Science Unlocked

Delta Variant vs. Case Fatality Rate in the UK

The takeover of the COVID-19 Delta variant correlates with a continuing decline in the CFR


In the UK, almost 100% of cases are now caused by the Delta variant, according to official data. There has been a lot of media speculation about the significance of Delta and whether, in addition to being slightly more transmissible, it might also be more virulent.


The Case Fatality Rate (CFR) of a disease is the proportion of the number of people who have died of that disease compared to the number of people diagnosed with it, over a certain period of time.  For example, a disease with 1 death out of 10 diagnosed cases in one day would have a CFR of 1/10, or 10%. It’s important to remember that the CFR can change over time depending on various factors including the susceptibility and health of the population, the virulence of the disease, and the availability and frequency of testing.

Delta vs. CFR

Thankfully, the data gathered in the UK since January not only fails to support the hypothesis that Delta is more virulent, it suggests the opposite: reduced virulence. The UK COVID-19 CFR has shown a strong, downward trend since January 2021 and is currently at an all-time low of 0.186% as of 10 July 2021. For comparison and adjusting for the number of tests, the current CFR (with cases comprised of almost 100% Delta variant) is about the same as it was at the beginning of September 2020, which is 90% below the beginning of the pandemic. According to this adjusted CFR, there were only two peaks: March-April 2020 and Jan-March 2021. 

This very low CFR could be the result of a combination of:

  1. reduced lethality of the Delta variant
  2. strong pre-existing immunity in the population acquired naturally – before or during the pandemic – or by vaccination, and
  3. younger people (who face a low risk of severe outcomes on infection due to their age) form a larger fraction of the population infected in this wave, as the older population in the UK has already been vaccinated

As deaths continue decoupling from the sharp increase in cases, the less virulent Delta strain paired with the success of vaccinations, have produced a very manageable situation.

The data gives us a strong indication that we have achieved focused protection of the vulnerable and the epidemic emergency is over.

Helpful terms

CFR 2w lag – This stands for ‘Case fatality rate, two-week lag”. Deaths typically trail cases by about two weeks, so we use this metric to pair deaths with the group of cases that caused them. To do this, we calculate the CFR using the current daily death count divided by the daily case count from two weeks prior.

CFR 2w lag AdjustedCFR 2wk lag Adjusted is the CFR 2w lag, normalized for the fact that the number of tests conducted per day has grown sharply over the course of the epidemic. It equals CFR 2w lag divided by an adjustment factor, which equals the ratio of the number of tests done today over the number daily tests conducted at the beginning of the epidemic. We do this so that we can accurately compare CFRs from different time periods during which there were different testing volumes.

7d Avg: This is the average of the CFR for the previous seven days. This is known as a ‘moving average’. We use moving averages to smooth out daily reporting variations that might make overall trends more difficult to spot.

Case Fatality Rate – The proportion of people who die from a disease compared to the total number of people diagnosed with that disease, over a certain period of time. (daily rate, in this case)

Endemic – When a disease is consistently present in a specific population or geographic area

Virulence – The severity or harmfulness of a disease

Data Sources

José Gefaell, BSc Biochemistry, MBA, is Managing Partner at Caelum Labs and has more than 30 years of experience in Corporate & Investment Banking, Economic Policy, Venture Capital and FinTech. LinkedIn profile:

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