Death rates from COVID-19 should allow for how many people are tested, the age of an infected population as well as such factors as whether the health care system is overwhelmed, the researchers at Washington University School of Medicine in St. Louis have suggested.
In a study posted on the school's website, the researchers said because of the way countries monitor pandemics like the coronavirus, the case fatality rate tends to decrease over time.
For example, when an outbreak begins and health officials are not looking for the virus, some people may die at home and never be diagnosed. That would lower the numerator and might lead to an underestimate of the case fatality rate, said Steven Lawrence, an infectious disease expert and associate professor of medicine with the school.
But a much more likely scenario is that early in an outbreak, testing is limited to people who are so sick that they wind up in the hospital. That means the only infections that get counted are in the people most likely to die. So the denominator is missing a huge number of infected people who survive, and that makes the virus appear much more deadly than it really is.
Another factor affecting coronavirus fatality rates is the characteristics of the population that is infected at any given moment, says Mary Bushman, a postdoctoral researcher at Harvard University's Center for Communicable Disease Dynamics.
In Washington state, the first cases appeared in nursing home residents, who tend to be extremely vulnerable to the disease. At one nursing home, 35 of 81 infected people died, which is a case fatality rate of 43 percent. But as Washington began testing for the virus outside the nursing home, it became clear the case fatality rate in the general population was vastly lower.
Across the United States, as testing has expanded to include younger and healthier segments of the population, the fatality rate has decreased to levels similar to those in South Korea.
It will take a different sort of test to assess how lethal coronavirus has been, Lawrence said.
Most current tests only detect active infections, when the virus is still present in the body. But a different type of test, now being developed but still probably months away from wide use, can reveal whether a person has ever been infected. And that is what scientists need to know to establish the true denominator for coronavirus and to find the true case fatality rate.
In the United States, it is likely that the case fatality rate from the coronavirus will end up somewhere between 0.5 percent and 1 percent, once a broad cross-section of the population has been tested, Lawrence said. Enditem
