Measles in adults in Canada and the United States: Implications for measles elimination and eradication

Background Despite the implementation of mass school catch-up campaigns for measles in Canada, an outbreak of measles occurred in early 1997 mostly af fecting the adult population. The higher incidence in Canada in adults led us to compare immunization policies and the evolution of measles among adul ts in Canada and the US. Methods Based on information gathered from both national immunization progr ammes and surveillance systems. Results Although the proportion of cases occurring in adults has increased tremendously in both countries in the past decade, there was no increase in measles incidence in these populations. The most likely factors to explain the higher rate of measles occurring in adults in Canada are the younger a ge at administration of first dose in Canada, the delay in implementation o f a second dose policy in Canada compared with the US combined with the lac k of prematriculation immunization requirements in Canadian colleges and un iversities, and the higher rate of overseas travel to and from Canada. The situation in Canada may also have been exacerbated by incomplete efforts to control measles for many years without attempting to eliminate the disease . Conclusions In order to prevent measles in adults, high-risk groups must be identified and catch-up for selected groups considered. Vaccination of int ernational travellers to endemic areas should be recommended until global e limination has been achieved. Appropriate measles control strategies in you nger populations seem to be effective in preventing measles in adults. The experience in Canada and the US suggests that measles transmission in adult s is unlikely to be a major impediment to regional elimination or global er adication.