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Editor's Correspondence |

Influenza Vaccination Effectiveness Is Not Proven in Younger Individuals at Risk—Reply

Eelko Hak, MSc, PhD; Erik Buskens, MD, PhD; Theo J. M. Verheij, MD, PhD
Arch Intern Med. 2005;165(16):1922. doi:10.1001/archinte.165.16.1922-a.
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Our prespecified primary objective was to evaluate the clinical effectiveness of influenza vaccination during an influenza period with epidemic activity among persons recommended for annual influenza vaccination. In the 1999-2000 winter season, influenza A activity was epidemic, whereas the 2 following seasons had only very mild influenza activity and were therefore unsuitable for our objective. The highly statistically significant relative risk reduction of 43% in general practice visits due to acute respiratory disease during the study season further supports the current guidelines to vaccinate children with high-risk medical conditions against influenza. Damoiseaux et al questioned our findings, referring to the trial by Bueving et al1 among children with asthma and a trial by Hoberman and colleagues.2 However, the findings from those trials were not convincing. The influenza vaccine trial by Bueving et al1 among children with asthma had serious flaws including inadequate statistical power and pooling of data over influenza seasons with varying influenza activity.3 The study by Hoberman et al2 among infants also clearly showed that when influenza activity is epidemic (1999-2000 season), the vaccine’s effectiveness was higher in reducing culture-confirmed influenza (66%) than in the 2000-2001 season with mild influenza activity (−7%). Also, the attack rates were 5 times lower in the latter season, hence pooling leads to invalid results. Further, Damoiseaux and colleagues argued that excluding cases with depressive disorders or diabetes from our case-control evaluation might have had an effect on the vaccine effectiveness estimates in these children. However, because there were only 2 children with a diabetes event and 1 with a depressive disorder, such a bias is very unlikely. Finally, we agree with Damoiseaux et al that influenza vaccination can only be effective during seasons with high influenza activity, and it is therefore important to evaluate the cost-effectiveness of influenza vaccination among high-risk children over a longer period. For such an evaluation, it is important to adopt the societal perspective and to model the possible effectiveness of vaccination in reducing costly hospitalizations and deaths as well as the productivity loss of parents.4

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