More than seven months into the global pandemic, the novel coronavirus has infected more than 4 million people in the United States, killing over 144,000. And as politicians increasingly reference new terms such as death rate, mortality rate, case fatality ratio and infection fatality ratio, it's important to understand exactly what those terms mean, and what they imply about the severity of the ongoing pandemic.
With any disease, it's useful for scientists and doctors to understand approximately what portion of people are expected to die if they become infected. However, there are several different ways to evaluate the death rate.
"While policy makers require various indicators of mortality to help inform public health decision-making, each metric has its own limitations and needs to be interpreted with caution," said Dr. John Brownstein, an epidemiologist, professor of biomedical informatics at Harvard Medical School, and chief innovation officer at Boston Children's Hospital.
The term death rate and mortality rate are often used interchangeably, but neither of these terms carries much significance. Instead, epidemiologists use metrics called infection fatality ratio, case fatality ratio, and deaths per capita to understand how many people are dying among those who are infected.
Infection fatality ratio (IFR) is the term most commonly used by epidemiologists. It is the number of people who die from a disease among everyone who has the disease, regardless of the presence of symptoms. Epidemiologists prefer this rate because it includes all people who have the virus -- not just the ones with symptoms -- and is there considered more accurate.
The problem is, we don't know the true infection fatality ratio in the United States because in many regions, only people with symptoms receive a test. Right now, the CDC's best guess is that the IFR is 0.0065, meaning that fewer than 1% of all people infected with the coronavirus -- regardless of whether they have symptoms -- die from the disease.
The next best estimate is the case fatality ratio (CFR), a metric that is used often in literature and by the media. This measures the number of deaths divided by the number of confirmed cases. In the U.S., confirmed cases generally implies that those individuals had symptoms.
Right now, the case fatality ratio is 3.6%, meaning that out of every 100 Americans diagnosed with COVID-19, about 3 to 4 are expected to die. By this metric, the U.S. is better off than some hard-hit European countries but worse than many other countries that have more successfully contained the virus and implemented widespread testing.
Still, experts warn, it's difficult to use the case fatality ratio to compare countries. "While a useful measure, we need to be careful in the interpretation of the CFR," Brownstein said. "Differences between countries could be driven by a range of factors including testing capacity, demographic characteristics of the population, and health care system quality and capacity."
First, countries who have been hit with the coronavirus for a longer period of time will likely have a higher ratio. Second, the CFR can vary widely by country, largely due to testing capabilities.
In the United States, the current case fatality ratio is likely more dramatic-sounding than the true death rate, skewed by the fact that the real number of COVID-19 cases in the country is likely 10 times higher than official estimates.
Another way to gauge the death rate is to measure how many people die per 100,000 people, also called "per capita." This is a number meant to reflect the general population, including confirmed cases and healthy people.
By this measure, the United States has the third-worst death rate, after the United Kingdom and Chile. That means that, relatively speaking, a greater portion of people die relative to our population compared to most other countries.
At this stage of the pandemic, these numbers will continue to change as we get better data, and as doctors become more adept at rescuing ailing patients. It's a dynamic value that won't be set in stone until we reflect back on this time. Nevertheless, these terms will continue to be recited by people in the limelight and should be interpreted with a grain of salt.
With a better understanding of these terms, it's easier to understand why these ratios are not directly comparable between countries -- and only time, more testing, and better information will help paint the complete picture of the true "death rate."
Shantum Misra, M.D., is a senior resident in internal medicine at Dartmouth-Hitchcock Medical Center and a contributor to the ABC News Medical Unit.
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