Data Snapshot: Flu season assessment, 2023-2024.
Some typical and atypical stuff happened.
Officially, each influenza season begins on October 1st. Usually, by April and May things have more or less simmered down.
The 2023-2024 season is just about in the books, though I will say that I saw a few ER patients in Boston over the past two weeks who needed to be hospitalized in part (if not entirely) due to influenza.
In this week’s Data Snapshot, let’s look at the 2023-2024 influenza season compared to the previous ten.
Just by the numbers, 2023-2024 was on the bad side, though the rates of hospitalization in the general population were not as high as the worst year in recent memory (2017-2018). As always, I am amazed when I look at the data from the 2020-2021 season (as well as 2021-2022). The low levels of flu during those years suggest that efforts to control Covid-19 had an impressive impact on other pathogens like the flu. (And, yes, we were testing for influenza during those periods.)
But there are a couple of interesting things hidden in the graph above. To reveal them, I graphed weekly rates going back to 2018-2019. Take a look and then we’ll talk on the flip side.
A few observations.
First, The peak in late December 2023 was the highest among the last six years (the 2017-2018 season is not shown in this graph, but the peak back then was a bit higher than the 2023-2024 peak). As I’ve noted before, the peak of the flu outbreak this season actually exceeded the Covid peak, though overall, Covid hospitalizations have been higher this season.
Second, we had a fairly long period with moderately high levels of flu-related hospitalizations, which lasted well into the late part of the traditional season. Some of this reflects averaging (Michigan had its peak in March, for example, while others had fallen off), but not all of it.
Third, we are actually not quite done. Notice that the rate for the most recent week we have data for (the week ending April 20th), is pretty high by historical standards for this time of year. April of 2019 had high rates too, but notice that this was clearly just the downtrend from a very late flu peak the 2018-2019 season (which occurred in March of 2019). So 2023-2024 seems to have had an unusual combination; an early and high peak in late December and yet we still had persistently moderate levels throughout March and at least half of April. In fact, in Connecticut (the closest site to me that the CDC data tracks), flu hospitalizations actually went up last week—which matches my anecdotal experience that I mentioned above.
10,000 foot view.
In the most concrete sense, we can say that the 2023-2024 influenza season had a higher than normal peak (at nearly 9 flu-related hospitalizations per 100,000 people in the population), a somewhat typically early peak (late December), and yet an unusually long tail, with significant levels lasting into April.
I do not know exactly what this all means, but I'm particularly interested in the long tail—that is, that moderate levels have persisted so long in many places. Relatedly, I’ve long been interested in whether high-risk people should get a two-dose series of the flu shot, one in early fall and another at the peak of winter. Two-dose influenza vaccines series have been studied in the past (among high risk individuals), but those looked at two doses given just a few weeks apart. I’d like to see a study done in high-risk individuals where the two-dose series cadence separated the doses by a few months, with one dose, say, in October, and another in early January.
Any researchers in the audience want to try this? There is “equipoise” for such a study, and this strategy could save a lot of lives if it worked. I’ll take middle authorship on the study, if you do it!
Questions? Feedback? Chime in below in the Comments section!
Thanks to Benjy Renton for maintaining the Inside Medicine Covid-19 and Respiratory Virus dashboard, which made this Data Snapshot possible!
We have been extremely careful to avoid Covid with consistent masking during peaks and not missing a single dose of offered vaccine. At age 63, we do not qualify for the stronger senior flu dose vaccine. This year, we asked for a second flu vaccine when they opened up a second Covid vaccine a few months ago. This gave us so much added peace of mind fir best case scenarios. These years of isolation have taken a toll. Having vaccine access and being careful during peak Covid and flu season, easy enough.
I came down with the flu almost two weeks ago. I’m just starting to get back to my regular activities, though I’m still coughing like crazy, though not quite as hard.