Purpose: In response to the 2009 H1N1 influenza pandemic, millions in the US were vaccinated, with state-specific coverage ranging from 8.7 to 34.4% for adults and 21.3 to 84.7% for children under 18; we study factors associated with higher vaccination coverage in a system where vaccine was in short supply.
Method: We used regression coupled with other statistical techniques to predict state-specific vaccination coverage of adults or children, using independent variables including demographics, and area (US Census Bureau); past seasonal adult or childhood vaccination coverage (Behavioral Risk Factor Surveillance System, National Immunization Survey); Public Health Emergency Response Funds (CDC); physician counts (US Bureau of Labor and Statistics); children’s health information (National Center for Health Statistics); H1N1-specific state and local data at the CDC (level of allocation control, type of allocation priority, participation of VFC providers, date of expansion beyond ACIP target groups, number of shipments, number of ship-to locations, lead time for allocation and ordering, peak week of Influenzalike illness activity); and degree of local autonomy of the public health system.
Result: The best models including only statistically significant variables explained over 70% of the variation in state-specific vaccination coverage of adults or children. We find that higher past seasonal influenza vaccination coverage of adults was associated with higher 2009 H1N1 vaccination coverage of adults and children, and accounted for 30% of the variation. In terms of supply chain factors, vaccination coverage changed positively with the number of shipments per location and negatively with the time to order allocated doses. For children, the proportion of the state’s population < 18 years was negatively associated with vaccination coverage.
Conclusion: Strengthening routine influenza vaccination programs may help improve vaccination coverage during a pandemic or other emergency. Repeated distribution to the same locations could represent underlying system differences related to efficiency or monitoring of usage to redistribute to providers who were vaccinating quickly. Ordering lag may be a function of system structure or of efficiency. This analysis suggests factors that public health agencies might consider monitoring in an emergency vaccination program during a supply shortage or aspects public health systems could consider when designing systems. In addition, accounting for the relative size of a State’s child population in allocating vaccine could improve vaccination coverage of children, in a scenario where children are targeted.