Official data seems to suggest that the flu nearly disappeared in the second half of 2020.
Did it really happen, or was it an artifact of not testing for the flu anymore?
Was it because we tested less?
In the second half of 2020, fewer people have been tested for influenza than usual – probably as everyone’s attention was focused on COVID.
However, the positivity rate of influenza tests also dropped, from about 30% in a usual December to about 1% in December 2020. This strongly suggests that the flu truly nearly disappeared.
Why would the flu disappear?
School closures and lockdowns meant fewer opportunities for indoor socialization, which is how the flu spreads. Moreover, the use of masks also helped.
If the flu disappeared, why didn’t COVID disappear, too?
That’s because COVID is more contagious. Imagine that, in normal conditions, the average influenza patient infects four people, whereas the average COVID patient infects ten people. And suppose that school closures, masks, and lockdowns reduce those numbers by 80%. In that case, the average influenza patient would only infect 0.8 patients – that’s less than one, so the number of infected people decreases over time. Conversely, the average COVID patient would still infect two people – that’s more than one, so the number of people increases over time.
Hence, it is possible that school closures, lockdowns, masks, and other precautions were enough to make the flu disappear but not enough to make COVID disappear. (This doesn’t mean they were useless, though – if they helped reduce the number of cases, they helped prevent hospital overwhelm and reduced the number of people who would get sick before having access to the vaccine.)