It’s been all about influenza type A in the United States (US) so far this annual 2017/18 Flu season. In particular, the influenza A H3N2 (A/H3N2) subtype has been dominating there as it did Down Under in our 2017 season.
But it’s worthwhile reminding everyone that all FOUR main Flu virus subtypes are circulating at the same time (co-circulating) and that they tend to have slightly different age profiles.
A trivalent or quadrivalent (which accounts for an extra FluB lineage missing from the trivalent vaccines) vaccine is really important to protect as much as possible against as many Flu virus subtypes as possible. Sure, protection afforded to 1 of the 4 subtypes might be lower this season, but it’s better against the other 3 viruses – the vaccine is definitely a good investment for such a small outlay.[1,2]When we look at the next graph (Fig.2) we can see that most (36%) A/H3N2 detections have so far occurred among the elderly (>64 year olds), with the 25-64yo age group not far behind at 32%. A quarter of the 5-24yo group are positive for A/H3N2 so far but only 8% of A/H3N2 Flu detections are from the 0-4yo age group.
So which Flu is the most common among the youngest age group? It turns out 19% of 0-4yo are A/H1N1 positive, 5% B/Yamagata and 12% B/Victoria, and that 8% of A/H3N2 above.If we look at Flu detections by region of the US, we see activity is highest in many southern States. Is that real (and if so what is the mechanism?) or is this a testing issue (these states tes more than the northern states) or some other issue like population density, climate, chance or a mix? Over in San Diego, California for example, Flu detections have well and truly taken off compared to previous years (thanks to @marlene_mmm for tweeting the image below).
#fluseason off to the worst start we’ve seen in years here in #SanDiego Multiple pharmacies completely out Tamiflu. Updating our #EHR protocols for #flu now to help speed up care. Thanks @MackayIM for heads up. pic.twitter.com/nP80rDb0nq
— Marlene Millen (@marlene_mmm) December 30, 2017
If we look at influenza-like illnesses (not Flu virus confirmed, but people presenting to a Doctor with fever + cough and/or sore throat (Fig.4)), then it appears as though the US Flu season is tracking to be the same as 2014/15 US Flu season. But from above, at least in some regions, it is clearly much worse among confirmed FLu cases.Always look at confirmed numbers, because ILIs can also be caused by other respiratory viruses which can co-circulate with Flu. Human respiratory syncytial virus (HRSV; see Fig.5) provides a nice example of this in the US at the moment. It favours younger children too. Some respiratory viruses tend to fade away when Flu is active (see Fig 6). This could be due to virus:virus interactions (also called “virus interference”). Or environmental changes. Or something else. References…