Polio, once among the most dreaded diseases even in the richest countries, has seemed on the verge of eradication for years now—butonly on the verge. In a small handful of nations, the virus is holding on stubbornly despite a major vaccination effort backed by the United Nations and philanthropists such as the Bill and Melinda Gates Foundation. Perversely, a current polio outbreak in the Democratic Republic of the Congo is caused by the polio vaccine itself. What explains this seeming paradox? What are the implications for polio eradication? To gain some insight, the Bulletin posed a few questions via e-mail to Christopher K. Brown, a scientist with the Occupational Safety and Health Administration. Brown holds degrees in biodefense, biostatistics and epidemiology, and biology—and just weeks ago wrote about Ebola vaccine for the Bulletin. A lightly edited version of his answers to the Bulletin’s questions appears below.
Bulletin of the Atomic Scientists: A current polio outbreak in the Democratic Republic of the Congo is caused not by a “wild” poliovirus but rather by a virus derived from oral polio vaccine. How does a vaccine end up causing the very illness it’s intended to prevent?
Christopher K. Brown: There are several different ways to make a vaccine, one of which involves weakening the pathogen that causes the disease so that, ideally, it triggers the vaccine recipient’s immune system to recognize and fight off the pathogen without actually causing a case of disease. Because weakened—or attenuated, as they are called—vaccines still include a live, virulent organism, they are in very rare instances linked to disease, whether in a single individual or as the culprit behind an outbreak. Scientists do not fully understand why the intentional infection that the oral poliovirus vaccine causes might lead to unintended disease in an extremely small number of vaccinated people, but it is likely that having a weakened immune system may contribute to someone experiencing paralysis after getting the attenuated vaccine.
The process by which an attenuated vaccine leads to a larger outbreak is more complicated. When someone gets the vaccine, the viruses it contains replicate in the small intestine. The weakened viruses can then spread from person to person. This can be a good thing in that it offers immunity to people who are exposed through this type of transmission even though they didn’t get the vaccine themselves—and thus it increases the proportion of the population that most likely will not get polio even if exposed to poliovirus (that is, it bolsters herd immunity). But if a vaccine-derived strain of the virus continues to spread and replicate, mutations in its RNA (the genetic material that controls the virus) can over time allow it to revert to a form capable of causing disease, including paralysis, as we are seeing among children in the Democratic Republic of the Congo. If most of a population is not immune to the particular strain, then more people may start to develop disease as a result of the vaccine strain becoming virulent again.