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Note: Activity levels are based on outpatient visits in a state compared to the average number of visits that occur during weeks with little or no flu virus circulation; Data: Centers for Disease Control and Prevention; Chart: Chris Canipe/Axios

This year's flu season caught many experts off guard with both its sustained prevalence and its virulence. At its peak, there was a higher level of flu-like illnesses reported than any other year during the past eight years. Watch in the visual as it hits its peak around Week 18.

Why it matters: Public health officials try to capture this data when developing the next year's vaccines. And, of course, they want to find better ways to prevent severe flu seasons. There's a "Strategic Plan" to develop a universal vaccine to protect against a wider range of influenza viruses, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, tells Axios.

"Over the past ten or more years, this seasonal flu season is clearly — if not the worst — is one of the worst seasons we've had," Fauci said.

The data: Based on the trajectory of reported influenza-like illness (ILI), most public health experts believed this season was following the track of the 2014–2015 season and would peak at the end of December or beginning of January. But the peak really didn't happen until early February, making it much worse than expected.

  • As seen in the graphic above, reports of influenza-like illness — defined as having a fever of 100°F or higher with a cough and/or sore throat — continued to be prevalent longer than in most other seasons.
  • For this year and last year, the flu season was considered to be active when 2.2% of all Americans who visited the doctor in a given week had flu symptoms.
  • This year's flu season started earlier than last year, and the level of reported illness was higher. At its peak, during the week ending Feb. 3, 7.5% of all Americans who visited the doctor likely had the flu. The cases have started to trail off, but not enough for flu season to be considered over.
  • Last year, the flu season peaked during the week ending Feb. 11, with 5.1% of all Americans visiting the doctor reporting flu symptoms. That season ended in mid-April.

What to watch: Until a universal flu vaccine is available, public health officials want to improve the current vaccines by eventually dropping the time-consuming egg-production method that's used now.

Current vaccination methods:

  • Egg-based: The main and cheapest method of creating the seasonal vaccine remains the 70-year-old process of using eggs — which takes six to nine months from start to finish and has multiple steps where things can go wrong. Fauci calls it "an antiquated approach."
  • Cell-based: This less-used method grows a more specific set of viruses in animal cells. It's faster than the egg-based method, but it's more expensive to make.
  • Recombinant technology-based: This is the fastest but newest method of producing the vaccine, which doesn't require eggs at all.

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