Over the course of the pandemic the death rate in people diagnosed with COVID-19 (the case-fatality rate) has declined. Unpacking all the reasons this may be the case can help us better understand and fight this disease. A few recent studies shed some light on this question. While there might be some encouraging news here, it highlights that this is still a “novel” virus and we have a lot to learn about the illness it causes.

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. So simply “flattening the curve” and reducing pressure on hospitals is one important factor.

The most encouraging reason for the decline is 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 same 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. This is the not good reason for the drop in deadliness of this pandemic.

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 healthier victims. And keep in mind, the average daily death rate of the pandemic is still very high in the US, about 700 deaths per day, 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 seems 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.

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 makes this a deadly illness.