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The continuous antigenic drift of influenza viruses requires annual adaptation of the vaccine. Protection depends largely on the match of the variants represented in the vaccine with the viruses actually known to be in circulation and may differ considerably from season to season. Therefore studies to assess the efficacy and effectiveness of the vaccine are conducted rather sporadically on an annual basis and it would be desirable to make use of routinely available data from surveillance programs. We compared two different approaches: (1) the "screening method" where cases are identified from laboratory data and controls are taken from data on vaccination rates and (2) a second method that uses the same cases, but controls were influenza-negative individuals with influenza-like illness (also identified from laboratory data). The sensitivity of the methods to confounders that were considered as relevant was tested with a simulation. Both methods were applied to the data of the German influenza surveillance data of the season 2004/2005. The estimated effectiveness over all age groups was rather low with both methods, but comparable with other estimations from the literature. We observed differences in certain age groups between the methods as well as large differences between particular age groups within one method. Possible explanations are random variations due to low numbers in age strata and other influences not yet considered. Therefore the estimations should be interpreted with care; however, relative comparisons among seasons may still be meaningful.

Applied infectious disease epidemiology/field epidemiology involves the application of epidemiologic methods to unexpected health problems when a rapid, on-site investigation is necessary for timely intervention. While field investigations of acute problems share the need of high methodological quality with prospectively planned studies, they may differ in several respects. In particular, because field investigations of ten start without a clear hypothesis, they may require the use of descriptive studies to generate a hypothesis before analytic studies are conducted. There also may be an immediate need to protect the population's health, pressure to intervene may conflict with the need to investigate and publicity may introduce bias. Outbreak investigations are paradigms of the use and methodology of applied infectious disease epidemiology/field epidemiology. The steps of an outbreak investigation include verification, confirming the diagnosis, developing a case definition and case finding, describing the data in terms of time, place and person, risk identification, formulating and testing of a hypothesis, planning for further studies, establishing control measures and communicating the findings.