This week, The Lancet continued the debate on polio eradication, complete with an editorial by Dr. Kimberly Thompson, the College of Medicine’s professor of preventative medicine and global health.
While the last case of polio was reported in the Western Hemisphere in 1991, the threat of the virus remains, and recent outbreaks have occurred in nations battling extreme civil unrest – Syria and Pakistan. Because Florida is a top destination for international travelers and America is a magnet for refugees worldwide, polio remains a real threat and “we need to make sure to immunize people who have not previously been vaccinated or exposed,” Dr. Thompson said.
Eradicating polio involves balancing the risks, rewards and costs of vaccines, and medical experts continue to debate the best way to finish the job. The United States and other high-income countries historically used the relatively inexpensive and easy-to-deliver oral poliovirus vaccine (OPV) to stop poliovirus transmission. However, as a live virus vaccine, OPV can cause paralysis in very rare cases, and can lead to outbreaks of vaccine-derived polioviruses if vaccine coverage decreases. To avoid the small but non-zero risks associated with OPV, the U.S. and other developed countries switched to inactivated poliovirus vaccine (IPV), although IPV does not provide intestinal immunity. For countries yet to stop transmission and focused on costs, OPV remains the vaccine of choice for routine immunizations.
The Lancet reported on a study conducted in Vellore, India that suggested that the injected vaccine would boost intestinal immunity in children who previously received OPV. One of the challenges of eradicating polio is that the virus can multiply in the gut and then be passed on to others through contact with infected fecal material. Such infection has “led to serious polio outbreaks in Asia, Africa and Europe over the last 10 years and is hampering efforts to eradicate the disease,” The Lancet reported. Some scientists had believed that IPV wouldn’t provide much protection to the polio virus in the gut. But the India study showed that providing children with the IPV vaccination after they had received the oral vaccine “actually boosted their gut immunity.”
“Our findings show that an additional dose of the injected vaccine is more effective at boosting immunity against infection than the oral vaccine alone,” said Nick Grassly, professor of vaccine epidemiology at Imperial College London, senior author of the study. “This implies that the IPV could be used to boost immunity in people travelling from or to polio-infected countries, such as Afghanistan, Pakistan and Nigeria. It could also replace some of the OPV doses in immunization campaigns to boost gut immunity, particularly in areas of poor sanitation.”
However, in her accompanying editorial, titled “Managing the polio endgame: Focusing on performance with or without IPV,”Dr. Thompson cautioned about extrapolation of the results from individuals in the trial to the population. With over a decade of experience modeling polio, which the Institute for Operations Research and Management Science recently recognized with its prestigious 2014 Franz Edelman Award, Dr. Thompson noted that based on an expert review her modeling already assumed that giving a dose of IPV to individuals previously-vaccinated with OPV would boost immunity, so the results of the study came as no surprise. She emphasized that national immunization policy decisions need to focus on population immunity, not individual immunity, and that stopping transmission depends on the aggregate level of immunity for all individuals in the population. Giving an extra dose of IPV to previously OPV-vaccinated children compared to no extra dose will lead to an “incremental increase” in population immunity, but the impact may be small and not cost-effective. Professor Thompson urges public health and policy experts to establish global performance goals for eradicating polio – whatever vaccine is used – and hold countries to those goals.
Despite continued progress toward eradication of all three types of polioviruses, Dr. Thompson noted that since 2000, more than 50 countries that had successfully stopped transmission of the polio virus reported one or more new cases that occurred due to “importations” from other countries. Dr. Thompson emphasized that independent of whether countries choose to include IPV or not in their immunization schedules, “it’s time to get the job done.”
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