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Time is of the essence: exploring a measles outbreak response vaccination in Niamey, Niger.Grais, RF; Conlan, A J K; Ferrari, M J; Djibo, A; Le Menach, A; Bjørnstad, O N; Grenfell, B T; Epicentre, 8 rue Saint Sabin, 75011 Paris, France. firstname.lastname@example.org (The Royal Society Publishing, 2008-01-06)The current World Health Organization recommendations for response during measles epidemics focus on case management rather than outbreak response vaccination (ORV) campaigns, which may occur too late to impact morbidity and mortality and have a high cost per case prevented. Here, we explore the potential impact of an ORV campaign conducted during the 2003-2004 measles epidemic in Niamey, Niger. We measured the impact of this intervention and also the potential impact of alternative strategies. Using a unique geographical, epidemiologic and demographic dataset collected during the epidemic, we developed an individual-based simulation model. We estimate that a median of 7.6% [4.9-8.9] of cases were potentially averted as a result of the outbreak response, which vaccinated approximately 57% (84563 of an estimated 148600) of children in the target age range (6-59 months), 23 weeks after the epidemic started. We found that intervening early (up to 60 days after the start of the epidemic) and expanding the age range to all children aged 6 months to 15 years may lead to a much larger (up to 90%) reduction in the number of cases in a West African urban setting like Niamey. Our results suggest that intervening earlier even with lower target coverage (approx. 60%), but a wider age range, may be more effective than intervening later with high coverage (more than 90%) in similar settings. This has important implications for the implementation of reactive vaccination interventions as they can be highly effective if the response is fast with respect to the spread of the epidemic.