Why does flu arrive in the autumn?

Above is a plot of deaths of children from flu in the USA, from 2017 to 2020. The data is from the USA’s CDC. I think the data in the UK will be similar, although the numbers will be lower as we have about a fifth the population of the USA. I just want to make three observations about this data.

The first observation is that during the present COVID-19 pandemic, some of the people who want to downplay, for example, the Omicron variant, dismiss it as being “no worse than the flu”. The plot above reminds us that flu does kill people, including children.

The second observation is that the seasonal nature of the flu is very clear from the plot. The plot shows four years of data and the numbers along the x-axis are dates in the form “year-week”. In the summers of 2017, 2018 and 2019, flu deaths were almost zero. Then in autumn, at about week forty-odd (so October), they rapidly increased. By about the next April, the deaths are back down to almost zero. This pattern repeats for the three years.

We are all used to flu being seasonal. But I don’t think scientists really know why flu is seasonal. Like a lot of these very familiar things that we take for granted, I think we tend to think that scientists understand it. But we don’t.

We do tend to be indoors when it is cold, which helps transmission, so there is almost certainly a behavioural component that helps transmission in the autumn and winter. But there may also be direct physical effects of the weather on transmission. There is speculation in the scientific literature about viruses surviving longer in lower humidity air. The air is less humid in winter. But we don’t really know enough about how viruses survive and spread, for it to be more than speculation.

Out lack of knowledge of why respiratory diseases – like flu and COVID-19 – are seasonal may be a bigger problem now, if COVID-19 also comes in waves every autumn.

The third observation is that the USA (and we did too) almost wiped out the 2020-2021 flu season. In the plot above, you can see just one death in 2020-2021, not the 195 in 2019-2020. This is most likely due to flu and COVID-19 being transmitted in similar ways, but with flu being less contagious. So the measures such as social distancing and masks that suppressed COVID-19 did an even better job of suppressing the flu. One of the few bonuses of that difficult winter.

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