Efforts to fight polio outbreaks by vaccinating adults as well as children are costly and potentially misguided. That’s the conclusion of an analysis of recent polio outbreaks in Tajikistan and the Republic of the Congo. Researchers wanted to know whether adults and older children had spread the virus and, more important, whether giving adults booster vaccines to prevent transmission would do any good once an outbreak started. To find out, the team analyzed real-world data with two computer models. The first was a standard model of polio transmission, in which people don’t lose their immunity over time. According to that model, adults in Congo, but not in Tajikistan, transmitted polio. That’s no big surprise, because polio is transmitted when fecal matter ends up in someone else’s stomach, and Tajikistan has generally better hygiene than Congo. More surprising was what happened when the researchers considered a second computer model, which took account of the fact that in the years since they were vaccinated, adults and older children may have lost their immunity and could therefore help spread polio. Despite the change, the model predicted about the same number of people would get sick, and the epidemic took about the same course, even if some people lost their immunity. That conclusion, reported online today in the Proceedings of the National Academy of Sciences, means that lost immunity isn’t to blame for adults spreading polio, and simply giving adults polio vaccinations won’t necessarily help stop an outbreak. In Congo, they say, adult boosters could have helped, but in Tajikistan it would have been costly and probably useless. In the future, the team argues, global health officials should spend their time and money on responding more quickly to the early signs of a polio outbreak. That way, children can get the protection of a vaccine before an outbreak gets out of control.