Flu shots contain vaccine for three influenza viruses that researchers predict will be the most common in a given flu season. However, plenty of other viruses (flu and otherwise) also are circulating and can make you sick. Many illnesses with flu-like symptoms are incorrectly attributed to the flu.
Consequently, the best studies use a lab to identify the specific virus that infects each of their sick subjects. These studies only count the subjects with confirmed cases of the three types of influenza virus. Effectiveness is defined as the reduction in these three influenza viruses among those who were vaccinated compared to those who were not vaccinated.
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