As the number of COVID-19 patients in Massachusetts hospitals slowly ticks down, another grim metric — somewhat under the radar — has steadily been going up. That’s the case fatality rate, the percentage of deaths among known COVID-19 patients.
It stands at about 6.6 percent, up from 1.6 percent on April 1.
This increase may seem alarming, but it does not mean the disease is getting deadlier. Here are three contextual things to know about this number:
1. The case fatality rate does not measure the overall risk of dying from the disease, experts say; it is more a measure of who we test for coronavirus and a signal that many infections probably have gone uncounted.
As of Tuesday, Massachusetts officials had reported 79,332 confirmed cases of COVID-19 and 5,141 deaths, meaning 6.5 percent of those with confirmed cases had died.
A World Health Organization report released Monday said 4,006,257 cases had been reported worldwide, with 278,892 deaths, for a case fatality rate of nearly 7 percent. The rate in China is about 5.5 percent, according to data maintained by Johns Hopkins University. The rate in New York state is about 6.5 percent; in New Jersey, 6.8 percent.
There is another, more telling number that is harder to know. That is the infection fatality rate of COVID-19: the percentage of deaths among everyone who gets infected, including people who are never tested for the disease and those who might not feel any symptoms. That number is probably between 0.5 percent and 1 percent, specialists say. That is still probably five to 10 times deadlier than the flu, but nowhere near the rates seen among confirmed patients.
2. So why is the case fatality rate so much higher? Probably for a combination of reasons:
The first is a simple lack of complete data. Many cases of COVID-19 are uncounted. “The cases we are detecting in the US are predominantly people who come to us with symptoms,” said Columbia University professor Jeffrey Shaman, who studies infectious disease transmission and forecasting. “There are a lot of people out there with mild or limited symptoms or with no symptoms. They don’t seek medical care because they don’t know they have the virus. Those cases are going to be undocumented.”
And that number could be big.
Justin Silverman, an assistant professor at Penn State’s College of Information Science and Technology, was part of a study that estimated — as far back as March — that at least 5 percent of Massachusetts’ population had been infected, he said, with the vast majority of those infections undocumented. There are uncertainties in the estimates, he said, but if that number is anywhere close to accurate 5 percent of the state population is about 345,000 people. Fewer than 6,000 cases had been confirmed in Massachusetts on March 30.
The upshot is this: If we had an accurate number of the total infections, the number of fatal cases would make up a much smaller percentage.
The second thing that could help explain the high rate relates to the people the state has been testing, said Dr. Michael Mina, a professor of epidemiology at the Harvard T. H. Chan School of Public Health.
“Massachusetts has made a very diligent effort, for better or worse . . . to really put all of our eggs in the basket of testing the most vulnerable in a way that other states haven’t,” Mina said. “What that means is a tremendous fraction of our tests are going toward nursing homes.”
There is a good argument for heavy testing in nursing homes, which have been hit hard by the disease. But when testing focuses on the elderly and people with serious medical conditions that require nursing care, you are recording infections among those most at risk of dying from the virus.
“It's an extraordinarily skewed representation of what the mortality rate looks like,” Mina said.
Put another way, if the state, for some reason, had tested vast numbers of healthy people in their 20s and 30s, the results would have shown a much lower COVID-19 death rate.
3. Will we ever know the true mortality rate of COVID-19 in Massachusetts?
Probably not. Not only do we not know exactly how many people were infected, but it appears we don’t know how many deaths are due to COVID-19. Early statistical studies on excess mortality — the number of deaths above what is normal for a certain time period ― suggest that some coronavirus deaths are going uncounted.
“You’re never going to be able to accurately calculate it,” said Shaman, from Columbia, speaking of the infection death rate.
But, in time, the data will get better, and so will the estimates. Antibody tests may provide decent numbers for how many people were actually infected. Studies on excess mortality will be refined to yield a more accurate death toll.
Martin Finucane of the Globe staff contributed to this story.