We read, during this 2020-2021 winter of the Covid-19 pandemic, that Covid case counts rose dramatically in December and January, but influenza (which typically runs rampant in the winter) is at very low levels. How can we understand this?

1. Perhaps more people got vaccinated for the flu this fall, to avoid the chance of contracting covid-19 and influenza either sequentially or concurrently, which would certainly be a greater challenge than either one, separately.

2. Perhaps being infected with covid-19 provides a protective effect against infection by the flu.

3. Perhaps patients with respiratory illness, not precisely diagnosed, are now being counted as "covid", whether they are or not.

4. Perhaps prior statistics for respiratory illness, not precisely diagnosed, were biased toward diagnosis as "flu", when some were coronaviruses (not covid-19, but not actually the flu, either).

5. Perhaps health practices adopted in response to covid-19 have been even more effective against the flu than they have been against covid.

6. Other ideas?

Notes:
1. only about 10% more influenza vaccines were used in 2020 than in a typical year. That helps, but we're seeing a much more dramatic decrease than 10%.

2. A healthy body fighting off an infection may produce a fever, and various other responses, which will prevent other infection. But the reaction only lasts a few days. That doesn't seem sufficient.

3. There is an element of moral hazard, in that hospitals can claim reimbursement from the federal government for "pandemic expenses" which they can't claim for "influenza expenses". Perhaps the PCR tests are being cycled until they produce the "right" result? Only the labs know.

4. Testing in a typical year doesn't just identify "the flu", but usually identifies a specific strain, to help formulate the vaccines for the next year. If a significant number of tests said "not the flu at all", other illnesses would probably be diagnosed.

5. One argument goes like this. "Virus particles, whether covid or flu, are tiny enough to penetrate our masks and drift over distances greater than six feet. So how can masks and distance defeat one without defeating the other?" I speculate that the answer lies in the contagion model's "R0" coefficient, the probability that one case will spread to another. R0 depends on many factors, both intrinsic to the virus, and dependent on the environment. But the key thing is that if R0 is greater than 1, the number of cases grows. If R0 is less than one, the number of cases shrinks. Suppose R0 is 1.5. 10 cases becomes 15, 15 becomes 23, 23 becomes 34... this is exponential growth (which doesn't necessarily mean "explosive" growth, by the way). On the other hand, if R0 is 0.75, 10 becomes 7.5, 7.5 becomes 5.6, 5.6 becomes 4.2, and so on. It fades away.

So, suppose that we start with an R0 of 3 for covid, and 1.5 for influenza, based on our vaccination and immune experience with the flu. Both will grow exponentially. But now apply masks, distance, and hand-washing, and suppose that action cuts each R0 in half. Covid still grows exponentially (R0=1.5), but flu (R0=0.75) never gets established! Any isolated case has a 1/4 chance of stopping right there. If another case develops, it may also "burn out" before it spreads.

Caveat: this essay is merely conjecture intended to promote consideration of alternative hypotheses. The numbers are for illustration only, and may not reflect reality at any particular time in history.