COVID – What is the mortality rate?

I have had some feedback from readers on mortality rate and why I have paid so little attention to it so far.


Why is the stated mortality rate around the world so different?

UK and Italy are on the high side with mortality rate around 13%.  On the low side, Germany at 3% and Norway at 2.3%.  The US is in between at 5%.  There are various issues with data collection but the biggest factor is how widespread testing is.

In relatively overwhelmed countries e.g. Italy and the UK there has been a limited roll out of testing, with a high proportion of those tested because they have symptoms already.  In the extreme case, if we only tested people for COVID who had already died with COVID symptoms, then it would have a mortality rate of approaching 100%.   In the UK a quarter of people tested test positive, which means we are far from seeing a random sample.

The countries with much high testing rates will give a much better idea of the actual mortality rate.  Norway has only 5% of tests come back positive and their mortality rate is much lower.  It does not necessarily mean that Norwegian people have better outcomes, it is just that they are better at catching the cases where people have COVID but are either asymptomatic or recover by themselves at home.

What is the mortality rate really?

Getting a narrow range on this estimate is not at all easy.  We really need random sampling and tracking of outcomes.  This has recently begun in Germany, but it will be a long time before we get decent results.  While it is known there is some underreporting in the number of deaths e.g. care homes, it is also clear that underreporting of cases from incomplete sampling/too few tests is a far bigger distortion.  This means that the mortality rate will be lower than the reported mortality rates at the moment.

The lowest estimate from a good source that I have come across is from the Diamond Princess cruise ship; adjusting for the average age of the passengers gives an estimate of 0.66%
https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.12.2000256

I have used 1% as my base case in my modelling, but 0.5% would also seem very reasonable.

Could the mortality rate be much lower?

There are some scientists arguing that the mortality rate is far lower.  Their argument assumes the virus was spreading earlier than we thought and it must be far more rapidly transmitted.   It follows that lots of us have been infected without knowing it and therefore the mortality rate is extremely low.

A recent highly publicised study in Santa Clara used antibodies tests to find that 50 times as many people were infected than we thought with 50 out of 3300 testing positive for antibodies associated with COVID.  https://www.nature.com/articles/d41586-020-01095-0

I find it very hard to reconcile these theories with the known data.  Countries that have done large scale testing such as Iceland have not found the large numbers of positives that would be consistent.

If the virus had infected 50 times as many people as we thought, then virtually all cases must not show any symptoms.  This is not consistent with other reliable studies.  Studies with a controlled set of patients such as Diamond Princess do show that perhaps half of cases are asymptomatic, which is why the mortality rate of people infected could be below 1%.  There is no evidence that a large majority of infected people are asymptomatic.

False positives

The more likely explanation for the Santa Clara result are false positives which occur in all medical tests.  The test for HIV is also an antibodies test i.e. we do not detect the virus directly but we can detect the antibodies which are associated with our immune response to the virus.  If 3300 people were tested for HIV using a rapid test, we would EXPECT to see 50 positive tests even if NO ONE has HIV.  This is because the test has a 1.5% false positive rate.

Can a treatment breakthrough reduce the mortality rate?

Another argument for a lower mortality rate is that we will develop treatments for the disease.  The stock market soared on “news” that Gilead had developed a treatment https://www.thetimes.co.uk/article/coronavirus-patients-given-us-trial-drug-are-off-ventilators-in-a-day-pb3jnzf3k

I see two problems with this

  1. It is overwhelmingly likely that it is not true. I have written before at length about the misuse of statistics and this is a classic example.  Every possible drug is being given to patients with a view to treating Covid, but these are not the controlled trials that would confirm effectiveness.  By random chance, some outcomes will be better than others.  If I dressed all patients in different coloured gowns, I would find that those wearing perhaps pink gowns had better outcomes than those wearing red ones.  Swapping all gowns to be pink would not then lead to lower mortality rates. Until we have some proper testing, I will assume this is a nonsense story.

  2. Even if it turns out to be true, this treatment only works for people on ventilators. It is only relevant for bringing down the mortality rate in a scenario where everyone has access to a ventilator i.e. the number of people infected is small. It does not change at all the need for lockdown or have any major economic implications as the story currently reads.  For a treatment to have an impact on lockdown policy, it would need to have a material impact on patients to stop them requiring hospitalisation.

Could the mortality rate be much higher?

If treatment makes a large difference to the outcomes, here there could be an important skew to the mortality rate higher.  Currently, the countries with the lowest mortality rates have, in common, a low number of cases and excellent access to healthcare.  There is good evidence so far that early treatment and access to oxygen makes a big difference to the outcomes.  If a patient is admitted late to ICU, then the prognosis is very poor.  With Boris Johnson for example, early intervention with oxygen led to a positive outcome.  For many this is not the case and if our health systems are overwhelmed then very few will get this level of treatment and the mortality rate could be far higher. My fear is that if this virus gets out of control, then the mortality rate will be far higher than we have observed so far with total deaths far higher even than the current high estimates.

If it is much lower what difference does this make to lockdown policy?

The reason that I have focused on R0 rather than mortality rate is that it is far more important for policy decisions.  A lower mortality rate is relevant in two cases:

  1. We aim for herd immunity.
    This was the initial idea for the UK and requires a lower mortality rate that we have estimated, and thus the number of people already infected to be far higher.  With current estimates, we only get herd immunity after perhaps 500,000 people die in the UK, which dramatically swamps the system.  The other big problem with this is there is only limited evidence on the level of immunity we get after recovering from COVID.  Looking at our response to other respiratory viruses, the current best guess is that it provides a decent level of protection for only a year or two.

  2. We let people die
    The idea here is that “losses” to the virus are acceptably low in comparison to the economic damage we otherwise take.  This is the common argument of the right and, particularly in the US.  I will leave it up to each reader’s own ethics on how they would make that trade-off.  But I will reiterate that we currently have no evidence that the mortality rate would be lower than 0.5% i.e. 1 million US deaths, and good reason to think it would be a multiple of that with this many cases and thus no medical treatment available.

 

 

 

 

 

 

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