The 2018-2019 season has been unusual because the flu came in two waves: one that peaked at the end of December, and a second that peaked in early March.

The two peaks were caused by two different strains of the flu virus, and the protection given by vaccination early in the season may have waned by the time the second strain appeared.

The just-ended 2018-2019 flu season was relatively mild compared to the last season, during which nearly 80,000 people in the U.S. died of flu-related illness, according to estimates by the US CDC. This year’s death toll is predicted to be about half, or, in the worst case, three-quarters of that number.