When is flu season - according to the CDC?

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Discussion Overview

The discussion centers around the definition and timing of "flu season" as referenced by the CDC. Participants explore whether specific dates are provided for flu season and how flu activity is characterized throughout the year. The conversation touches on the implications of flu season for vaccination recommendations and public health messaging.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants note that the CDC does not provide specific dates for "flu season," suggesting that it is generally understood to start in early Fall and peak between December and February.
  • Others argue that flu activity can occur at any time of the year, though it is more common during the designated flu season.
  • A participant mentions that flu season is defined by an increase in flu cases, which can vary based on factors like weather and location.
  • Some express uncertainty about how the CDC defines the end of flu season, with references to activity potentially lasting until May.
  • There is a suggestion that different researchers may use varying thresholds to analyze flu activity, complicating a standardized definition of flu season.
  • Participants discuss the implications of vague recommendations regarding vaccination timing, advocating for more specific guidance from health authorities.
  • Concerns are raised about the timing of vaccine production and its impact on vaccination strategies, particularly regarding strain identification and vaccine efficacy.

Areas of Agreement / Disagreement

Participants generally agree that there is no universally accepted definition of "flu season" with specific dates, and multiple competing views remain regarding how flu activity is characterized and analyzed.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about flu activity and the variability in definitions used by different researchers. The conversation also highlights the dependence on external factors such as weather and viral strain changes, which affect flu season dynamics.

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TL;DR
Does the CDC website give specific dates for "flu season"?
In browsing the CDC website ( https://www.cdc.gov/flu/season/index.html ), I find all sorts of information about "flu season" For example:


However, I don't see where any dates for a "flu season" are defined. I also don't see any procedure that defines "flu season" in terms of the flu activity chart or other statistics. Can someone point out where the dates of "flu season" are established? Is there more than one flu season per year?
 
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I have no reference, but in the USA "flu season" starts in the early Fall and goes for months. BUT ... you can get the flu any old time, it's just much more common during the "season". You should still get a flu shot if you didn't last fall according to what CVS told me.
 
Stephen Tashi said:
Summary:: Does the CDC website give specific dates for "flu season"?
As far as I know there is no specific date. In general, the 'season' is expected to start by November (on the northern hemisphere): but the 'activity' itself depends on too many things, like weather, location, luck.
 
Flu season is just a name for when the occurrence of people getting the flu rises. No special dates. Just colder weather.
 
russ_watters said:
https://www.cdc.gov/flu/about/season/flu-season.htm

That page narrows down the months of flu season:

The exact timing and duration of flu seasons can vary, but influenza activity often begins to increase in October. Most of the time flu activity peaks between December and February, The exact timing and duration of flu seasons can vary, but influenza activity often begins to increase in October. Most of the time flu activity peaks between December and February, although activity can last as late as May

This implies "flu season" can be defined for a particular year, but avoids saying how this is done. Perhaps the implied definition is that "flu season" begins the month when influenza activity increases. When does it end? The phrase "although activity can last as late as May" suggests there is a definition for "no activity" vs "activity".
 
Stephen Tashi said:
That page narrows down the months of flu season:

This implies "flu season" can be defined for a particular year, but avoids saying how this is done. Perhaps the implied definition is that "flu season" begins the month when influenza activity increases. When does it end? The phrase "although activity can last as late as May" suggests there is a definition for "no activity" vs "activity".
I don't think they bother with a general/technical definition but rather analyze and describe the data for what it is. You don't need to specifically label "flu season" to do that. See:

WHONPHL02_small.gif


Different researcher may use different thresholds when doing their analysis. The most useful I can think of would actually be from one nadir to the next, which would be on average one year, but will vary from season to season.
 
One way to visualize the seasonality of flu infections is to look at data that the CDC collects about influenza testing:

1579799979223.png

(source: https://gis.cdc.gov/grasp/fluview/fluportaldashboard.html)

Form these data from last year about positive influenza tests, you can clearly see most of the infections occurred from ~ week 50 of 2018 to ~week 15-20 of 2019, with very few cases occurring after week 20 of 2019.

For reference, here is the data for this year's flu season:
1579800931421.png
 
russ_watters said:
I don't think they bother with a general/technical definition but rather analyze and describe the data for what it is.

I agree.

Since that is the case, I think they should be more moderate in using the phrase "flu season" in their pages and pages of health advice. To get vaccinated "early in flu season" or "before flu season" is not a specific recommendation. Advising people to get a flu shot in September is specific.
 
  • #10
Stephen Tashi said:
...To get vaccinated "early in flu season" or "before flu season" is not a specific recommendation. Advising people to get a flu shot in September is specific.
When should I get vaccinated?

You should get a flu vaccine before flu viruses begins spreading in your community, since it takes about two weeks after vaccination for antibodies to develop in the body and provide protection against flu. Make plans to get vaccinated early in fall, before flu season begins. CDC recommends that people get a flu vaccine by the end of October.
From here.
 
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  • #11
Stephen Tashi said:
Since that is the case, I think they should be more moderate in using the phrase "flu season" in their pages and pages of health advice. To get vaccinated "early in flu season" or "before flu season" is not a specific recommendation. Advising people to get a flu shot in September is specific.
That I'm not sure of, there are some competing considerations shaping the guidance:
-Consistency/clarity (your point)
-Getting as many people vaccinated as possible
-Identification of strain and vaccine efficacy

The third is the tough one. The timing can vary and the vaccine itself is sometimes changed mid-season due to changes in strains.
[edit: I'm not sure that is correct. But it has been picked/produced late.]
 
Last edited:
  • #12
russ_watters said:
-Identification of strain and vaccine efficacy

The third is the tough one. The timing can vary and the vaccine itself is sometimes changed mid-season due to changes in strains.
[edit: I'm not sure that is correct. But it has been picked/produced late.]
About 6 months.

from: https://www.cdc.gov/flu/prevent/vaccine-selection.htm
How long does it take to manufacture seasonal influenza vaccine?

It takes at least six months to produce large quantities of influenza vaccine. For vaccine to be delivered in time for vaccination to begin in the fall, manufacturers may begin to grow one or more of the vaccine viruses in January based on their best guess as to what viruses are most likely to be included in the vaccine. For information about flu vaccine production, see How Influenza (Flu) Vaccines Are Made. https://www.cdc.gov/flu/prevent/how-fluvaccine-made.htm
 

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