The United States has been suffering from many influenza virus infections. And it looks like those are translating into a “flunami” of hospitalisations, which has resulted in the US CDC classifying the 2024-2025 flu season as the highest severity season since 2017-2018, with flu hospitalisation rates at their highest in at least 15 years.

High numbers of flu infections

American clinical laboratory-confirmed flu-positive numbers show Week 6 to be a bit smaller than Week 5 but still huge, with a third of tested samples being positive for a flu virus. That’s a lot!

It was a smaller week than last. However, the percentage of positives appears to be stabilising (not getting higher). We’ll know that for sure next week because Flu may not be peaking even if it looks like it is right now…and here’s why.

These are mostly flu A viruses – 55:45 A/H3N2 and A/H1N1, similar to last week. However, the proportion of flu positives that are flu B has gone up a little since last week. A few days ago, I wrote about how the vaccine matches what’s circulating in The US 2024-2025 flu season and the vaccine.

High proportions of flu-related hospitalisation and death

Among the Influenza Hospitalization Surveillance Network (FluSurv-NET), confirmed flu positives per 100,000 people among those hospitalised have hit a height (12.9 per 100k) that surpasses any previous data shown, which goes back to the 2009-2010 season. The next biggest rate was 10.2 per 100k from the also big 2017-2018 season.

If we compare this graph to one from 2017-2018 and include the 2009-2010 pandemic influenza season, we can confirm the size of the 2024-2025 season at its apparent peak in Week 6 (those data will be added to in the Week 7 report). I’ve compared every year listed on the CDC FluView interactive site, and 2017-2018 was the next highest rate after the current season, going back 15 seasons (data not shown).

Lastly, here is some evidence that even notoriously lagging death-due-to-infection data (=it can take time to die from infection and record causes of death) highlight the severity of the 2024-2025 influenza season (Figure 4). Week 6 of 2024-2025 recorded 2.58% of 26,437 total US deaths associated with influenza. The 2017-2018 season peaked (Figure 5) at Week 3 with 2.51% of 64,646 total deaths that week.

Total deaths were higher in the 2017-2018 season, but remember, the data are faaar from finalised, and the flu season is nowhere near over in the current season.

And so it’s no surprise that the Week 6 CDC Report states, “Based on data available this week, this season is now classified as a high severity season overall and for all age groups (children, adults, older adults) for the first time since 2017-2018“.

What could be driving it?

No one knows for sure, but it could relate to some of a wide range of things, including but not limited to:

• The nature of the two main virus subtypes—changes between seasons may have made the latest variants of one of both a bit more transmissible, harmful, or better able to replicate.
• The relationship between these viruses and the flu vaccine – perhaps these variants have mutated away from the vaccine’s protective effects.
• There have been low flu vaccination rates over several years, which in the US are currently at 45% for both children and adults, according to the FluVaxView summaries.
• Immunity – to everything – wanes (fades over time). Among those unvaccinated, 60% of Americans may be a lot of people who haven’t been vaccinated in years and may have been lucky enough to avoid infection, so their immunity could be very low
• Vaccine production problem: Could the viruses have changed during production? A previous example occurred in the 2017-2018 season, when an egg adaptation lowered the effectiveness of a vaccine ingredient (read Flu is a slippery virus, a tough vaccine target and a challenge to contain for more). Could something similar have happened with a cell culture vaccine virus? This point will take a bit of time to study.
• It could have been helped by a small showing from SARS-CoV-2, which may have been holding some sway over flu seasons when it’s been circulating in high numbers, creating a protective inflammatory shield of infected people preventing flu from getting a good toehold as a precursor for a big season.
• The season ramped up very quickly, suggesting rapid transmission, but it’s been sitting at peak levels for a long time already (see the width of the red peak in Figure 2). This means many cases for longer, overwhelming healthcare and sick workplaces, which we know will still have workers ‘soldiering on’ even when schools close. And we haven’t really started the descent from this peak yet. Will it be equally abrupt, or will it drag out? If the rising flu B levels continue, they could fuel a long tail, slowing the peak’s decline.
• The size of the season may have been related to a slightly later start to the 2024-2025 US flu season, like the start times of some other seasons that went on to be large, like 2017-2018 and 2014-2015. This may have lined up better with a return to school from the Christmas break at the start of January. Flu season starts, people take home a virus for the holidays, fill the house with it, and then inject fresh home-acquired infections into school and workplace pools.
• Perhaps more children were susceptible to these particular viruses this season. You can see below (Figure 6) that children lead the rise of infections and adults follow.
• Seasonal flu viruses cause harm at both ends of the age spectrum – children and adults, so when ‘the stars align’ – it’s not hard to have a huge flu season. Sometimes, past immunity can enhance the impact of a current flu virus and this is influenced by the combination of age, past infections and vaccinations. Have a read of H3N2 is 50 years old and still going strong – especially the section titled “Past infection, immunity, age and imprinting”.

In other words – there are lots of things to implicate!

All in the past

Of course, these are data from the week ending 08-FEB-2025, which is more than a week ago. They’re already reporting on history.

Surveillance reports often lag, so we don’t know exactly what’s happening on the ground right now. But something is much better than nothing, and these are the data available.

What can you do?

Masks help reduce the viral dose, distance exposes you to fewer infectious airborne particles, reduced time indoors reduces the risk of inhaling enough virus, and vaccination can provide a degree of new immunity or a boost to pre-existing immunity. If enough people were to do these things, they might be able to head off a flunami such as this.

One thing’s certain: doing none of those things will do nothing to help. Of course, when something works to prevent harm, you don’t see its absence, and our wonderful brains usually forget what it was like when it was there. So it’s hard to convince people things work when they only notice that their absence doesn’t!

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