FIG 4.

Effect of vaccination under different assumptions of waning immunity. A hypothetical scenario is illustrated of immunization at 3 years into the epidemic, of 50% of the population with a vaccine that is 50% effective (∼1 million individuals in the Alberta population move from susceptible to immune). (A) The cumulative number of deaths plateaus as the epidemic is extinguished under conditions where vaccine-induced immunity is permanent (solid line, γ = 0) or wanes slowly (dashed line, 1/γ = 1 year). Under conditions of rapidly waning immunity (dotted line, 1/γ = 1 month), the number of deaths continues to rise despite vaccination. (B) The number and proportion of immune individuals increases stepwise at 3 years, remaining stable if immunity is permanent (solid line, γ = 0), or subsequently declining with waning immunity over a time scale of years (dashed line, 1/γ = 1 year) or months (dotted line, 1/γ = 1 month).