aims for polio to be eradicated by 2018, the problem of imported cases remains a threat in the meantime.
There was concern reported in The Lancet in November 2013 that Syrian refugees would import polio to Europe after an outbreak was confirmed in the conflict region. See "Syria polio outbreak could hit Europe, say experts."
More recently, in June 2014, WHO reported the detection of poliovirus in a sewage sample in Brazil, possibly imported during the influx of people to the World Cup.