Flu in the US – Week 51

Posted onAuthorIan M Mackay, PhD (EIC)3 Comments

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]

Figure 1. Positive influenza tests, type, subtype and week. Graph compiled by Centers for Disease Control and Prevention (CDC), National Center for Immunization and Respiratory Diseases (NCIRD) and posted at https://www.cdc.gov/flu/weekly/ [3]

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.

Figure 2. Influenza subtype by age.Graph compiled by Centers for Disease Control and Prevention (CDC), National Center for Immunization and Respiratory Diseases (NCIRD)CDC and posted at https://www.cdc.gov/flu/weekly/ [3]

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?

Figure 3. Influenza activity by State. Graph compiled by Centers for Disease Control and Prevention (CDC), National Center for Immunization and Respiratory Diseases (NCIRD)CDC and posted at https://www.cdc.gov/flu/weekly/. [3]

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).

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.

Figure 4. Influenza-like illness (ILI) by State. Graph compiled by Centers for Disease Control and Prevention (CDC), National Center for Immunization and Respiratory Diseases (NCIRD)CDC and posted at https://www.cdc.gov/flu/weekly/. [3]

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.

Figure 5. Respiratory syncytial virus (HRSV) US surveillance. https://www.cdc.gov/surveillance/nrevss/rsv/natl-trend.html [4]

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.

Figure 6. Non-influenza respiratory viruses at Wisconsin Laboratories. http://www.slh.wisc.edu/wcln-surveillance/surveillance/virology-surveillance/respiratory-virus-activity/ [5]

References…

  1. https://virologydownunder.com/flu-is-a-slippery-virus-a-tough-vaccine-target-and-a-challenge-to-contain/
  2. https://theconversation.com/heres-why-the-2017-flu-season-was-so-bad-86605
  3. https://www.cdc.gov/flu/weekly/
  4. https://www.cdc.gov/surveillance/nrevss/rsv/natl-trend.html
  5. http://www.slh.wisc.edu/wcln-surveillance/surveillance/virology-surveillance/respiratory-virus-activity/
  6. https://virologydownunder.com/human-respiratory-syncytial-virus-an-example-of-why-calling-them-many-usually-outweighs-calling-them-few-or-one/

3 thoughts on “Flu in the US – Week 51”

  1. It could be a quite severe season in the US. But elsewhere, things are going different. In Italy, for example, so far the season is characterized by: an early start (similarly to past year), a very high incidence among children (0-4 and 5-14 years old) and a prevalence of B type and H1 subtype (this especially worrysome if one considers the negligible vaccine coverage among children and young adults). In addition, we have a large – and unchecked – avian influenza epizootic that spreads into the massive poultry industrial sector by lack of bio-containment measures. The combined big farming industries (pigs, chickens) are a formidable incubator for any kind of reassortant, a situation that is completely out of the radar of public opinion (big business makes big moneys and influences mass-media and the idiocracy at power).

  2. I would only add that the flu epidemic in Italy is getting worse. Not only the H1pdm09 is causing a surge in ILI cases never seen since 2009 pandemic, but other respiratory pathogens are pushing GPs outpatient visits on record high: after Xmas eve, about 10% of population saw its family doctor for ARI/ILI. So far, only for the seasonal flu, more than 2 million of people fell ill and we are only at the beginning of the epidemic. Since the Italian surveillance system is faulty and the overall state of the health infrastructure is in bad shape, we do not know well how bad the things are going to become. With 0% of pediatric population vaccine coverage, another 0% for the young adults and only a half of the elderly vaccinated, I cannot figure out the damage this epidemic will do in the next weeks.

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