From the beginning of the COVID epidemic, the working construct has been that the spread of the coronavirus would inevitably result in a certain level of serious disease and fatalities.  And early on, the estimates for both were very high.   The early outbreaks in places like New York, Spain and Northern Italy seemed to confirm those gloomy projections.

But shortly after those initial outbreaks, the ratio of serious illnesses and fatalities to confirmed infections began to decline and has continued to do so ever since.  You see this growing divergence between the infection and serious illness/fatality curves everywhere you look. Let’s start with the COVID Tracking Project’s data, which is a compilation of COVID data from the states’ health agencies.  Three data streams CTP has been tracking are the cumulative positive test results, hospitalizations and fatalities.  In mid-April, 13% of positive tests were resulting in hospitalizations.  But that ratio has been falling ever since, reaching 6.6% this week.  Since this is a cumulative ratio, the current ratio must be running significantly less than 6%.

You see a similar curve by comparing cumulative positive tests to fatalities, falling from a high of just over 5.5% to barely over 3% today.

Some believe that the fatality rate is a lagging indicator and the currently falling rate is tied to the decline in new infections in May and June and will soon be reversed by the more recent rise in positive tests.  However, I did a 4-week lag analysis, comparing fatalities for a particular week to the number of positive cases from four weeks prior.  You see the same decline.  Last week, it fell to 1.6%.

The World Health Organization’s tracking of daily new cases and fatalities shows the same divergence, with new cases going up much more steeply than fatalities.

What can explain this evidence that the virus has become less lethal over time and what does it imply for the future?  Of course, the short answer is that we do not know.  But there are several emerging theories that make some sense.

Better Clinical Care – It is unquestionably true that healthcare professionals are getting better at treating COVID.  With more clinical experience, techniques have improved, producing better outcomes.  For example, doctors reduced the reliance on ventilators, discovered that patients’ lungs did better lying prone than on their backs, and a number of therapeutic drugs have proved to be of some benefit.

Better Protection of Vulnerable Populations – Early in the outbreak, public health officials were unaware of how dangerous COVID was going to be to the elderly and those with certain comorbidities, as evidenced by New York discharging patients with active COVID infections to nursing homes.  Since then, public health officials and senior care facilities have done a much better job of protecting their residents.

Also, many people with conditions that expose them to greater risk have modified their behavior in an attempt reduce their potential exposure to the virus. This have particularly been the case with older Americans. As a result, there has been a marked reduction in the mean age of those infected over time.

Declining Susceptible Population – There is a natural variation in individuals’ susceptibility to pathogens.  Some individuals’ immune systems will completely block an infection; in others, it will contain the infection to mild or moderate symptoms; and in some, it fails to provide any protection and the pathogen will produce serious disease.  Pathogens naturally infect the most susceptible first.  But because there are a limited number of highly susceptible individuals, as an epidemic wears on, there are fewer highly susceptible targets for the pathogen to find.

Mutation – Pathogens tend to mutate to less lethal strains because less lethal strains are favored by evolution.  While scientists have found many mutations of the SARS-CoV-2 virus, including one that made it much more highly contagious, so far no one has found a mutation that makes it less lethal.  However,  doctors and researchers still are struggling to understand how this virus affects the body, so it may be that there has been a beneficial mutation – the effect of which we do not yet understand.

Of course, this divergence has been good news for reducing the toll COVID is taking on the world.  But, as I have warned before, just because we can identify a trend in the past does not necessarily mean it will continue into the future.  Let’s keep our fingers crossed that it does and continue to take sensible precautions.