Influenza did not magically disappear. This is a common retort by those who don’t understand how influenza moves around the world, how mitigation measures work, or immunology. It’s incredibly counterintuitive to see one respiratory virus surge while another vanishes. It’s easy to see why this sparked confusion. However, the explanation comes down to basic virology and math. In addition, influenza surveillance dropped early in the pandemic because labs were overwhelmed. This didn’t cause the disappearance, but it amplified the appearance of disappearance. It didn’t disappear, it just went to record lows for the reasons described below. RSV followed the same pattern for the same reasons.
The Environment
Influenza tends to be very seasonal, mostly in the winter. This is because schools are in session, people tend to stay indoors in colder weather, and because both humidity and temperature are lower. Influenza doesn’t survive as well on fomites in high humidity and or temperature.
The Exponents
Another big determinant in influenza activity has to do with activity in the opposite hemisphere prior to the typical influenza season. That’s simply an exponential function. When the starting numbers are low, the peak will also be low if all other variables remain constant.
As a simple illustration, assume that each person infected with influenza infects two others. Under five generations of spread, we get case counts like this if starting with a single case
1 – 2 – 4 – 8 – 16 – 32, or 63 cases total.
If we start with five people initially infected, the numbers rise much more dramatically over the same time period.
5 – 10 – 20- 40 – 80 – 160, or 315 cases total.
The Behavior
Early non-pharmaceutical interventions early in the pandemic not only slowed the spread of COVID, but also of influenza.
Respiratory protection, social distancing, and closing businesses are obvious examples. One that many people overlook that is very important in this is airline travel. Fewer people were flying which also helped limit spread. We saw how air transportation was strongly correlated with spread of the concerning variant of H1N1 influenza in 2009.
All of these variables work synergistically with each other and dramatically slowed the spread of influenza.
The Immunology
People often then respond asking why this didn’t work with COVID. There’s a very simple answer. The primary strain of influenza that is circulating has been a descendent of the Spanish Flu from 1918. Even when there are genetic changes, the immune system retains some activity against influenza year over year, although this diminishes over time. Hence, everyone that has had influenza (or vaccine) in recent history has some protection. This also contributes to herd immunity.
This wasn’t the case with COVID. SARS-CoV-2 is what is known as a novel virus and until someone is infected or vaccinated, they will have very little protection. That meant that there was a very high level of susceptibility in comparison to influenza.
A “Magic” Disappearance
Oddly enough though, their claim is partially right, but not in the way they intended. Because of all of the reasons mentioned above, one strain of influenza (B/Yamagata) went extinct as far as we know. It wasn’t magical though, there were very clear reasons:
- Non‑pharmaceutical interventions (NPIs) during COVID‑19 sharply reduced transmission.
- Conserved antigenicity meant B/Yamagata wasn’t generating new drift variants to escape immunity.
- A large 2017–2018 outbreak had already depleted the susceptible population.
Science Illiteracy
The common theme in these types of retorts is lack of understanding of something that seems counterintuitive. As I have been writing about these myths, I’ve seen that people keep saying the same things without any real understanding of the reality or any knowledge of science. They are simply rehashing comments of misinformation spreaders, further contributing to the problem.
