NEJM -- April 12,2007

When we eradicate the world of wild poliovirus and start immunising with injectable polio virus(IPV) instead of Oral Polio Vaccine (OPV), will IPV actually be useful? This study is the first field study to evaluate the efficacy of IPV after eliminating the confounding factors of OPV use and wild type poliovirus infection.

I find this entire area of public health fascinating. Unfortunately for me I was born after global smallpox eradication( I never got a shot) and polio affords me an opportunity to follow the eradication of a disease in real time. I remember giving OPV drops to kids on National Immunisation days while in college but this study addresses a far more important issue.

The WHO in 2004 published a report suggesting the use of IPV following global eradication of polio and the last detectable case of Vaccine-derived poliovirus (VDPV). This is a major policy decision that has to be made and policy and guidelines are being developed since 1998 when the first meeting to discuss post-eradication scenarios took place.

The fundamental issue pertains to the use of any polio vaccine once eradication occurs. The reason OPV is not favoured is because of its propensity to cause VDPV strains which cause poliomyelitis and its ability to spread like wild type poliovirus. So, we use IPV,yes? Not yet, because we dont have enough data about what kind of regimens are efficacious or whether it can prevent an outbreak of wildtype poliovirus. The Cuban study addresses this issue by evaluating IPV efficacy against a challenge of OPV in a setting of 0% prevalence of poliovirus.

The study shows that IPV induces antibodies against the three serotypes of poliovirus but maternal antibodies (induced by OPV vaccination) may decrease protection to poliovirus serotype 2. It also challenges the notion that IPV prevents the excretion of poliovirus and shows that previous data which postulated IPV being as efficacious as OPV might have been influenced by contamination of subjects with either OPV or wild type poliovirus.

What is quite interesting is that although the data suggests that perhaps IPV might not prevent the excretion of poliovirus in stool, it might decrease the amount of poliovirus excreted.

But, lets be realistic about the results. As far as clinical trials for this study is concerned Cuba is a ideal situation and not really representative of countries like India where wild type poliovirus is more prevalent and possibly of higher virulence and therefore the study authors finally suggest careful consideration based on scientific evidence before we blindly substitute OPV for IPV in tropical countries endemic for poliovirus.

If you are interested in this topic also read this Science paper which tackles post-eradication era policies.