Does South Korea Data support a lower headline CFR than the China published 3.4 percent?

The WHO published a tentative mortality rate of 3.4% based off China data on the 3rd of March. Since then numerous commentators have suggested that real mortality rate of the SARS-COV-2 could be substantially less. Some of these commentators such as the President of the United States of America suggested a mortality of “way less than 1 percent based” off a hunch.

Other experts at least initially suggested the rate could decrease substantially due the presence of a material amount of asymptomatic cases. The theory being promoted is that the 3.4 percent death rate was real but rather than being caused by the 40,000 identified cases there might be 400,000 “real” cases with the majority not being identified in China because they never had symptoms and testing did not reach them. To buttress this theory commentators have been pointing towards South Korea’s reported 0.7 percent mortality rate (Deaths / Cases) as indication that these asymptomatic cases exist and the virus is not as dangerous as first feared.

South Korea is used because the country has undertaken an enormous amount of testing for SARS-COV-2 and for this reason their data set should capture most asymptomatic cases. If South Korea’s mortality rate is 0.7 this will provide evidence of a lower global risk profile than the original China data suggested and enable different policy responses.

I have used publicly available data in order to interrogate whether this 0.7 headline rate is an accurate reflection of the expected mortality and have specifically investigated two hypotheses

1. Impact of age on South Korea’s mortality rate

2. Whether the delay between case identification and mortality is biasing the headline rate.

The analysis has determined that both factors are contributing to South Korea’s apparent lower mortality rate and if you adjust for these factors South Korea’s true mortality rate comes in line with what has been observed in China. SAR-COV-2 across a population will have a mortality rate of around 3.4 percent.

Impact Of Age

I am sure most readers have least seen the following table around fatality of SAR-COV-19. In theory we can use this table to see whether Age may be having impacting on the South Korean calculated mortality rate.

First step is to validate these number and to do this I used Australian demographic data utilizing the results of the latest census. To my shock these numbers generated a mortality rate of 1.8 percent vs the expected 3.4 percent.

At face value there is no way Australian Age demographics should differ by that degree to that calculated by WHO. This means that the 3.4% estimate must have come from a different source.

Digging into the published papers this commonly published mortality rate by age originated from data sourced on the 11th February vs the 3rd of March age mortality rates used in the calculation of 3.4% published by WHO. For those interested I have included a link to this study at the bottom of the page. Basically, the mortality by age data is materially out of date. One month during a fast-moving pandemic is a long time.

The obvious question is whether the mortality data can be refreshed using public sources. Unfortunately case level data is not available, but a lot of insight can be gained by the published cumulative totals.

The chart above demonstrates that the number of deaths relatively to number of cases has increased dramatically since the study. A significant proportion of these “unexpected deaths” needs to be attributed to the cases present on the 11th of February. If we take the conservative assumption that cases present on 11th of February have the same mortality rate as cases after (this is conservative as post 11th of February cases are likely to still be tallying deaths) then the deaths associated with the 11th data set should have been 1.54 times higher.

With public data I can not attribute these accurately but if you assume, they are spread evenly over all buckets the new death rates look like this by age.

With these updated number we can now look and compare expected mortality rates.

Mortality Rate with new factors (Chinese 11th Feb data set) - 3.5%

Mortality Rate with new factors (Australian Demographics) - 2.8%

Mortality Rate with new factors (South Korea Cases 4th Mar) - 2.5%

As you can see a significant proportion of the observed South Korea’s lower mortality can be explained by Age demographics. For those infected on the 4th March the expected mortality rate is 2.5% which is substantially below the WHO estimate of 3.4%.

Mortality Delay Impact

There is anecdotal evidence that there is a significant delay between case identification and mortality with Chinese sources suggesting mortality occurred 2 to 8 weeks after case identification. We can also see this from the mapping of mortality vs Case numbers above that this delay is real.

What does this mean for South Korea?

Effectively the large number of new cases is inflating the denominator and reducing head-line mortality rate.

One way to adjust this why the epidemic is occurring is to exclude the new cases. (i.e. cases identified T numbers of days ago are unlikely to have had a chance to have fatalities so we can remove them from the denominator.) To do this we will use deaths as per the 9th March and Case numbers for different T numbers.

We can conclude that new cases are dragging down the mortality rate however at this point we do not have the sample size to determine what level of delay to put in.

It is however clear that the headline mortality rate of 0.7% used to justify lower mortality for SARS-COV-2 was incorrect. In fact, as the number of new cases slows, as observed in the last couple of days the mortality rate is rising and is now 0.8 percent.

Conclusion

South Korea is not an example of lower mortality rate. Once mortality delay and age demographic differences are adjusted for South Korea is demonstrating similar mortality behavior to China.

It is expected that South Korea headline mortality rate will trend in a similar fashion to China’s over time. Where a substantial proportion of fatalities occur 2 to 8 weeks after case identification.

Finally, due to the difference in Age data the South Korea death rate should remain below what was observed in China but is still expected to rise to 2.5%.

#Analytics #COVID #SARS-COV-2 #Coronavirus #COVID-19

For those interested I have used the following data sources

11th of February Study to estimate mortality rates can be found at http://weekly.chinacdc.cn/en/article/id/e53946e2-c6c4-41e9-9a9b-fea8db1a8f51

Cumulative Cases and Deaths daily time series all regions can be found at https://github.com/CSSEGISandData/COVID-19

South Korea Age Profile 4th of March https://www.cdc.go.kr/board/board.es?mid=a30402000000&bid=0030 (Press release 171)

Australian Age Demographic information https://profile.id.com.au/australia/five-year-age-groups