T his year marks the centennial of the 1918 infl uenza pandemic, which killed 50 million people worldwide. Three more infl uenza pandemics and annual epidemics have occurred since then, with other signifi cant interim events (Table 1). 1 The 2017-2018 epidemic was particularly severe and long. 2 This centennial year update focuses primarily on immunization, but also reviews epidemiology, transmission, and treatment. ■ EPIDEMIOLOGY 2017-2018 was a bad season The 2017-2018 infl uenza epidemic was memorable, dominated by infl uenza A(H3N2) viruses with morbidity and mortality rates approaching pandemic numbers. It lasted 19 weeks, killed more people than any other epidemic since 2010, particularly children, and was associated with 30,453 hospitalizationsalmost twice the previous season high in some parts of the United States. 2 Regrettably, 171 unvaccinated children died during 2017-2018, accounting for almost 80% of deaths. 2 The mean age of the children who died was 7.1 years; 51% had at least 1 underlying medical condition placing them at risk for infl uenza-related complications, and 57% died after hospitalization. 2 Recent estimates of the incidence of symptomatic infl uenza among all ages ranged from 3% to 11%, which is slightly lower than historical estimates. The rates were higher for children under age 18 than for adults. 3 Interestingly, infl uenza A(H3N2) accounted for 50% of cases of non-mumps viral parotitis during the 2014-2015 infl uenza season in the United States. 4