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Influenza vaccine strain selection strategies

Currently, the influenza vaccine is updated only when there is at least a 4-fold difference [measured by the hemagglutination inhibition (HI) assay] between the current vaccine strain and an expected epidemic strain. It is not clear if this is the optimal strategy, especially for individuals vaccinated every year. Using a computer model, we determined the vaccine efficacy resulting from 12 different vaccine strain selection strategies in four categories: stay strategies that sometimes leave the vaccine unchanged, follow strategies that always update the vaccine to the expected epidemic strain, variant strategies that sometimes choose strains that are different from both the current vaccine and the expected epidemic strain, and skip strategies that sometimes skip vaccination. Judicious application of the follow, skip, and variant strategies each produced improvements over the current strategy in vaccine efficacy in repeat vaccinees ranging from 13 to 24%. The best strategy was to choose a “variant” strain when the expected epidemic strain was the same as the previous vaccine strain, and choose the expected epidemic strain otherwise. Had this strategy been applied during the years of the Hoskins [1] and Keitel [2] studies the vaccine efficacy in repeat vaccinees would have increased by 37%. Efficacy in first-time vaccinees however is reduced, by 6% when a variant-strain is chosen. These results support the current strategy to update the vaccine to the expected epidemic strain when there is a 4-fold or more difference between the current vaccine and the expected epidemic strain. If there is a less than 4-fold difference, these results suggest that a deviation from the current strategy would increase vaccine efficacy in repeat vaccinees: on a 2fold difference update the vaccine to the expected epidemic strain, and when there is no difference update the vaccine to a variant strain 2-fold different from the expected epidemic strain.