Mortality Outcome transmission

Is there any evidence to suggest that Covid-19 is becoming less severe ?

Although in the USA and many other countries deaths from Covid-19 are not showing any major decline, deaths in Europe including the UK have been falling steadily since the peak of the epidemic in April.  In contrast to this positive news about falling death rates, there is no sustained slowing in the number of new cases detected.  This raises the question that possibly the infection may be becoming less serious as time passes.  In this post, I take a detailed look into the available data, and consider whether there is any support for such a possibility.

How large is the decline in deaths? 

The graph below shows the decline in total reported deaths from Covid-19 in Europe, which has occurred  since the peak in late March/early April (the blip in June was probably due to changes in recording deaths)


How does the  decline in deaths compare with the rates of infection?

  • The decline in number of deaths could, of course,  be explained by a similar drop in the numbers who are infected.
  • I have used the official UK data to show the patterns of  decline in both deaths and number of cases. 
  • To make the comparison easier, the figures in the graph below are the percent of deaths and cases  at each of the 4-week periods relative to the peak (which I called 100%).  Thus a figure of 50% percent for any 4-week period means that the number was half that of the peak
  • The graph shows that relative to the peak, deaths are falling at a greater rate than the fall in the number of cases.
  • However before concluding that the virus is becoming less severe, there are a number of other explanations that should be considered first.

What information do we need?

  • What is really needed is the ‘case fatality’ rate.  This is the proportion of people who develop the disease who then die.
  • Thus, a case fatality rate of 50% means that half of the people who develop Covid-19 are dying from the disease 
  • In order to calculate the case fatality rate, we need accurate data on the number of new cases. I have discussed this in my last post*. On one estimate, only about 1 in 10 cases in England are actually diagnosed and recorded
  • The impact of the under-reporting on the case fatality rate is large. For example, in the UK today (6th August) there have been 308134 cases and 46413 deaths.  At face value this suggests a case fatality rate of 15%.  That is ludicrously high.  We do not know what the true figure is, with the best available data being from China suggesting a rate of under 1%


Could the trends still be accurate?

  • The trends might be accurate even if the absolute numbers of cases are not
  • However, there is a problem given that at the beginning of the epidemic fewer people got tested, so more cases would have been missed 

What about other approaches to measuring severity?

  • Of course, death is not the only marker of how bad the infection is.  How many are admitted to intensive care is also a useful measure. 
  • That number though depends on how many intensive care beds are available and the threshold for admitting a person
  • Data from the UK indeed show a steady decline in the number of admissions to ICUs


If the decline in deaths and ICU admissions is real, what are the explanations?

There are a number of possible explanations for the decline in the severity profile and deaths.  These are:

  • Relatively fewer older people, and relatively more younger people, are getting the infection, probably because of greater protective behaviours in the older age groups
  • Similarly, those with underlying health problems will also be avoiding risk more than those without
  • The treatment for those most seriously affected is improving.  This is indeed the case.  At the beginning of the epidemic only around 50% of cases admitted to ICUs survived in the UK.  By the end of May this had increased to 70% and is probably higher today.

Is there any possibility that the virus itself is becoming less deadly?

  • All of the above explanations are reassuring; in part suggesting our preventive measures and improvements in health care are working 
  • What would be really exciting would be if the virus itself was changing to become less severe!
  • As they spread in the population, viruses can change their genetic structure, or ‘mutate’.  This particular virus seems to be mutating approximately twice a month 
  • Indeed, one of the early changes was to generate a form of virus that would more easily transmit to humans.
  • Covid-19 is not mutating as much, though, as  is seen with many other viruses.  This is good news insofar as it might mean that those working to develop a vaccine would not need to worry about entirely new strains 
  • It can be argued that for the virus to survive, it would not do itself any favours if it killed everyone: the argument being that the milder the disease, the more likely that the virus gets passed on.  This is a possibility, but there is no evidence yet that this is happening with Covid-19 
  • Interestingly, the SARS epidemic in 2002 died out after about 8 months, for reasons that are not totally clear 


  • There are no definitive data that show the virus is causing less severe infections 
  • We are probably succeeding in reducing the spread in those most at risk and in the treatment of those with the most severe forms
  • It is not impossible that over time the virus itself may change to become less severe