Effect of vaccination campaign start date on overall attack rate. For a given vaccination campaign start date, the percent reduction in final attack rate relative to that observed in the absence of vaccination is presented for campaign lengths of between 4 and 12 weeks. Vaccination campaigns were implemented weekly, starting July 5, 2009, with the last campaign started November 22, 2009. The start of the Vancouver influenza season on September 6, 2009 is indicated by a vertical line. All simulations assumed R
0 of 1.4, latent period of 3 days, and infectious period of 7 days.