Tag: ILI

Has This Really Been the Longest Flu Season in a Decade?

Many folks were probably surprised by reports that this has been the longest flu season in ten years.

After all, just about everything about this year’s flu season has likely seemed mild compared to last year.

“There have been 21 straight weeks of elevated flu season in the U.S., making the current 2018-2019 flu season the longest in ten years.”

CNN

But here we are.

Has This Really Been the Longest Flu Season in a Decade?

What does it really mean to have 21 straight weeks of elevated flu season?

Where is it elevated and by how much?

“Influenza-like-illness levels have been elevated for 21 weeks this season, breaking the previous record of 20 weeks set during the 2014-2015 flu season.”

Situation Update: Summary of Weekly FluView Report Week 15

It’s likely that the media reports have been generated by a statement in the latest Weekly FluView Report that influenza-like-illness levels have been elevated for a little longer than usual this season.

What does that mean?

Well, influenza-like-illness levels are the proportion of outpatient visits for influenza-like illness (ILI), or basically, how many people are going to the doctor with flu symptoms. Once we get above the national baseline of 2.2%, we know that flu season has started.

“The proportion of outpatient visits for influenza-like illness (ILI) decreased to 2.4%, but remains above the national baseline of 2.2%. Seven of 10 regions reported ILI at or above their region-specific baseline level.”

Situation Update: Summary of Weekly FluView Report Week 15

And it ends when we get back below 2.2%.

Flu season is the time we spend above the national baseline level of influenza-like illness activity.

What’s missing in the talk of the longest flu season, is that it doesn’t tell you much about the severity of the flu season. For example, the peak ILI this year was well below that of last year.

And flu season doesn’t start and end at the same time all over the country.

The bottom line? This has been an average flu season and a lot of people still died, including at least 91 children.

So whether it is a long or short flu season, severe or mild, get a flu vaccine and be protected.

More on the Longest Flu Season

Understanding Flu Season

Flu season starts.

A lot of folks get sick as flu season peaks.

Flu season eventually ends.

What’s to understand?

Understanding Flu Season

While we see flu activity at epidemic levels every year, some years are clearly much worse than others.

What kind of flu season will we have this year?
What kind of flu season will we have this year?

And while the severity of a flu season is easy to see after it is over, many of us could use a little help making sense of things when we are still in the middle of it.

  • Geographic Spread of Influenza Viruses – when you hear that there is widespread flu activity in a lot of states, this is what they are talking about. The only problem is that this doesn’t really tell you anything about the severity of a flu season. We get widespread flu activity in all states, or almost all states, each and every year as flu season peaks.
  • ILI Activity Indicator Map – like the maps showing the geographic spread of the flu, the ILI activity indicator map can help you tell where flu is spreading, but since it doesn’t necessarily represent the whole state, it can be misleading.
  • Influenza-like Illness Surveillance – the proportion of outpatient visits for influenza-like illness (ILI), or basically, how many people are going to the doctor with flu symptoms. Once we get above the national baseline of 2.2%, we know that flu season is starting in an area. How high can ILI get? Recently, it has peaked between 3.6% (2015-2016) and 7.5% (2017-2018). But that’s nationally. Because of wide variability in regional level data, you might see much higher ILI numbers in your state. For example, the regional baseline in Texas is 4%, while it is just 1.1% in Idaho. Still, widespread flu activity with a high ILI likely means a bad flu season.
  • Flu-Associated Hospitalizations – laboratory-confirmed influenza-associated hospitalizations. Overall, as flu season peaks, this can range from 4 to 5 per 100,000 population in a typical flu season, to 9 or 10 in a more severe flu season. This is also reported by age group. During a bad flu season, flu-associated hospitalizations will be high.
  • Mortality Surveillance – the proportion of deaths attributed to pneumonia and influenza. At some point in flu season, we get above an epidemic threshold and more people die with the flu, especially during a bad flu season.
  • Pediatric Deaths – pediatric influenza-associated deaths have been a nationally notifiable condition since the 2004 flu season and since then, on average, about 118 kids die with the flu each year. Last season was especially bad, with 185 pediatric flu deaths.

So how do you really know if it is a bad flu season?

Look for a high ILI%, high flu-associated hospitalizations, which will almost certainly be followed by a high mortality surveillance.

You also want to check viral surveillance data. Are the majority of influenza viruses being tested antigenically and genetically similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses? If not, that could mean a vaccine strain mismatch and a worse flu season.

And don’t be surprised by reports of widespread flu activity or rising ILI. That’s just flu season.

More on Understanding Flu Season