Exactly How Deadly is this Coronavirus?
Note: Please see the latest information on the COVID-19 death rate and other questions in my new Coronavirus FAQ. This article was updated March 24 (update noted below in italics).
Two primary factors determine how many people die from a disease outbreak, epidemic or pandemic: How quickly and easily it spreads; and how deadly it is. Evidence suggests COVID-19, the coronavirus racing around the world by hopping from one person to another, spreads relatively easily. The mortality rate is less clear, given the disease has been infecting humans only since December and there’s been little time for rigorous study of its ultimate effects.
Before we dive into the numbers, some baseline figures to compare with:
- The seasonal flu kills about 0.1% of those infected. Because influenza strains typically spread easily and a lot of people catch it, flu deaths range from 12,000 to 61,000 every year in the United States.
- Some flu strains are worse than others. The 1918 global flu pandemic killed about 2% of the people infected. And because it spread easily — and there were no vaccines or effective treatments, more than 50 million people died, including 675,000 in the United States.
- MERS, a coronavirus discovered in 2012, kills 36% of its victims.
Now some coronavirus data (including newer research lower down):
A Feb. 24 study in the Journal of the American Medical Association (JAMA), looking at 72,528 early COVID-19 cases in China, found an overall death rate of 2.6%. But that figure was skewed by the fact that the bulk of the cases were inside Hubei province, the epicenter of the disease, where the death rate was found to be 2.9%. For cases outside Hubei, the fatality rate was 0.4%.
Also, the researchers noted that the total number of cases was likely higher than what they knew about for their study, given the number of people who likely had mild symptoms or no symptoms and never saw a healthcare professional. All the figures in this study “need to be interpreted with caution and more research is required,” the authors wrote.
A slightly more recent study, involving 1,099 people across China confirmed to have the disease, found a mortality rate of 1.4%. But that result, published Feb. 28 in the New England Journal of Medicine, should also be considered preliminary. The researchers cited incomplete medical records among other limitations to their study and, again, the unknown number of people who’ve had the disease but have not been counted.
“We no doubt missed patients who were asymptomatic or had mild cases and who were treated at home, so our study cohort may represent the more severe end of Covid-19,” the researchers say.
A corresponding editorial Feb. 28 in the journal — by Dr. Anthony S. Fauci, director of the National Institutes of Allergy and Infectious Diseases, his NIAID colleague Dr. Clifford Lane, and Dr. Robert R. Redfield, director of the U.S. Centers for Disease Control and Prevention — speculates that the fatality rate could turn out to be much lower than what research shows so far.
“If one assumes that the number of asymptomatic or minimally symptomatic cases is several times as high as the number of reported cases, the case fatality rate may be considerably less than 1%,” they write. “This suggests that the overall clinical consequences of COVID-19 may ultimately be more akin to those of a severe seasonal influenza (which has a case fatality rate of approximately 0.1%) or a pandemic influenza (similar to those in 1957 and 1968) rather than a disease similar to SARS or MERS, which have had case fatality rates of 9 to 10% and 36%, respectively.”
UPDATE March 24: Newer data paints a rather bleak picture, however, and illustrates why it’s so important for science to keep iterating, and why the true death rate will only be accurate with more hindsight. The study, published on March 17 by the Journal of the American Medical Association, finds the following death rates by age groups, based on 22,512 known cases in Italy:
- Under 30: 0
- 30–39: 0.3%
- 40–49: 0.4%
- 50–59: 1.0%
- 60–69: 3.5%
- 70–79: 12.5%
- 80–89: 19.7%
- >90: 22.7%
- Total: 7.2%
That data, as you can see, shows the death rate to be higher than seasonal flu for everyone 30 and older. END UPDATE
But remember, there are two factors in the equation. If only a small percentage of people die from a given disease, but many are infected, the end result can be worse than if the reverse were true. MERS, while killing about a third of those infected, has since 2012 infected only 2,494 people. COVID-19 has already killed more than 16,000 people as of March 24, infecting more than 90,000 in at least 58 countries.
Concern over super-spreaders
Among the most worrisome traits of the coronavirus is its apparent ease of spread, fed by the fact that people infected with COVID-19 can be contagious before symptoms appear, and some carriers feel few or no symptoms. There are mounting numbers of cases, including in the United States, of the coronavirus spreading without a known source, meaning a newly infected person has not traveled to an outbreak area or been known to have contact with someone who has the disease.
Health experts fear COVID-19 can be carried in some cases by so-called super-spreaders, people who don’t know they have the disease and who then potentially infect dozens if not hundreds of people at a time in crowded settings like hospitals or public events. It happened with past coronavirus outbreaks (SARS and MERS), and it seems to be happening with COVID-19. The big challenge is it can then be days or weeks before the newly infected people know they’re sick, so they’re out passing the disease along to others.
The authors of the Feb. 28 New England Journal of Medicine study say their findings echo others that reveal outbreaks in family clusters and transmission from asymptomatic patients. “Our study cannot preclude the presence of patients who have been termed ‘super-spreaders,’” they say.
“This coronavirus is not containable,” Dr. Amesh Adalja, an infectious disease expert at the Center for Health Security at Johns Hopkins University, told me last week. “It has achieved efficient human-to-human spread; it has been doing so for several months now.”
That’s a fact that other nations already knew, and which is now becoming clear in the United States.
What’s the rate of reproduction?
Scientists are eager now to figure out COVID-19’s rate of reproduction, the number of people infected by each infected person. For the common flu, it’s about 1.28, according to a 2014 study in the journal BMC Infectious Diseases. In the 1918 flu pandemic, it was about 1.8. The rates can fall off through efforts at containment or, in the case of the flu, when warmer spring and summer temperatures disrupt the disease’s natural cycle.
The latest research indicates an estimated reproduction rate of 2.2 for COVID-19, Fauci and colleagues explain. It remains to be seen if COVID-19 will have a seasonal component, or how effectively it can be contained by potential quarantines, event cancellations, school and business closures or other mitigation efforts, though the declining rate of new cases in China suggests such efforts can combat the spread.
Until the reproduction rate falls below 1.0, Fauci and colleagues say, “it is likely that the outbreak will continue to spread.”
That sounds like a number we can rely on.