Beyond the death rate, Part III: Flu versus Covid
Beyond the death rate, Part III: Flu versus Covid
In Part II of our brief review of some Covid data, we used an infographic to visualize your risk of being hospitalized if you catch Covid, depending on your age. In Part I, we explored why looking at hospitalizations as a risk metric makes sense at all. In Part III, we’ll look at how dangerous Covid is compared to the flu by using both hospitalization and death rates. I chose to compare the 2017–2018 flu season since it was a particularly severe year of the flu.
Chances are, by now you’ve heard someone compare Covid to the flu. Most of these comparisons focus on the death rate, or CFR (Case Fatality Rate). Influenza has a CFR of about 0.1%, whereas Covid-19’s CFR is about 3.5% (varying slightly depending on the data source). But Covid’s CFRs vary vastly by age group. For example, 30–39 year olds have about a 0.3% CFR from Covid. That number feels pretty low, and no doubt encouraged many to take full advantage of the reopenings by being more social. If you’re 30–39, and Covid is only 3x as deadly as the flu, does it warrant the additional precautions that health experts are recommending?
Hospitalizations: Covid versus the flu
The short answer is: yes. CFR doesn’t paint the whole picture. This is especially true when comparing CFR for your age group (0.3% for 30–39 year olds) to CFR for an entire population (0.1% for the flu). Comparing hospitalization and death rates of Covid to the flu by age group reveals a few interesting things. For example, compared to the flu, Covid especially dangerous for people aged 50–64.
For both flu and Covid, the hospitalization rate increases with age, as we’d expect. Covid hospitalizations are higher than for the flu for all ages. But the two bars representing ages 50–64 clearly stick out. If you’re between the ages of 50–64, you’re about 12 and a half times more likely to be hospitalized for Covid as for the flu. And recall that this comparison uses the 2017–2018 flu season, which the CDC categorized as “high severity“.
The blue chart shows the hospitalization rate for Covid divided by the hospitalization rate for the flu. As you can see, 50–64 year olds are at greater risk of being hospitalized relative to their risk of being hospitalized for the flu, compared to any other group. Another interesting observation from this chart is the 65+ years category. 1.3x might seem sort of low. It means that people in this age group get hospitalized at similar rates for both the flu and Covid.
Death rates
As we know, 65+ year olds face the greatest risk of dying from Covid compared to any other age group. This is also true of the flu — 65+ year olds are more likely to die from the flu than younger adults. However, the difference between 65+ year olds’ chance of dying from Covid compared to the flu is striking when seen side by side.
Similar to the blue hospitalization chart above, 50–64 year olds face a unique risk from Covid. 50–64 year olds are 52 times more likely to die from Covid than they are from the flu, more than any other age group. The outsized increase in risk for 50–64 year olds is not a story I’ve seen told elsewhere (but please shoot me an email at xander@reconsidermedia.com if you have)
Even for young adults Covid presents an order of magnitude increase in risk of dying compared to the flu.
Covid’s potential disease burden
Comparing the hospitalization and death rates of Covid to the flu shows how different the risk profiles are for each pathogen. However, we haven’t yet looked at how many people in total were infected by the 2017–2018 flu. Comparing the total amount of people infected by the flu in 2017–2018 can give us insight into what sort of risk Covid could pose to the entire United States if a similar number of people are infected by Covid.
As of late July, approximately 4.2 million people have confirmed cases of Covid. A much larger number of people contracted the flu in 2017–2018 — nearly 45 million. If 45 million in the US were to contract Covid, and the Case Fatality Rate hovered near what it is now, about 1.7 million Americans would die.
To me, this is a staggering amount of death. It would, I believe, make Covid the single greatest mass casualty even in US history:
Hospitalizations would also be very high. If 45 million people caught Covid, there would be almost 6 times as many hospitalizations as for the flu. Yet the US has just about 900,000 ICU beds, which would only cover a small fraction of the nationwide Covid hospitalizations. This would depend, of course, on how quickly it spread, which is why the threat of overwhelming our health system has been a central part of the Covid debate (i.e. flattening the curve). If a large number of flu cases hit in a short amount of time, the US health care system would at least stand a chance of accommodating them (depending, of course, on all the other reasons people are hospitalized at any given time).
However, as we now know, Covid is more contagious than the flu during an uncontrolled outbreak. That makes it more likely that a large number of Covid cases happen in a short amount of time than with the flu, adding to the risk of overwhelming acute care hospitals. This is the darkest version of the Covid pandemic: hospitals becoming so overwhelmed for such a long period of time that death rates for other illnesses and accidents (car accidents, heart attacks, stroke, cancer, etc) start to increase as well.
Covid presents a completely different risk profile to us than the flu. From risk of hospitalization and death to the rate of its spread, Covid acts differently than the flu, and we must therefore force ourselves to think differently about it.
Data detail
For folks who read Parts I and II, you may wonder why the age categories were different in Part III. That’s due to the CDC’s data for the 2017–2018 influenza containing different age groups. Total population estimates for the 2017–2018, provided by the CDC, are estimates of symptomatic illnesses based on confirmed medical visits and hospitalizations. Covid cases, however, are confirmed cases, since the Covid-19 pandemic is being tracked in greater detail than the flu.
Sources
COVID-NET: COVID-19-Associated Hospitalization Surveillance Network, Centers for Disease Control and Prevention. WEBSITE. Accessed on July 17, 2020.
Cases and deaths in the US can be found on the CDC’s website. As of the evening of 7/20/2020, total US case count has already increased by over 200,000 since July 17th, when the data for this infographic was collected.
2017–2018 influenza hospitalization rates, death rates, and symptomatic illness count: CDC.
Further reading:
CDC on how Covid-19 is different than the flu
John Hopkins on Covid-19 versus the flu
