摘要：Objectives. We assessed short-term responsiveness of influenza vaccine demand to variation in timing and severity of influenza epidemics since 2000. We tested the hypothesis that weekly influenza epidemic activity is associated with annual and daily influenza vaccine receipt. Methods. We conducted cross-sectional survival analyses from the 2000–2001 to 2004–2005 influenza seasons among community-dwelling elderly using the Medicare Current Beneficiary Survey (unweighted n = 2280–2822 per season; weighted n = 7.7–9.7 million per season). The outcome variable was daily vaccine receipt. Covariates included the biweekly changes of epidemic and vaccine supply at 9 census-region levels. Results. In all 5 seasons, biweekly epidemic change was positively associated with overall annual vaccination (e.g., 2.7% increase in 2003–2004 season) as well as earlier vaccination timing ( P < .01). For example, unvaccinated individuals were 5%–29% more likely to receive vaccination after a 100% biweekly epidemic increase. Conclusions. Accounting for short-term epidemic responsiveness in predicting demand for influenza vaccination may improve vaccine distribution and the annual vaccination rate, and might assist pandemic preparedness planning. Seasonal influenza disproportionately affects the population aged 65 years and older. Approximately 90% of the 36 000 influenza-related deaths and 63% of the 226 000 influenza-related hospitalizations per year in the United States occur among the elderly. 1 , 2 Despite the significant disease burden of influenza disease and the benefits of influenza vaccination, 3 , 4 influenza vaccination coverage levels among the noninstitutionalized elderly population have fluctuated between 60% and 67% since 1997. 5 Influenza vaccine coverage rates tend to be lower when vaccine supply delays or shortages occur as observed in several seasons since 2000 ( Table 1 ). 6 – 8 Fortunately, vaccine production capacity has improved, in part because the number of influenza vaccine manufacturers supplying vaccine to the US market has increased from 3 in 2004 to 5 in 2007. 9 During this period, the vaccine supply has increased from 57 million to 130 million doses. However, problems with vaccine supply remain as evidenced by the suspension of 46 million doses from one manufacturer during the 2004–2005 season (Centers for Disease Control and Prevention [CDC], unpublished data, 2006), 9 and a problem with regulation compliance by manufacturers. 10 TABLE 1 Influenza Vaccination Rates Prior to and During an Influenza Epidemic Period Among the US Community-Dwelling Medicare Elderly Population: 2000–2005 Vaccination Rate Based on Claims Data Influenza Season Epidemic Start Datea Vaccine Supply Problem From Sept 1 to Epidemic Startb During Epidemic Periodb From Sept 1 to May 20 Vaccination Rate Based on Survey Data 2000–2001 Dec 3 Severe delay 37.2% 9.81% 47.0% 69.7% 2001–2002 Dec 16 Moderate delay 45.8% 2.66% 48.5% 71.5% 2002–2003 Dec 15 None 50.4% 0.54% 50.9% 72.9% 2003–2004 Oct 12 Relative shortage 34.5% 18.3% 52.8% 73.7% 2004–2005 Dec 5 Severe shortage 35.5% 7.93% 43.4% 67.1% Open in a separate window aStart date defined at the national level (influenza survey laboratory data percent positive ≥ 5%). bStart date defined at 9 census region level (influenza survey laboratory data percent positive ≥ 5%). Despite progress in vaccine production capacity, supply and demand for influenza vaccine remains a “chicken and egg” problem 7 in which fluctuating or sporadically low demand for the vaccine leads vaccine manufacturers to reduce their supply or exit the market. 7 , 9 To address this problem, several policy options have been proposed to motivate vaccine manufacturers, distributors, and providers to remain in the system. 9 These options include extending vaccination efforts into January and beyond, reducing the financial burden for patients and providers, 9 and instituting a public “buy back program” for unused doses to reduce financial risk for manufacturers. 11 These options could also include improvement in the distribution of influenza vaccine, because often a large number of doses remain unused even during seasons with vaccine supply problems. 9 It is difficult to predict vaccine demand, particularly late-season demand, for 2 reasons. First, manufacturers and distributors take vaccine orders as early as January of the prior season, because the vaccine production takes 8 or 9 months. 9 Second, demand may decrease when influenza epidemic activity is perceived to be mild, whereas demand may increase when an influenza epidemic is perceived to be severe or occur early. 7 Epidemic activity usually affects late-season demand because influenza activity peaks after January in most seasons (84% of the seasons from 1976 to 2006). 12 Although the effect of ongoing influenza epidemics on vaccine demand was suggested qualitatively by Layton et al., 7 to the best of our knowledge it has not been measured quantitatively in the literature. Previous studies have noted long-term responsiveness of influenza vaccine demand to epidemic activity with a 1-year lag, e.g., past year's epidemic level. 13 – 15 We sought to measure short-term responsiveness of influenza vaccine demand to ongoing influenza epidemic levels and timing. We tested the hypothesis that weekly influenza epidemic change is positively associated with overall annual influenza vaccine receipt as well as daily vaccine receipt. We analyzed a 5-year period since 2000 during which influenza seasons varied with respect to the timing and severity of the epidemics, and vaccine supply. This information could improve efficiency in distribution of influenza vaccine, particularly after the onset of an epidemic. Knowledge of this association might also help predict short-term, late-season vaccine demand in different geographic areas, thereby enabling better vaccine distribution and redistribution and thus improving the overall vaccine coverage level. Furthermore, measurement of short-term responsiveness to epidemic activity might be helpful in pandemic planning because of the possibility of insufficient vaccine supply compared with the demand for vaccine. 16 There is also the potential for policies that target younger populations who may experience higher attack rates during a pandemic than in seasonal influenza epidemics. 17
