The incidence of poliomyelitis has dropped precipitously over the last decade. However, persistent transmission in three countries and outbreaks elsewhere challenge the end-2014 eradication target. The Global Polio Eradication Initiative is considering expanding the age range of vaccination campaigns even in the absence of adult cases, because of concerns about imperfect, waning intestinal immunity. The success of this approach will depend on the contribution to transmission by older ages, which we estimated during two large outbreaks affecting adults. Their contribution was found to depend on the setting, but there was no evidence for imperfect immunity contributing to the transmission of infection. Even small gains in the speed of vaccination response were found to have substantially greater benefit compared with expanded age range campaigns.