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Over the course of the pandemic the death rate in people diagnosed with COVID-19 (the case-fatality rate) has declined. It’s not easy to unpack all the reasons this may be the case, but it may help us understand this pandemic. This is an extremely contagious and deadly disease, and nothing has changed that, but there is evidence we are making some progress.

One recent study looking at the case fatality rate in the New York region from March to August found that the death rate for those admitted to the hospital dropped from 27% to 3%. They also found many possible reasons for this dramatic decrease. One is the fact that in March New York hospitals were overwhelmed with COVID cases. They did not have enough ICU beds or ventilators, and doctors were crushed beneath the initial wave of cases of a disease they had no experience with. In fact, out of desperation, some hospitals were placing two patients on a single ventilator. So simply “flattening the curve” and reducing pressure on hospitals is one important factor.

Another study out of the UK found that from March to June the fatality rate of COVID patients admitted to the ICU decreased from 41% to 21%. This was even after controlling for demographic and medical history factors, so this was likely not due to patients being healthier at baseline. The study attributed the improvement to two factors – the decrease in stress on hospital systems (again), and improvements in the care of critically ill patients with COVID. This latter reason is the most encouraging reason for the decline – the steep learning curve of knowing how to treat those who are seriously ill with COVID. Doctors have learned through direct experience how to better manage COVID patients, and many interventions became standard practice between March and August. For example, it is better to rest patients on their stomach than their back, and it is better to delay ventilation as long as possible. The discovery that steroids can reduce the risk of cytokine storm was perhaps a significant improvement. Some patients now get convalescent plasma, something that obviously could not have happened early on. Remdesevir was given emergency use authorization, but a recent study by the WHO found no survival benefit from this drug (or from hydroxychloroquine; a combination of the anti-HIV drugs lopinavir and ritonavir; and interferon).

While we still do not have a cure for COVID-19 or a proven effective anti-viral, management has significantly improved and this has definitely contributed to survival. However – this is not the only effect, and may not even be the major effect.

The recent WHO study also found that as the pandemic progresses, younger and healthier people are being infected. They naturally have a higher survival rate, even if they become sick enough to get admitted to a hospital. It seems from this data, to put it bluntly, that the first surge of the pandemic in the New York region killed many vulnerable people, and now there are simply fewer of them alive to catch and die from the virus. Now we are seeing larger numbers of people infected, but they are healthier at baseline, so the case fatality rate drops.

As a side point, some have used the fact that the pandemic tends to kill older sicker people to minimize the significance of the death toll, as if these are mostly people who would have died soon anyway. But that is not the case. The pandemic does not mostly kill people who were already on death’s door (although certainly that describes some victims). The chronic conditions that make people vulnerable to dying from COVID, such as hypertension, obesity, or asthma, are also compatible with years of quality life. Anyone with an elderly parent who has chronic illnesses but is still a valued and loved member of the family would bristle at the suggestion that their death is no big deal.

But now, while the case fatality rate is lower, the pandemic has moved on to younger and healthier victims. And keep in mind, the average daily death rate of the pandemic is still very high in the US, over 700 deaths per day on average, because new cases remains high and is increasing as we enter what looks like a third surge.

Another recent study identifies a different potential reason for the decline in the case fatality rate – the protective measures being used to reduce spread of the virus. If, for example, you social distance and wear a mask but still get sick (it’s possible, because these protections are not perfect, people do not completely adhere to them, and the virus is very contagious) you will likely get a lower initial viral load than someone who was hugging someone, without a mask, who was infected. What the study found was two things – that the initial viral load correlates with the risk of death, and that as the pandemic progresses people are presenting with a lower viral load on average. So protective measures seem to be shifting to lower exposure, so many people do not contract the virus and those that do will tend to have lower viral loads. This will tend to generate lots of milder cases, some of which will be diagnosed and even admitted to hospital but have a lower fatality rate.

This study also finds that there is likely a significant effect from earlier diagnosis. We are doing more testing than in March, which means some people will be diagnosed at an earlier phase of their illness, be treated earlier, and have better outcomes. This is a good thing, and is another reason to have aggressive testing – to prevent spread of the virus, but also to treat people early. Ironically, some have suggested that the numbers are being inflated by aggressive testing – but this testing reduces the case fatality rate by increasing the number of mild cases.

In addition to all this, is the virus mutating and becoming less deadly? The short answer is no. The virus is mutating very slowly (which is typical for this family of viruses) – about two changes per month. There are many variants of the virus, which helps virologists track the spread, and one variant seems to be increasing in frequency. Unfortunately this variant carries a mutation of the spike protein that potentially allows the virus to be more infectious. So if anything, the virus has become worse over time. So hoping for the virus to have mutated into a more benign form is not a good strategy. It mutates slowly, and may, if anything, become worse.

The available studies don’t have the power or rigor (because it’s hard to control for confounding variables in ecological studies) to determine with precision the relative contribution of all these factors. But they are all plausible and well-established phenomena in general, and the evidence does show they are all playing a role. People are surviving more because our care is improving and we are diagnosing cases earlier, but also because the pandemic has moved on from the most vulnerable to healthier victims and our public health measures are not only preventing new cases but shifting cases toward the milder end of the spectrum through lower viral loads.

None of this should make us complacent. The death rate remains high, and even a 3% case fatality rate, or 21% death rate for ICU admissions, makes this a very deadly illness.

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Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.