Vaccine policy and incidence of influenza.

To the Editor: Influenza outbreaks in the United Kingdom typically occur seasonally from October through March. Data from the early 1990s show that outbreak size and virulence have varied substantially over age, calendar year, and geography. We estimated the rate of clinically diagnosed influenza according to age, calendar time, and region from July 1996 through June 2007. Monthly rates were calculated by dividing the number of cases of influenza recorded each month by the number of patients active in each of approximately 350 general practices in that month. The location of practices ranged from Belfast in the north to Southampton in the south. We also derived the frequency of influenza vaccination according to age and calendar time. The study was based on General Practice Research Database, which provides complete and accurate clinical information. 6 We identified more than 40,000 persons diagnosed with “influenza” or “influenza-like symptoms” during the 11-year period of study. Because the database represents a representative sample of about 5% of the UK population, more than 800,000 cases of influenza were presumably diagnosed by general practitioners nationwide during this time. The monthly rates of influenza are shown in the Figure. Few cases were recorded during the spring and summer months (April–September) of any year. Higher-than-normal seasonal outbreaks occurred in 1996–1997 (6885 cases), 1998–1999 (7452 cases), and 1999–2000 (9005 cases)—mainly in December and January. Smaller outbreaks in 1997–1998 and 2000–2001 peaked in February. Rates of influenza fell on average more than 70% to background levels of less than 2700 cases per year from July 2001 through June 2007. Rates of influenza were consistently highest in persons age 20–54 years and lowest in those age 70 or older. They were consistently higher in Wales and Northern Ireland and lowest in southern regions of the United Kingdom. The dramatic fall in influenza rates after 2001 was evident in all regions. A previously undetected influenza virus was identified in the 2001–2002 season; nevertheless, the level of diagnosed influenza was low. From 1996 through September 2000, the frequency of influenza vaccination was 35%–45% in persons age 70 or older, 18%–22% in those age 55–69, 5%–6% in those 40–54, 2%–3% in those age 20–39, and less than 2% in those under 20 years. Vaccination frequency rose by more than 75% in all age groups after October 2000, when the government initiated a policy to vaccinate the elderly and others at high risk for influenza. In the past 2 decades, outbreaks of influenza in the United Kingdom have been restricted to the fall and winter months of October through March. The severity, age, and geographic distribution have varied from season to season. The largest outbreak, considered “epidemic,” occurred in the 1989 –1990 season. Three outbreaks in the 1996 –1997, 1998 –1999, and 1999 –2000 seasons were described as “higher than seasonal average.” Other smaller outbreaks before 2001 are described as “normal seasonal activity.” Since October 2001, the seasonal rate of influenza has fallen by an average of more than 70% through June of 2007. The recent spring outbreak of A/H1N1 (swine) influenza was exceptional. Following a comprehensive review of published studies, Simonson et al concluded that the effectiveness of the influenza vaccine in the elderly is still under debate. In our descriptive study, a major increase in influenza vaccination frequency in October 2001 has been followed by a dramatic decrease in the rate of influenza in all age groups, and this decline has continued through June 2007. Whether this time-trend association is coincidental or, at least in part, due to increased influenza vaccination remains to be determined. Additional results are available as an eAppendix (http://links.lww.com/EDE/A361). Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com).