Modeling the effects of updating the influenza vaccine on the ef- ficacy of repeated vaccination

Background: The accumulated wisdom is to update the vaccine strain to the expected epidemic strain only when there is at least a 4-fold difference [measured by the hemagglutination inhibition (HI) assay] between the current vaccine strain and the expected epidemic strain. In this study we investigate the effect, on repeat vaccinees, of updating the vaccine when there is a less than 4-fold difference. Methods: Using a computer model of the immune response to repeated vaccination, we simulated updating the vaccine on a 2-fold difference and compared this to not updating the vaccine, in each case predicting the vaccine efficacy in first-time and repeat vaccinees for a variety of possible epidemic strains. Results: Updating the vaccine strain on a 2-fold difference resulted in increased vaccine efficacy in repeat vaccinees compared to leaving the vaccine unchanged. Conclusions: These results suggest that updating the vaccine strain on a 2-fold difference between the existing vaccine strain and the expected epidemic strain will increase vaccine efficacy in repeat vaccinees compared to leaving the vaccine unchanged.