I’ve heard numerous people saying that the flu is worse or that this will just be like a bad influenza season. I’m going to try to dispel this notion. It’s a false equivalency.

1. The Reproduction Rate (R0)
One way to put this in perspective is to review the R0 for all of the influenza pandemics since 1918. This was done in a meta analysis of multiple studies.

1918: 1.80
1957: 1.65
1968: 1.80
2009: 1.46

Since the argument is being made that it is simply similar to a bad influenza season, the best value to compare it to is the median found in 24 studies looking at seasonal influenza epidemics, 1.27. That’s the best value to use for a comparison.

One very valuable unintentional laboratory to calculate the value for COVID-19 was the Diamond Princess cruise ship. The researchers calculated the R0 value as 2.28. The WHO estimate is 2.0-2.5.

2. Susceptibility
Since influenza circulates widely around the world, most populations have had some exposure in prior years. “There may be some level of immunity existing in populations even to novel influenza strains with pandemic potential.” In addition, influenza vaccination hovers around 40% each year in the US, meaning that a large proportion of the population has some immunity. In addition, others who weren’t vaccinated benefit from herd immunity.

Figure 3. Flu Vaccination Coverage of Adults 18 years and older,  United States, 2010−2019

3. Case Fatality Rate (CFR)
Seasonal influenza has a CFR of 0.1%. My calculations using the Johns Hopkins data set at the time of this writing at 3.52% globally, but there is a large range across the nations I’ve been tracking so far:

Iran: 3.31%
Italy: 5.05%
South Korea: 0.61%
USA: 3.64%
China: 3.85%

Granted, these numbers are possibly higher than the true figures assuming we do not have good data on the numbers of people with mild illness or are asymptomatic. However, other countries have tested much bigger proportions of their populations than the US.

There is something very important here to note though. Influenza testing also is likely to miss large number in the denominator. That effectively means that the CFR for seasonal influenza may just as likely be lower as it is for COVID-19.

There are three reasons identified by the CDC that the number of influenza deaths may be inaccurate:
1. The sheer volume of deaths to be counted
2. The lack of testing (sound familiar?)
3. Different coding of deaths

4. The Tip of the Iceberg
The other piece that doesn’t make sense in trying to draw this comparison is that influenza is already common all over the world. We have decades of influenza data to calculate annual mortality figures. COVID-19 is an emerging disease starting about November (using genetic calculations) and is only just making it’s way into populations and has exposed a very small proportion of humanity. For example, let’s assume that 10 million people have been exposed (a very high overestimate in my judgement at this time), that would mean that only 0.13% of the global population has been exposed to this disease. To me, this is the most important reason that these diseases cannot be compared at this time.

If you hear people trying to draw this comparison, please send them a link to this and dispel this myth.

I found another way to show the difference since I first wrote this.

5 responses to “Flu…NOT!

  1. Could you explain the Reproduction Rate comparison between other pandemics and the Diamond Princes cruise ship? It would seem to be a poor comparison in regards to sample size and being a fairly artificial environment. Thank You!!

    • Yes, they are different environments. However, there aren’t extraneous variables like individuals entering and leaving the population to adjust for, so it made a very interesting lab type of analysis. In any population chosen for an event like this, there are a number of reasons that results may not be generalizable to the whole population, so there are pretty wide margins of error in any model until we get much larger and complete data sets.

  2. Hello,
    Thank you for posting some details about the Covid-19 being different from the Flu. I’m hoping you might be able to provide some additional perspectives and specifics with some of the points you highlighted?
    Reproduction Rate: I’m following your logic and details comparing previous pandemics to each other… makes perfect sense. Where I get confused is those pandemics being compared to the Diamond Princes cruise ship? Not only does this seem like a small sample, it also seems VERY select & specialized populations? Given your starting point of meta-analysis of multiple studies, how does that apply to the DP population?

    Susceptibility: Again, I’m tracking with the facts that you are sharing regarding people having various levels of prior exposure to influenza. However, isn’t that one of the main factors with previous pandemics? The 2009 Swine Flu for example… many different populations didn’t have immunities or antibodies for this strain, despite prior exposures.

    Again, thank you for your post(s)… very informative and appreciated.

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