Maybe it’s a sign that I’ve been looking at charts for too long, but this bar chart cracks me up.
On the left is a bar representing the number of influenza positive tests in Utah last flu season. That number was 14,678.
On the right is the number of influenza positive tests this flu season. That number is 47.
It’s almost ridiculous, right? Like, if you asked 15,000 people “Is the sky blue, or is the sky a chocolate-chip cookie?,” you’d still somehow get more than 47 people picking cookie. You just don’t see out-of-the-blue 99% reductions in anything.
And yet, that’s what we have here. We’ve seen a near elimination of the flu season, much smaller than any flu season on record. Here’s another incredible chart comparing this year’s flu season to previous years in Utah using data from the state Department of Health:
Flu hospitalization incidence in the last five flu seasons compared to last year. (Utah Department of Health.)
The red line is this year. I had to chat with our graphics designer to figure out how to make sure we differentiated that red line from the x-axis in print. This is not a problem we frequently have!
So what’s going on? Let’s break it down.
Why did so few get the flu?
The first thought I had, as a frequently suspicious person, is that we’re just not really testing for influenza anymore. After all, people might just suspect any flu-like symptoms are due to COVID-19, and proceed from there.
But it turns out that testing isn’t at all the problem. This flu season, we’ve tested 1.15 million people in the United States for flu so far, compared to 1.2 million last year. We’re doing about the same number of tests. But get this, last year, 1 in 5 tests came back positive for flu, while now it is 1 in 625.
The real reason flu is down is that all of the precautions we’ve taken to mitigate the coronavirus have decimated the flu, because the flu is a much less contagious disease than COVID-19.
We’ve introduced the key stat known as R0 before: It is the average number of people any sick person will infect. An R0 above 1 and a disease spreads rapidly. An R0 less than 1 and a disease will eventually peter out. For seasonal influenza, R0 is usually about 1.3. For the swine flu pandemic of 2009, it was 1.46.
For COVID-19, R0 is estimated to be about 3.3 without intervention. Scary.
But of course, we’re doing all sorts of things to change that: limiting groups, wearing masks, washing hands more, etc. Right now, with our interventions, our R0 for coronavirus hovers at 1 — now, with vaccination, we’re getting below 1. But those same interventions that cut coronavirus disease transmission by 70% mean the flu never had a chance.
By the way, influenza isn’t the only bug that was nearly eliminated because of COVID-19 precautions. According to Intermountain Healthcare’s Germ Watch, they’ve seen “minimal activity” of rhinovirus, adenovirus, and seasonal coronaviruses (the ones that aren’t COVID-19), and just plain “no activity” of RSV, enterovirus, and pertussis in the last week.
Estimating the impact of reduced flu
You might have noticed that I’ve been referring to laboratory positives of the flu, and you know as well as I do, not everyone who gets the flu gets tested for it, not by a long shot. Luckily, we have a 2019 study from the Centers for Disease Control and Prevention and the Utah Department of Health that expressly tried to figure out how many flu cases there were in Utah in both the 2016-17 and 2017-18 seasons, from the limited data we do have.
Essentially, these researchers used the counted number of influenza hospitalizations and extrapolated from there, separating by age group. To give you an idea, there were 2,169 confirmed hospitalizations in 2017-18, but the state knows that frequently hospitals don’t test patients for the flu, so they estimated 3,900 hospitalizations that season. They also know most flu patients never go to a hospital, and estimated there were 160,000 people who saw a doctor, nurse, or pharmacist for the flu. Of course, some just deal with it on their own and never see a medical expert at all; they multiplied again and estimated there were 338,000 cases of flu that year in Utah.
By the time you get to that number of multiplications, there’s a pretty wide confidence interval: they estimated with 95% confidence that the true number of flu illnesses was between 285,000 and 391,000. Essentially, between 9% and 13% of Utahns got flu that year. It was about 7% to 11% the year before.
If you do those same calculations with the number of hospitalizations this year — remember, a paltry 14 — you end up estimating that about only 2,180 Utahns have gotten the flu this year. Wow. We saved about 330,000 flu illnesses.
How does the reduction in flu illness compare to the increase in COVID-19 illness? The number of confirmed COVID-19 cases in Utah is about 385,000. Now, we know from previous research that the total number of Utahns who have been sick with the coronavirus is much higher than that.
But remember, the COVID-19 is more dangerous than the flu. We generally see somewhere between 300 and 400 deaths to flu and pneumonia every year in Utah, and we’ve seen 2,122 confirmed deaths since the pandemic began due to COVID-19. The state doesn’t release total flu deaths until after the flu season, but with only 14 hospitalizations, you can safely estimate flu deaths to be able to be counted on one hand. It could potentially even be zero.
What does this mean for the future?
Believe it or not, but some people are worried that we haven’t had more people sick with the flu. In particular, this Politico article, entitled “The pandemic dramatically reduced flu cases. That could backfire.” lays out the argument.
Essentially, because there are lots of different kinds of flu — regular influenza A, influenza B, swine flu, and so on — we rely on each year’s flu data to guess at what’s most likely to be circulating next year, and then create the flu vaccine that stops what we think is going to be most common. But with limited data, will we be able to do that?
Even in this pessimistic article, though, there were pretty clear signs that it’s likely going to be OK. First, prior low flu seasons (albeit they were nothing like this year) haven’t meant subsequent higher flu seasons in the past. Second, a member of the Food and Drug Administration’s vaccine advisory panel actually said that even the “low levels of the virus this season still yielded enough circulation” for the team to pick strains for the vaccine. He wasn’t worried about it.
Second, even when things return to normal, I suspect some number of people are going to be more cautious about infectious diseases than they were in years past. Coming in to work with a cough isn’t going to be generally accepted or even encouraged in the way it used to be. We will see some people — not a majority, but some — wear masks in airports or in large crowds. This won’t have a huge impact, but when seasonal flu has a transmission rate only slightly larger than 1 in the first place, even small changes matter a great deal.
We’ll see next winter to what extent precautions are taken, for both flu and the coronavirus, which will still be with us. But there’s no doubt, the reduction in flu cases and deaths this season is one silver lining to the dark cloud that is COVID-19.
Andy Larsen is a data columnist. He is also one of The Salt Lake Tribune’s Utah Jazz beat writers. You can reach him at alarsen@sltrib.com.
