Pakistan, Afghanistan and Nigeria are the three remaining countries where polio is endemic, and Nigeria had been free of wild poliovirus for three years. Highly contagious, the poliovirus largely strikes children under age 5 and can cause permanent paralysis. Vaccinations must be sufficiently widespread to break the chain of transmission.

Unfortunately in Pakistan, political instability, population mobility and violence driven by intolerance have made it hard to reach all children for vaccination. Two policemen were killed Dec. 18 while on their way to accompany a polio vaccination team in the Lower Dir district, about 112 miles northwest of the capital, Islamabad. Since 2012, at least 98 people have been killed in attacks on Pakistan’s polio vaccination campaigns.

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Hard-line Islamist forces still spread irrational beliefs that vaccines are contaminated or a Western plot. Social media in Pakistan has accelerated false rumors about vaccination, leading to waves of refusals by parents. According to a recent account by Al Jazeera, another worrisome trend has arisen lately: Villages are using vaccination as a bargaining chip to win other needs from the state, such as water, natural gas, electricity and jobs. No global eradication effort can be successful without Pakistan. This year must see progress against these serious obstacles, which are caused by human behavior, not virology.

At the same time, new problems have cropped up in the global vaccination effort. Three poliovirus strains have existed: wild types 1, 2 and 3. The wild type 3 was declared eradicated last year. The wild type 2 had not been sighted since 1999. Vaccines use an inactivated version of the wild strains to trigger an immune response by the human body. Sometimes, the inactivated strains pass into the environment and revert to active. When wild type 2 disappeared, a decision was made to omit the type 2 component from the oral polio vaccine used by 155 countries.

The “switch” in April 2016 was made with a realization that some type 2 outbreaks were possible if the vaccine-derived strain reverted. Now this has occurred, to a far greater extent than anticipated, especially in Africa. There have been 47 outbreaks in 20 countries since the switch, some very difficult to stop. By comparison, there were only eight outbreaks in five countries over three years before the switch. Existing supplies of a type 2-only vaccine are insufficient. Global health officials are rushing to get ready by this summer a new type 2 vaccine that won’t revert to active and is believed to be effective. Hopefully it will work. Otherwise a major strategy shift may be required in one of the world’s most difficult battles against disease.