While studies have found influenza vaccination to be cost-effective in older adults (65 years or older), they have not looked at how the vaccine's economic value may vary with the timing of vaccine administration. We developed a set of computer simulation models to evaluate the economic impact of vaccinating older adults at different months. Our models delineated the costs and utility losses in delaying vaccination past October and suggest that policy makers and payors may consider structuring incentives (< or =$2.50 per patient) to vaccinate in October. Our results also suggest that vaccination is still cost-effective through the end of February.