Tuesday, July 14, 2015

From the Field - Guiding Future Polio Vaccination Strategy

by Mike Famulare

We are taught about immunity as if it were a binary concept: we are protected from disease when we are immune, and we are susceptible when not. But immunity is a continuum. People can have different levels of immunity and different kinds of immunity, and each person’s degree of protection depends on their history of vaccination and infection. For many infectious diseases, immunity increases with each exposure—this is why many vaccines are given in multiple doses and with “booster shots”. So, if you’re trying to eradicate a disease, an important question is what combinations and timing of vaccination produce the most immunity in the most people?

This is an especially interesting problem for polio. Polio has been driven to the brink of eradication by global administration of the oral polio vaccine (OPV). OPV has been a fantastic tool for public health: it is affordable, easy to administer, and provides transmission-blocking immunity and protection from paralytic disease. However, now that most of the world is wild-polio-free, the imperfections of OPV have become important. OPV doesn’t always work, especially in children with malnutrition and other diseases of poverty. Also, OPV contains live polioviruses that are attenuated to be very safe, but the modifications are genetically unstable. So, viruses that are descendants of the vaccine can evolve back toward wild-type and can cause outbreaks of paralytic disease.

The alternative is the famous Salk vaccine, also known as the inactivated polio vaccine (IPV). IPV provides superior safety and individual protection from paralysis, but it is more expensive, more difficult to administer, and doesn’t provide much transmission-blocking immunity.

Many of the countries where cost is not an issue and transmission is no longer a concern have switched to using IPV exclusively. Soon, the WHO will be directing the transition to IPV for the rest of the world, including places where poverty, population density, and sanitation practices keep the risk of polio transmission high even in the absence of wild poliovirus. The transition will happen in stages to mitigate the evolving spectrum of polio risks, and so the people involved in the global polio eradication effort are looking to find the best vaccination regimens to take us to the post-polio future.

So, we are working with colleagues at the Institute for Diarrheal Disease Research in Bangladesh (icddr,b) and University of Virginia to test candidates for the new polio vaccine regimen in the developing world ahead of the global policy shift. We are conducting a cluster-randomized study in 67 villages in rural Matlab, Bangladesh to compare the levels of protection from transmission offered by different mixed regimens of OPV and IPV introduced to routine infant immunization. Together, we are measuring the impact changes to vaccination policy on polio transmission risk, and we are pioneering approaches to environmental surveillance to observe transmission in low infrastructure settings. Our study will help guide future polio vaccination policy and help build the legacy of the polio eradication campaign.