Patterns of seasonal influenza activity in U.S. core-based statistical areas, described using prescriptions of oseltamivir in Medicare claims data

Abstract

Using Medicare claims data on prescriptions of oseltamivir dispensed to people 65 years old and older, we present a descriptive analysis of patterns of influenza activity in the United States for 579 core-based statistical areas (CBSAs) from the 2010-2011 through the 2015-2016 influenza seasons. During this time, 1,010,819 beneficiaries received a prescription of oseltamivir, ranging from 45,888 in 2011-2012 to 380,745 in 2014-2015. For each season, the peak weekly number of prescriptions correlated with the total number of prescriptions (Pearson's r ≥ 0.88). The variance in peak timing decreased with increasing severity (p < 0.0001). Among these 579 CBSAs, neither peak timing, nor relative timing, nor severity of influenza seasons showed evidence of spatial autocorrelation (0.02 ≤ Moran's I ≤ 0.23). After aggregating data to the state level, agreement between the seasonal severity at the CBSA level and the state level was fair (median Cohen's weighted κ = 0.32, interquartile range = 0.26-0.39). Based on seasonal severity, relative timing, and geographic place, we used hierarchical agglomerative clustering to join CBSAs into influenza zones for each season. Seasonal maps of influenza zones showed no obvious patterns that might assist in predicting influenza zones for future seasons. Because of the large number of prescriptions, these data may be especially useful for characterizing influenza activity and geographic distribution during low severity seasons, when other data sources measuring influenza activity are likely to be sparse.

Keywords: Antivirals; Influenza; Medicare.

Fig. 1.

Seasonal maps of severity as measured by the moving epidemic method.

Fig. 2.

Seasonal maps of peak timing as measured by the average week weighted by the number of therapeutic prescriptions of oseltamivir. Red points represent early influenza activity, before the 1st week of the New Year (within an influenza season, week 1 comes after week 52). Violet points represent late influenza activity, after winter ends. The spectral colors between represent intermediate times.

Fig. 3.

Distribution of average week weighted by the number of therapeutic prescriptions of oseltamivir stratified by seasonal severity for core-based statistical areas.

Fig. 4.

Seasonal maps of relative timing as measured by the time lag corresponding to the largest first singular value of the correlation matrices. Red points represent early influenza activity relative to the other core-base statistical areas (CBSAs). Violet points represent late influenza activity relative to other CBSAs. The spectral colors between represent intermediate times.

Fig. 5.

Seasonal maps of influenza zones from hierarchical clustering. Red colors represent earlier relative timing, and yellow colors represent later relative timing. Intense colors represent very high severity severe seasons, and dull colors represent low severity seasons.

Fig. 6.

Seasonal time series of the weekly number of therapeutic prescriptions of oseltamivir (cases) per 10,000 population. Seasonal time series were aggregated by influenza zones from agglomerative hierarchical clustering. Population estimates obtained from the U.S. Census Bureau (2016).