Giving children under 5 years old an extra dose of inactivated polio vaccine (IPV) helps to boost their immunity to the poliovirus and should be added to vaccination programmes in polio-endemic countries and those facing a high risk of imported cases, suggests new research published in The Lancet.
Lead author Dr Jacob John from Christian Medical College, India explains, "Adding a supplementary IPV dose to children already vaccinated with oral poliovirus vaccine (OPV) may hasten polio eradication by boosting herd immunity in endemic regions, act as a booster to prevent international spread by travellers, and minimise the risk of polio outbreaks due to imported wildtype or vaccine-derived polioviruses."*
Mass vaccination with OPV has successfully eliminated wild poliovirus from most of the world, although it remains endemic in Afghanistan, Nigeria, and Pakistan, and imported cases have led to large outbreaks in Africa, Asia, and Europe.
Although OPV is highly effective, easy to administer, and relatively inexpensive, 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.
In an attempt to increase protection in children whose immunity might have waned, scientists from India and the UK examined the effect of an additional dose of IPV on both systemic and intestinal immunity in children from Vellore, India (aged 1 to 4 years) who had received at least five doses of OPV as part of routine immunisation at least 6 months previously. Children were randomly assigned to receive a dose of IPV (225 children) or no vaccine (225) at enrolment. The researchers used shedding of the virus (testing stool specimens) after a challenge dose of bivalent OPV containing serotypes 1 and 3 poliovirus to measure the immune response.
The additional IPV dose substantially boosted levels of protective antibodies in the blood and intestinal immunity against poliovirus compared with no vaccine. One week after challenge with OPV, 43 (19%) and 57 (26%) children given no vaccine shed serotype 1 or 3 poliovirus compared with 27 (12%) and 17 (8%) of those receiving IPV. Among children in the no vaccine group, the first dose of bivalent OPV did not reduce poliovirus shedding following a second challenge dose of this vaccine.
According to Professor Grassly, senior author of the study from Imperial College London, UK, "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 programme."*
Writing in a linked Comment, Professor Kimberly Thompson from the University of Central Florida College of Medicine, USA discusses the results in the context of national immunisation strategies saying that, "Giving an extra dose of IPV or OPV to already OPV-vaccinated children with waned immunity will provide some incremental individual benefit…[however] the effects on overall population immunity and cost-effectiveness of an extra dose remain uncertain…Some results from models that explore the potential addition of an IPV dose at the same time as the last OPV routine immunisation dose suggest that the benefit on overall population immunity and thus on poliovirus transmission might be small."