This year’s annual influenza (flu) H3N2 epidemic in the United States (US) has peaked. It’s been a big season in the US and the United Kingdom and in Canada, as it was a record-breaking one in Australia.

Referring to my earlier post that showed about a 3-week lag before the bars on the CDC graphs (laboratory-confirmed influenza infected people) settled in,[2] we can say that the past 3 reports show the beginning of a definite downward trend in total laboratory confirmed influenza cases.

Because the peak was so high (lots of infected and ill people) this season, it will take a while for the epidemic to cool down; there will still be lots of human cases (influenza infected and ill people) for weeks to come.

The graphs above show a few things that combine to show the peak of lab confirmed cases is behind us.

• I’ve used a red star to mark the current week bar (which week is listed under the graph).
• The blue triangle marks Week 4’s bar in each graph
• From Week 5 I’ve drawn an orange dashed line just above Week 4’s bar.
• The sharp-eyed among you will have noticed that the y-axis remains stable from Week 5, at 20,000 positive specimens
• The orange line lets us see that in Week 5, Week 4’s graph is still rising, From Week 6, Week 4’s bar remains at a stable peak (that roughly 3-week settling in period again).
• The orange dotted line tracks let’s you see Week 5’s, 6’s and 7’s data settle in, in reference to the stable Week 4 peak. Just for interest. 
• From Week 8 we can see that Week 5 and Week 6 have remained below Week 4’s peak; Week 6’s total is less than Week 5 and Week 7’s less than Week 6’s (although these haven’t passed the magic 3-week mark yet.

This is the case for lab-confirmed cases. Whether the actual number of infected people who were not ill enough to get tested has also peaked is unknown. You can get an estimate by looking at “influenza-like illness” (ILI) numbers (ill people but not necessarily lab-tested)  although this can be a trap in the absence of other virus testing, because other viruses can cause ILI.

But, if you hypothesize that the number of people who are ill with the lab-confirmed flu is a representative sample of the whole community who are infected, you would also expect that the peak has passed in general.

I’m hoping we see some more information about the vaccine issues in the wash-up to this season. In particular, were cell-line-based vaccines more effective than egg-based vaccines? And what might be done to improve vaccine uptake among those most at risk of severe influenza; the young (with the hospitalization rates), the elderly and those with underlying disease. The US has a range of vaccine available is they may be in a good position to look into this.

Whats next? The southern hemisphere flu season is coming!!

References…

1. Influenza: Past Weekly Surveillance Reports
   https://www.cdc.gov/flu/weekly/pastreports.htm
2. Flu may not be peaking even if it looks like it is right now…and here’s why
   https://virologydownunder.com/flu-may-not-be-peaking-even-if-it-looks-like-it-is-right-now-and-heres-why/