An estimated 52,000 people died from the flu in the United States during the 2017-2018 flu season, making it one of the deadliest seasons in recent history. Because flu seasons span two calendar years (roughly October through April), the calendar year 2018 actually overlaps with two separate seasons: the tail end of 2017-2018 and the start of 2018-2019. The following season, 2018-2019, was significantly milder, with an estimated 28,000 flu deaths.
Why 2017-2018 Was So Severe
The CDC classified the 2017-2018 flu season as “high severity” across every age group: children, adults, and older adults. It was the first season to receive that classification in all three categories since the CDC began using its current severity framework in 2003. For context, out of roughly 20 seasons assessed, only four have been classified as high severity overall, and none have reached “very high.”
Beyond the 52,000 deaths, the season produced staggering numbers across the board: an estimated 41 million symptomatic illnesses, 21 million medical visits, and 710,000 hospitalizations. The 2018-2019 season, by comparison, saw about 29 million illnesses and 380,000 hospitalizations, nearly half the burden.
What Made the Death Toll So High
Two factors drove the severity. First, the dominant circulating strain was H3N2, a subtype of influenza A that historically causes more severe illness and higher death rates than other flu strains. H3N2 seasons tend to hit older adults especially hard.
Second, the flu vaccine that year offered limited protection. Overall vaccine effectiveness was just 36%, and against H3N2 specifically it dropped to 25%. That means roughly three out of four vaccinated people exposed to H3N2 could still get sick. Even in a good year, flu vaccines typically perform better against other strains, but the mismatch in 2017-2018 was particularly costly.
Children Were Hit Hard
The 2017-2018 season was especially devastating for children. A total of 183 lab-confirmed pediatric flu deaths were reported to the CDC, one of the highest counts on record. That number represents only confirmed cases. The actual toll was likely higher, since not every child who dies from flu complications receives a lab test.
How These Numbers Are Calculated
The figure of 52,000 deaths is an estimate, not a direct count, and understanding why matters. Flu deaths are significantly undercounted in raw data because many people who die from flu-related complications (pneumonia, for instance, or worsening of heart disease) never receive a flu test. Their death certificates may list the complication rather than influenza as the cause.
To get a more accurate picture, the CDC uses a mathematical model built on hospitalization surveillance data collected from a network of hospitals across the country. That network captures only a subset of all flu hospitalizations, so the model scales the numbers up based on population data and adjusts for undertesting and underreporting. The result is a burden estimate that reflects total impact rather than just confirmed cases. This is the standard method used by public health agencies worldwide.
How 2018 Compares to a Typical Year
Globally, seasonal influenza causes an estimated 291,000 to 646,000 respiratory deaths each year. The U.S. share of that burden varies significantly from season to season. In mild years, domestic flu deaths can drop below 20,000. In moderate years, they fall in the 25,000 to 35,000 range. The 2017-2018 season’s 52,000 deaths placed it well above the recent average.
The 2018-2019 season that followed brought the numbers back down to 28,000 deaths, closer to a typical moderate year. That kind of swing is common with influenza. Season severity depends heavily on which strains circulate, how well the vaccine matches those strains, and how much immunity exists in the population from prior exposures. A brutal year like 2017-2018 can be followed by a relatively calm one without any change in public health strategy, simply because a different virus subtype takes over.
Who Was Most at Risk
Adults 65 and older bore the heaviest burden during the 2017-2018 season, consistent with H3N2-dominant years. Older adults account for the majority of flu-related hospitalizations and deaths in most seasons, but H3N2 widens that gap. Their immune systems respond less robustly to both the virus and the vaccine, creating a compounding vulnerability.
Young children, particularly those under 5, faced the next highest risk. Pregnant people and those with chronic conditions like asthma, diabetes, or heart disease also experienced higher rates of severe illness and hospitalization. The pattern held in 2018-2019 as well, though the absolute numbers were lower across every age group.