Interpreting the Covid-19 Death Rate: You Asked, We Answered

Apr 23, 2020 by


AS THE CORONAVIRUS pandemic continues to spread, Undark readers have been sending us insightful questions, comments, and observations on the subject. We’ve asked Pulitzer Prize-winning science journalist and Undark’s publisher, Deborah Blum, to dedicate some time to responding — both as a reader service, and as another way for Undark to cast some light into the darkness of misinformation, rumor, fear, and conjecture now percolating through the information commons.

You can find previous iterations of this feature here. And if you have questions, comments, or would like to see some other aspect of the Covid-19 crisis explored in a subsequent feature, please write us at: (Reader questions and comments below have been edited and sometimes combined for clarity and brevity.)

In very preliminary antibody testing, the Los Angeles County Department of Public Health indicated this week that approximately 4.1 percent of the county’s adult population has antibodies to the virus. If true, this would mean that there are hundreds of thousands more infections in the county than previously thought, suggesting in turn that the percentage of those who actually die from the virus is significantly lower. Is this good news?

Yes, this is definitely encouraging. But for the moment, let’s call it possible good news.

The Los Angeles County study belongs to a new wave of tests analyzing participants’ blood for traces of antibodies — a signal of previous infection — to the SARS-CoV-2 virus. Tests in Santa Clara County, south of San Francisco, suggested that between 2.5 and 4 percent of the population may have been infected. On the East Coast, even higher numbers have been seen. Antibody testing in New York state estimated an infection rate of nearly 14 percent. And one very small antibody study of a Boston-area hotspot found a remarkable infection rate of 32 percent. Such studies, both in the U.S. and in Europe, suggest that the official case rate for Covid-19 by far underestimates the number of people actually infected. And this, as you point out, would be good news regarding the lethality of the virus. It would appear, after all, that far more people than we originally realized are surviving the infection — in some cases not even noticing that they are infected at all.

Most epidemiologists have predicted this: The infection fatality rate (IFR) is usually lower than the case fatality rate (CFR) which tends to dominate early reports. That is, if we only look at confirmed cases — usually those people sick enough to require medical treatment — then the percentage of deaths will be higher. You’re starting with a smaller population, and your population tends to be sicker at the start. When we calculate based on an expanded number of infections in the broader population, however, which includes many milder illnesses and even asymptomatic cases, the death rate is invariably lower.

This means that early numbers tend to be extreme. In China, for instance, the case fatality rate itself fell from an estimated 17 percent in the early days of the crisis to 0.7 percent, as accurate counts of case numbers increased. The preliminary antibody studies in California looked even better, suggesting an infection fatality rate between .1 and . 2 percent — not much different than the flu.

So taking the good news where we find it, the antibody tests do suggest that SARS-CoV-2 is less deadly than we first feared. But there are caveats.

A review of the studies in Science magazine raised concerns about the methodologies of the California studies, especially regarding recruitment strategies. Some of the approaches appear likely to select for people who were infected, meaning that the rate of infection may be over-stated. And questions have been raised about the reliability of the antibody tests themselves, many of which have not received certification from the Food and Drug Administration. Finally, many researchers add a cautionary note about all of this: While the current testing does suggest that many more people have been infected with relatively mild cases of Covid-19, scientists are still unsure about whether they are immune-protected from reinfection — and even if they are at least temporarily immune, no one yet knows how long that will last.

At this point, the numbers should mostly remind us that we are likely surrounded by people who might be infected. And even the leaders of that Los Angeles County study suggest that the right response to these findings is to continue to practice protective measures, and to maintain social distancing.

I’m a California resident and I may have a very mild Covid-19 case. For three-plus weeks, I’ve had a persistent dry cough and an off-and-on very low-grade fever. Otherwise I feel fine and have no desire or need to seek treatment. But I am wondering: If I am positive for the virus, would my blood be useful? If so, should I get checked or is that using up a test much needed for someone sicker?

Well, first, I hope the infection clears up soon and that you are well. But, yes, the shortage of Covid-19 tests has been an ongoing, severe, and for many of us, embarrassing problem in the United States during this pandemic. But California has been ramping up its testing capability and was, in fact, the first state in the country to begin offering tests to high-risk but asymptomatic people. So it’s probably worth at least contacting your local public health department and find out if a test is easily available.

There are a few medical trials looking at whether antibody-rich blood from people who were infected with the coronavirus might be useful in creating serums to treat sicker patients and other researchers who believe such blood samples could be useful in searching for cures. So if everything aligns, when you get tested, be sure and ask if the laboratory is collecting any blood samples for research purposes. You might find that you could play at least small part in helping to solve the problem.

Recently, I’ve seen more and more stories warning of a second wave of coronavirus infections. What does that really mean and how worried should I be?

That’s been a question of the week, especially after the director of the U. S. Centers for Disease Control and Prevention (CDC) issued a warning that a second wave of Covid-19 this fall was likely to wreak even more havoc in the United States than has occurred this spring.

But let’s tease that apart a little. The term “second wave” in an epidemic generally refers to a point when, after an initial outbreak is contained, the virus bounces back, bringing on another unwanted round of infection. This can happen fairly quickly. For instance, the country of Singapore initially contained the first wave of SARS-CoV-2 infections, but this month, due to introduction of the virus into cramped worker housing, it rebounded, leading to thousands of new infections. In China, where the virus originated and where strict social distancing seemed to suppress it, researchers see signs that relaxing restrictions has led to a new spike in illness.

But for outbreaks of respiratory viruses, second wave also tends to refer to a seasonal cycle. Viruses such as influenza, for instance, tend to spread more readily in the colder months when people spend more time indoors, in close quarters, and when the drier air makes it easier for virus remain airborne. Although this particular coronavirus seems to be less affected by the seasons, as mentioned in last week’s Q&A, scientists still expect something of a reduction during the milder summer months. And, they suspect, the coronavirus will gather new impetus in the fall, partly due to seasonal changes and partly due to the relaxing of the strict social distance measures currently widely in place.

The warning from the CDC director, Robert Redfield, also underlined fears that a Covid-19 second wave would come in tandem with the return of seasonal influenza, which also sickens and kills thousands of Americans every year. “We’re going to have the flu epidemic and the coronavirus epidemic at the same time,” Redfield said, adding that he worried the combination could once again overwhelm the health care system.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, also warned of a second Covid-19 wave in the fall, but emphasized that its severity will depend on our response. And since we are unlikely to have a vaccine that protects against the coronavirus by fall, this means it’s probably smart to continue practicing social distancing as appropriate and, of course, to get your flu shot in the fall and contribute to reducing those infections as much as possible.

People keep comparing the current pandemic to influenza in 1918. Does that even make sense? Are there actually any meaningful lessons for today from 100 years ago?

When you hear such comparisons, the reference is often to the deadly second wave of that earlier pandemic. Let’s first take some comfort in the differences though. The 1918-19 pandemic was a lot more lethal than this one appears to be. It was also not a coronavirus: Rather, it was a particularly virulent version of the H1N1 swine flu, and it’s estimated that some 500 million people were infected, and at least 50 million were killed worldwide.

It’s also important to remember that these were very different circumstances. The deadly influenza pandemic occurred during the devastating global conflict of World War I. In fact, one of the theories regarding that lethal second wave in the fall of 1918 is that it was largely spread by soldiers from countless countries as they were sent from battlefields back to their home cities as the war ended. Those infections spilled outward as standard community spread through everyday person-to-person contact.

“Knowing the history of 1918, I seriously fear a second wave of the virus,” wrote historian Kenneth Davis, author of a book about the influenza pandemic, titled “More Deadly than War,” in an email message. Davis added that there are a number of important lessons for us today in that pandemic. These include the critical role played by social distancing and the importance of not relaxing those rules too soon. Davis noted that two of the important lessons from the earlier pandemic — be prepared for the next one and don’t downplay the risks — have unfortunately not been adopted fully in the United States. “My own take on the lessons of 1918/1919 are that lies, censorship, and propaganda can kill,” he wrote. “Ignoring sound science can be lethal, and misplaced priorities — for instance ‘business as usual’ over public health — are deadly.”

Indeed, it’s thought that some of these practices contributed to another aspect of the influenza pandemic that we will all want to avoid: a third wave of infections.

That’s it for this installment, but if questions or observations occur to you as this pandemic moves forward, don’t hesitate to contact us. We’ll do our best to include your input or otherwise provide you with realistic answers — and we can promise you they’ll be well researched ones. Please email us at

Deborah Blum is the director of the Knight Science Journalism Fellowship Program at MIT and the publisher of Undark.

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