The Polio Vaccine Is Causing Polio. Oops?

by Alanna Shaikh

category: Basics

Published August 16, 2009 @ 09:25AM PT

(Nigerian kids. Photo credit: OziAfricana)

124 children in Nigeria have been paralyzed by a type of polio that was actually caused by the polio vaccine. It's the result of using a cheaper, more effective type of polio vaccine. This could be bad news for that vaccine - if it's causing that much polio, we may have to stop using it.

Developing countries use a type of polio vaccine - a live, oral vaccine - that causes a very mild form of the virus, and therefore the infection.  The mild form triggers an immune response which protects from the serious, wild form of the virus. The mild virus also spreads from child to child and provides some protection to kids who've never gotten the vaccine. It's used in developing countries because it can be given by mouth, it's cheaper than the vaccine made with an inactivated (as opposed to live) virus, and because of that protection it provides to unvaccinated children.

Everything, of course, has a catch. In the case of the live oral vaccine, the catch is that every once in a while, it causes a more severe form of polio. Not often - it's usually about 1 case of polio-paralysis in every 2.5 million doses of vaccine, which makes the odds worthwhile from both an individual and a population perspective. And the more severe form happens in children who have not actually been vaccinated. Sometimes the vaccine virus can mutate in its host, and then cause a more severe form of polio in the child who catches it. If you can vaccinate almost all kids, then there is an even tinier risk of vaccine-related polio paralysis.

42 million children have been vaccinated for polio in Nigeria. This year, there have 124 cases of polio caused by the vaccine. That is double what we saw last year. Experts are blaming low vaccination rates for the spread, and they are right - to a point. If more children were vaccinated, then this wouldn't be happening. But accepted wisdom has always been that vaccine-caused polio virus didn't spread the way wild polio does. The Nigerian outbreaks cast some doubt on that.

If the oral polio vaccination is only effective when given to nearly 100% of the population, then it changes the whole cost-benefit discussion. It starts to look like there's no real point to using the oral vaccine except to save money. That makes cases of vaccine-induced polio paralysis a lot harder to accept.