It all comes down to the reproduction number, r0, and a little bit of math.

There’s been a lot of people talking about how there doesn’t appear to be much seasonal influenza at the moment. There are people falsely claiming that this is because flu cases are being counted as COVID-19 cases, but that’s not true at all. We know from the details of the genetic testing method used (reverse transcription-polymerase chain reaction, or RT-PCR) that the COVID-19 test does not mistake influenza for SARS-CoV-2, the virus that causes COVID-19. So what’s really going on?

I and others have pointed out that the failure of people to properly wear masks, social distance, avoid large indoor gatherings, and wash hands regularly (collectively known as control measures) is likely the cause of the increasing numbers of COVID-19 cases. But this argument raises a good question – how can there be essentially no influenza cases while COVID-10 cases are still rising?

Let’s ignore the baseless conspiracy theories that are swirling around this question and instead look at how it’s entirely possible for COVID cases to rise due to insufficient control measures while those same control measures could effectively stop the spread of influenza. The answer is the reproduction number, r0 (pronounced “r-naught.”).

According to the Centers for Disease Control (CDC), the reproduction number is the number of new cases of a disease that a single disease carrier would be expected to generate. An r0 of 1 means that a single ill person is expected to infect one other person, while an r0 of 10 means that a single ill person is expected to infect 10 other people. Reproduction numbers greater than 1 mean that an illness is going to spread and worsen in a community, while numbers less than 1 means that the illness will eventually die out. And for illnesses that have r0 values greater than 1, control measures are used in order to reduce r0 below 1.

[Note: this is a simplified explanation of r0. For a more complete examination of r0 and the challenges of using it properly, please read this CDC paper on the subject.]

At the moment, the CDC’s current best estimate for the r0 value of SARS-CoV-2 is 2.5 (scroll down to Table 1). For comparison though, the r0 value of seasonal influenza is more like 1.3. So seasonal influenza is generally less contagious than SARS-CoV-2 is.

This difference is key to clearing up the apparent conundrum.

For SARS-CoV-2, an r0 value of 2.5 means that control measures need to be greater than 60% effective. We calculate this by dividing 1 by 2.5 (0.4), converting this into a percentage (40% ineffectiveness) and then subtracting that from 100% to get the minimum effectiveness (60%). Using the same method but for the seasonal influenza r0 of 1.3, we calculate that control measures need to be greater than 23% effective in order to stop the spread of seasonal influenza.

The difference between the two numbers is the region in which control measures effectively limit the spread of influenza but fail to stop the spread of SARS-CoV-19.

Let’s show how this works with a couple of examples.

Example 1: In this example, we’ll assume that control measures are 80% effective. From what we’ve seen so far, this means that the controls should reduce r0 of both SARS-CoV-2 and influenza.

Multiplying the r0 for each virus by 20% (0.2) we get that the controlled r0 of SARS-CoV-2 is 0.5, well below one, and low enough that the disease would die out. The controlled value of r0 for influenza is even lower – 0.26 – meaning that seasonal influenza would die out nearly twice as fast as COVID-19 under these controls. So clearly, any controls that will limit the spread of SAR-CoV-2 will also limit the spread of seasonal influenza.

Example 2: In this example, we’ll assume that the control measures are only 45% effective. From what I’ve said above, these controls should still allow SARS-CoV-2 to spread while not allowing seasonal influenza to spread.

Multiplying the r0 for each virus by 55% (0.55), we find that the controlled r0 for SARS-CoV-2 is 1.375. This is still above 1, and that means that the spread of the virus has been slowed significantly, but the disease will continue to spread to more and more people. Alternatively, we find that the controlled r0 for seasonal influenza is 0.715. This is below 1, and that means the spread of the virus has been reduced enough that the seasonal flu will eventually die out.

As we can see, there are control measures whose effectiveness would slow the spread of COVID but not completely stop it while simultaneously stopping the spread of seasonal influenza.

So what does this mean? Given we haven’t seen much spread of seasonal influenza yet, it’s very probable that the control measures currently in place to limit the spread of SARS-CoV-2 are proving effective against flu. But given that cases of COVID-19 are rising dramatically in Colorado, the United States, and globally, it’s clear that the methods that are in place are not sufficient to completely control the spread of SARS-CoV-2. Only by taking the current measures more seriously (more people wearing masks, for example) or by imposing new measures (partial or complete lockdowns) will the spread of SARS-CoV-2 be brought back under control.

This also means that any attempt to further relax controls must be vigorously opposed. We’re seeing new cases increase globally, which means that our existing controls are not sufficient. Relaxing the our controls will only serve to further increase cases of COVID-19 along with the corresponding increases in hospitalizations and, tragically, deaths.