Flu at highest level in 15 years, CDC data show
Key takeaways:
- The CDC estimates that at least 24 million cases and 13,000 deaths have occurred during the 2024-2025 influenza season.
- Data suggest that flu is at its highest level since the 2009 swine flu pandemic.
CDC data show that seasonal influenza activity is significantly elevated across the United States, and that influenza activity is at its highest level in more than 15 years.
According to the CDC, there have been at least 24 million illnesses, 310,000 hospitalizations and 13,000 deaths — including 57 pediatric deaths — related to influenza this season.
One way the CDC tracks the severity of influenza seasons is by estimating the percentage of outpatient and ED visits that are attributed to influenza-like illness (ILI), which the CDC updates weekly. ILI rates climbed as high as around 7.4% during the 2022-2023 season and 7.7% during the 2017-2018 season, but the 2024-2025 influenza season has peaked even higher at 7.75% in the most recent week with available data — the highest rate since the 2009 swine flu pandemic.
Additionally, the rate of clinical lab tests that were positive for influenza reached a high of 31.6% as of Feb. 1, up from 29.4% the week before and 18.6% at the start of the year.
“To my recollection, this is the most severe flu season we've had in the last 15 years,” Aaron E. Glatt, MD, MACP, FIDSA, FSHEA, chairman of the department of medicine and chief of the division of infectious diseases at Mount Sinai South Nassau and professor of medicine at the Icahn School of Medicine at Mount Sinai, told Healio.
Most cases reported and subtyped in the most recent influenza surveillance report were influenza A viruses H1N1 (53.7%) and H3N2 (46.3%). The CDC noted that no cases were H5 bird flu, and that there has yet to be a case of H5 human-to-human transmission.
Glatt said he does not believe there is one specific factor causing the surge in influenza cases this season. Instead, he explained that there could be many contributing factors, including the end of masking, social distancing and other COVID-19 pandemic-era precautions that “tremendously suppressed” respiratory illness transmission, as well as people becoming more comfortable being in large groups in small spaces again.
Another possible reason is a lack of immunity from not having influenza during the previous few seasons because of these and other precautions, Glatt said.
“Obviously, that doesn't apply to everybody and that doesn't apply as an absolute blanket statement, but that must be contributing to it,” he said.
“There are probably other factors as well, but it's not unusual,” he added. “We've had bad flu seasons in the past, prior to COVID.”
Glatt’s message to providers and patients is that the vaccine is safe and can have a huge impact on illness severity.
“They have to realize that the flu vaccine is not a perfect vaccine, but it's a safe vaccine. It may not prevent you from getting the flu, but it certainly will have a role in preventing you from getting much more serious flu, and I think that's very, very important for people to realize,” he said. “Flu is a fatal disease — not for everybody, but it can be. Therefore, it's very important that people get vaccinated and take the appropriate precautions.”
References:
- Weekly US Influenza Surveillance Report: Key Updates for Week 5, ending February 1, 2025. https://www.cdc.gov/fluview/surveillance/2025-week-05.html. Published Feb. 7, 2025. Accessed Feb. 11, 2025.
Perspective
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We are in the midst of a very large influenza outbreak right now here in the U.S. It covers essentially the entire country, and it looks to be among the most serious influenza outbreaks over the last 20 years.
It's certainly true that my hospital’s ICUs and EDs are full of patients who are seriously ill with influenza. This is conventional seasonal influenza, not bird flu. The two leading viruses are both influenza A viruses — H1N1 and H3N2.
It's affecting, as you can imagine, the entire spectrum of the population — even healthy people can be put in the ED within 48 hours by influenza — but it hits most severely the people in high-risk groups, including people aged 65 years and older; people who have an underlying chronic medical condition of any kind, including heart disease, lung disease, diabetes — you name it — and people who are immunocompromised.
Many people among them, of course, are unvaccinated.
It's very late, but if you haven't been vaccinated, do so today. Don't wait. And make a resolution this fall in preparation for the next influenza season. Everyone aged 6 months and older should take advantage of influenza vaccine.
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Weekly US Influenza Surveillance Report: Key Updates for Week 5, ending February 1, 2025. https://www.cdc.gov/fluview/surveillance/2025-week-05.html. Published Feb. 7, 2025. Accessed Feb. 11, 2025.
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