"Studies have shown that the Infection Fatality Ratio (IFR) of Covid-19—the number of people who die if they catch the disease—is quite low - maybe even as low as seasonal flu, at somewhere around 0.1%. We don't shut the country down for that. So why do so for Covid?"

1. That's not the IFR of the flu - it's the CFR. Covid sceptics often rely on slippy terminology here, so let's clarify what we mean. The Infection Fatality Ratio (IFR) is the number of people who die if they're infected with a disease. But lots of people can be infected by a disease and remain asymptomatic - so we (and they) might never find out they're infected. With Covid, we're doing lots of random testing, so we have a better idea of how many infections there are, but in normal years we don't do so much random testing for diseases like seasonal flu.

If we just look at the number of people who have symptoms of a disease and who die of it, we have a different number, called the Case Fatality Ratio (CFR). This number—the number of deaths divided by the number of symptomatic cases—is normally a lot higher than the IFR, because people who have symptoms, almost by definition, have a worse disease than those who are asymptomatic.

Many of the numbers often quoted as the "IFR of the flu" are actually the CFR of the flu - so they can be used to make a flu infection sound much more deadly than is the reality. That 0.1% number, which can be calculated using, for example, the numbers on the CDC website, is really the CFR, not the IFR, of seasonal flu (at least in the US, in specific years).

For example, using those CDC data from the US, we can take flu deaths in 2018/19 (34,000) and divide them by the number of symptomatic cases (36,000,000). This gives us a CFR of 0.094%, which can be rounded to 0.1%.

1. The CFR of Covid-19 is much higher than the CFR of flu. A current estimate of the CFR of Covid in the UK is 2.1%. That means a symptomatic case of Covid is twenty one times deadlier than a symptomatic case of seasonal flu (if we use the CFR estimate that we calculated above).

2. The IFR of Covid-19 is much higher than the IFR of flu. Let's try to estimate the IFR of seasonal flu - the number of people infected, symptomatic or asymptomatic, who die. A systematic review study from 2018 concluded that perhaps 50% of flu cases (in unvaccinated people) are asymptomatic. If this is the case, we can double the case numbers above to get an estimate of total infections, and thus derive an IFR of 0.047% (in reality, IFR calculation is far more complicated than this, but these are broad estimates).

A systematic review study estimated the IFR of Covid at 0.67%. If we take the flu IFR estimate of 0.047%, it means that a COVID infection is more than fourteen times as deadly as a seasonal flu infection.

There are at least two caveats. First, this really is just talking about seasonal flu - other strains of flu (those that tend to be called "pandemic" influenza, like swine or avian flu) vary in their CFRs and IFRs. Second, don't get us wrong - seasonal flu causes a great deal of suffering and death every year, and we don't wish to diminish that. But the numbers—even very crude ones like we just calculated—clearly show that Covid is more deadly on average.

Looking at intensive care unit data can also help illustrate this. A short animation by John Burn-Murdoch shows how normal winter admissions into intensive care for flu compare with admissions for Covid-19. The numbers of people requiring intensive care for the two diseases are entirely different, with Covid admissions outstripping those for flu by a dramatic margin.

1. Covid is more contagious than the flu. Even if Covid had the same CFR or IFR as the seasonal flu, there are other differences between the diseases that makes Covid more of a problem. As described on this CDC page, Covid is more contagious than flu, and those infected might remain contagious for a longer time.

2. A single number for fatality risk might not be very useful. Covid is a disease where the fatality risk increases with the age of the infected person. We discuss this on another page, but in summary, taking one number—the average across all ages—makes it easy to forget the differences in risk across different ages. Two caveats: first, we should also acknowledge that flu is more dangerous to children than Covid. Second, although there are differences in fatality risk across ages, it's not as if people of one age are more or less valuable than others: we should be working to prevent as many deaths as possible from any disease, whether Covid or the flu.

Page added on 19 January 2021