The flu is down. Way down. Influenza infections are down ninety percent compared to last year.
Most years, millions of Americans get the flu. Thousands are hospitalized and some people die of flu, every year. But not so much, this year. Deaths from respiratory illness are up—way up—but that’s coronavirus, not influenza virus. Flu has plummeted.
So what’s going on?
Last year, in epidemiology circles, people wondered how COVID lockdown protocols might impact the transmission of other infectious diseases, and now we have a few answers. We’re nine or ten months into better practices as a global society, and those practices include social distancing, mask-wearing, and hand-washing. We’re seeing less of each other, and enough so to change the transmission of flu. Although the numbers do seem to beg the question: How can flu be falling so dramatically while coronavirus keeps surging?
The answer to that, is that coronavirus is a highly infectious little bugger. The r-nought value (R-0) of the COVID-19 coronavirus, according to the Lancet, is between 2 and 3.5. This means for each infected person, we expect as many as 3.5 more people to become infected. And new coronaviruses variants cropping up are perhaps fifty percent more infectious than that—they have a higher R-0.
Imagine halving the coronavirus R-0, through actions like masking and distancing and better hygiene. Imagine bringing the overall R-0 down to a value of 1- 1.7 (or more, with the new variant.) Within that range, we’ve still got an ongoing, increasing, surging pandemic. Each infected person will infect, on average, between one and one-point-seven people. The pandemic will continue, as long as R-0 is greater than one.
Now, take the flu virus. Influenza infectivity varies year-to-year, with an R-0 usually between 0.9 and 2.1. Bear in mind that we have flu vaccines, which more people have gotten this year because of general health concerns, because of coronavirus. But the same societal behaviors that curb coronavirus transmission (masking, social distancing, good hygiene) also curb flu transmission—as well as transmission of many other pathogens, including other viruses.
So using math alone, it’s easy to see how we’ve so effectively cut down flu. It’s thanks to our behaviors, which lower the R-0. Cutting the R-0 of flu in half (an assumption for the sake of this post), it’ll land between 0.45 and 1.05.
An R-0 less than 1 means each person with flu will infect between zero and one of their contacts. This means each cycle of infection sees fewer, and fewer, and fewer cases of flu. Influenza virus would eventually die out. Imagine that!
Placing all of this infectious disease ‘stuff’ in terms of math is fantastic if for no other reason than this—math clarifies why science makes its recommendations. In this case, masking, social distancing, good hygiene. That clarity, from math, is fantastic. And another fantastic thing is that despite the awfulness of this pandemic, it’s clear that our collective behavior profoundly impacts community health.
Wash those hands. Wear your mask. Keep on trucking.