The degree of protection afforded by any vaccine can be understood in terms of its efficacy and its effectiveness. The distinction between the two terms is important. Efficacy refers to the reduction in biologically proven illness (detected serologically or via culture of the virus from the patient), whereas effectiveness refers to the reduction in clinical consequences, which, for infectious disease like influenza or Covid-19, includes pneumonia, hospitalization, and death.
The influenza vaccine has a variable efficacy; it is not terrific at preventing infections (50%) in years where the vaccine is not well matched to circulating strains of the virus, better (90%) when it is. But the more important purpose of getting vaccinated is in preventing hospitalizations and death. The flu vaccine reduces hospitalization by 90% in healthy adults. More importantly, it decreases the risk of death in the elderly, where most deaths occur, by 20-50%.
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