Weekly Update: Flu Season is Coming, What Does That Mean?

Flu season is about to begin in the Northern Hemisphere. I got the flu vaccine this week to get ready.  

It seems likely that catching both the flu and COVID-19 at the same time would increase risk of both mortality and of serious side effects. Worse, an increase in flu patients could increase the pressure on an already stressed healthcare/hospital system. 

Several news outlets have been predicting a crisis as the flu and the coronavirus collide. 

While I think that there are risks associated with the convergence of COVID-19 and the flu, I was skeptical that there would be a massive flu burden in the US this season all the way back in March when this fear was first mentioned. I am even more skeptical now. Why is that? 

There are many reasons, but the most significant is that the level of physical distancing needed to keep COVID-19 under control should crush the flu into near nonexistence. And recent data from the Southern Hemisphere that just finished their flu season confirms this hypothesis. 

Here are the numbers we have on this. 

SARS-CoV-2 is Roughly Twice as Infectious as the Flu:

The "initial reproductive rate" R0 is a measure of how many people each infected person infects on average, in a fully susceptible population with no immunity, and no physical distancing. 

The flu has an R0 value of about 1.3, meaning that each person with the flu gives it to 1.3 other people on average. 

In contract, the CDC now estimates the R0 of COVID-19 as being somewhere between 2 and 4, with a most likely value of 2.5. That means that COVID-19 is roughly twice as infectious as the flu. 

Physical distancing, masks, increased hygiene, etc. are all used to reduce the R0 of a disease to an Rt (the reproductive rate at a given time). When the Rt is below 1, the number of infected people will fall over time. The mitigation approaches used to reduce the Rt of COVID-19 will also reduce the Rt of the flu. 

We currently estimate that the Rt of COVID-19 in the United States is 0.98. The amount of transmission mitigation needed to reduce the R0 of COVID-19 from 2.5 to an Rt of 0.98 should utterly crush the flu, which is far less contagious. 

This is what I predicted back in March, and there is a lot of evidence that it is happening. 

In Australia, flu vaccination rates also increased in response to COVID-19. In response to record demand, flu vaccine supply increased from 13 million in 2019 to 18 million in 2020. I haven't yet found good numbers on the rate of vaccination compliance from Australia, but we know it was up significantly. 

With any luck, something similar will happen in the US and that will help even more. 

Data From the US:

According to the CDC that is exactly what we have already seen in the US, where the number of flu tests submitted fell 61% in March 2020, while the percent of those tests that were positive fell 98%. (from a median of 19.34% to 0.33%). So even in the off flu season we saw an obvious decline in flu rates due to COVID-19 mitigations.

Data From Other Countries Just Finishing their 2020 Flu Season:  

The Economist has collected data from several countries in the Southern Hemisphere, and they all show marked decreases.

Impact on Excess Mortality Calculation: 

Understanding this fact will be important for understanding the excess mortality data which is used to determine the true cost in lives from COVID, because the decrease in the flu this year will mean that tens of thousands of deaths that would normally come this year from the flu, will not happen. To understand the true cost of COVID-19, we will need to take that into account.

Conclusion: 

There is risk involved with a convergence of the flu and COVID-19, both on a personal level, and from the larger perspective of the healthcare system. 

However, a combination of physical distancing and an increase in vaccination compliance (if we follow Australia's example) seems likely to prevent a major outbreak of the flu this year, and apocalyptic predictions about the convergence of COVID-19 and the flu seem unlikely. 

But this conclusion does depend on what we do, and I hope that people will still take reasonable precautions this year, including getting vaccinated for the flu, and continuing to take precautions to reduce the spread of COVID-19. 

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