Two Down, One to Go: A Second Polio Type Is Eradicated From the Human Population

The World Health Organization has officially declared Type 3 polio virus eradicated, leaving only Type 1 as the viral cause of poliomyelitis in Afghanistan and Pakistan. However, as The New York Times points out, two big obstacles remain before Type 1 can be eradicated and mark the end of polio as a disease:

“Although that brings the world another step closer to eradication, the effort has taken far longer than was ever anticipated. When the campaign began in 1988, most public health officials and donors expected the battle to be over by 2000.

But two major obstacles emerged.

First, millions of families around the world have not let their children have the drops because of persistent false rumors that the vaccine is a Western plot to sterilize Muslim girls or do other harm.

Second, in some countries viruses used in the oral vaccine itself have mutated into a form that can be passed on in diapers and sewage, and can paralyze unvaccinated children. That has contributed to fear of the oral vaccine, even though full vaccination is the only protection against such mutant viruses.

Just in the last two months, cases of paralysis caused by mutant vaccine viruses have been reported in the Philippines, Zambia, Togo and Chad. Because paralysis occurs in only about one in every 200 cases of polio, experts assume many more children have been infected.

Stopping such outbreaks typically requires vaccinating hundreds of thousands of children with both the injectable vaccine, which contains killed virus that cannot mutate, and the oral vaccine. The latter contains weakened viruses that normally cannot cause disease but provide better protection than killed viruses.”

Indeed, one of the most shared talking points by anti-vaccine organizations is that “the vaccine causes more polio than the wild type virus,” without giving any context as to why that is, of course. If the recommended number of children were vaccinated on time, every time, we would probably be talking about polio eradicated entirely, a feat only done once with smallpox virus in the 1970s. So there is still some work to be done.

After polio, the next virus that could be eradicated is measles. For a viral disease to be eradicated, three things need to happen. First, the vaccine against the viral infection must be highly effective. The MMR vaccine pushes 97% to 98% effectiveness if the two recommended doses are given. Second, humans must be the only reservoir of the infection. Measles is only found in humans. Other primates may contract the disease incidentally, but they are not reservoirs. (Compare this to influenza, where pigs and birds, along with humans, are the main reservoirs of the influenza virus.) Third, the vaccine needs to be widely accepted by the public.

That last part is the hardest part about measles eradication, and one that we might be fighting for the foreseeable future given how ardently a very small — but significant when it comes to community immunity — segment of the population advocates against the measles vaccine around the world. Nevertheless, this giant step toward polio eradication shows that it can be done, and that millions of children’s lives can be saved.

Author: René F. Najera, DrPH

I am the editor of the History of Vaccines site, an online project by the College of Physicians of Philadelphia. All opinions expressed on these blog posts are not necessarily those of the College or any of my employers. Check out my professional profile on LinkedIn: https://www.linkedin.com/in/renenajera Feel free to follow me on Twitter: @EpiRen

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