Coronavirus: two months later, what do we know about the case fatality rate of Covid-19?

Harvard historian Jill Lepore recently told the magazine The New Yorker which when democracies sink into crisis, the question that comes to mind is: "where are we going?" ". As if we were relying on the equivalent of a weather forecast to tell us how healthy our democracy will be tomorrow. Quoting the Italian philosopher Benedetto Croce, the historian also writes that “political problems are not external forces beyond our control; these are forces that we can control. For that, we just have to make up our minds and act ”.

IThe same goes for the coronavirus epidemic. How big will it be? How many people will be infected? How many citizens will die? The answers to these questions are not set in stone. They are partly under our control, provided that we act with determination, speed and solidarity.

As an epidemiologist with eight years of field experience (including being at the forefront of isolation and quarantine efforts during the 2009 swine flu pandemic), I felt like the month last that reports of deaths from Covid-19 in China gave a very imprecise picture of the case fatality rate of the disease. In an articleIn particular, I wrote that the case fatality rate from an emerging disease is always high in the early stages of an epidemic, but is likely to decline once more comprehensive data becomes available. After waiting eight weeks, I am now worried that this new data - which would establish that the case fatality rate of this new coronavirus is low - will never arrive.

Young passengers wear masks on a high-speed train in Hong Kong on Wednesday, January 22, 2020. At that time, it was unclear how dangerous the virus would be.
Kin Cheung / AP

Do not confuse case fatality rate with infection fatality rate

As of January 31, 2020, China had reported a total of 11 cases of Covid-821 and 19 deaths, or a case fatality rate of 2%. Two weeks later, the number of cases had passed to more than 50, and deaths, at 1, which corresponds to a mortality rate of 524% (this increase was expected because deaths are always counted later than cases). However, a case fatality rate of 3 or 2% is extremely dangerous when it concerns an easily transmitted disease.

However, it should be noted that these case fatality rates of 2 and 3% had been calculated on the basis of numbers of "official" cases (respectively 11 and 821). However, these only take into account people who

  • have symptoms;
  • decide that their symptoms are severe enough to warrant a visit to the hospital;
  • choose a hospital or clinic capable of testing and reporting coronavirus cases.

It can therefore be reasonably assumed that hundreds of thousands of cases, perhaps even as many as a million, have simply not been counted.

Before continuing, let's take a look at some definitions from Steven Riley, an infectious disease specialist at Imperial College. The case fatality rate of an infection represents the probability of dying for an infected person, whether or not they go to the hospital. The case fatality rate (implied clinical) relates to the probability of dying for an infected person who is sick enough to come to a hospital or clinic. This rate is therefore higher than the case fatality rate from infection, because people who present to the hospital are usually more seriously ill.

Imagine that China's statistics for mid-February, either 1 deaths, involved one million infections by Covid-19 rather than 50 (counting, for example, all symptomatic and asymptomatic infections). In this case, the case fatality rate from the infection would have been 000%, about three times that of the seasonal influenza virus; such a virus is a matter of concern, but not a major health crisis.

The case fatality rate is much more difficult to estimate than the case fatality rate. The reason is that it is difficult to count people who are mildly ill or who have no symptoms. To hope to succeed in determining the proportion of asymptomatic, mild, symptomatic and serious infections in a population, it is indeed necessary to be able to count and test all its members. This is sometimes possible, for example on a cruise ship or in a small community.

This is how the scientists of the London School of Hygiene and Tropical Medicine, theImperial College and Institute for Disease Modeling, in London, to estimate the case fatality rate of SARS-CoV-2 infection.

Currently, the figures indicate that it would be between 0,5% and 0,94%. In other words, Covid-19 is about 10 to 20 times more deadly than seasonal flu. These conclusions are consistent with the data from the genomics and large-scale testing. The only good news at the moment is that in Korea, the epidemic could ultimately present a case fatality rate lower than the epidemic in China.

A cruise ship quarantined in Japan in the port of Yokohama, near Tokyo, on February 9, 2020. Cruise and airline bookings are on the decline due to the coronavirus.
Eugene Hoshiko / AP

Impact of the epidemic in the United States

In the United States, new cases of Covid-19 are now detected daily, which means it is too late to stop the initial wave of infections. The epidemic is likely to spread across the country. The virus seems to be about as contagious than that of the fluHowever, comparing the two is complicated because having never encountered SARS-CoV-2 before, we have not developed immunity against it.

All things considered, it's reasonable to think that Covid-19 will infect as many Americans over the next year. that the flu does in a normal winter - or between 25 and 115 million people. Maybe a little more if the virus turns out to be more contagious than we thought. Maybe a little less if we put in place restrictions that minimize our travel and social and professional contacts.

The bad news is, of course, that these infection numbers could translate into 350 to 000 deaths in the United States alone (with an uncertainty range of 660 to 000 million deaths). The good news is, this isn't about the weather forecast: we can influence the size of the epidemic, in other words the total number of infections. By changing our habits to limit our contacts and by improving our hygiene, for example. If the total number of infections decreases, the number of deaths will also decrease.

Science cannot tell us, as of now, which measures will be most effective in slowing the epidemic and reducing its impact. If I stop shaking hands, will it halve my likelihood of infection? A third ? Nobody knows. If I work from home two days a week, will this reduce my likelihood of infection by 40%? Perhaps. But we don't yet know the precise answer to this question.

What is certain is that we must now prepare, by all possible means, to reduce our exposure to SARS-CoV-2, that is to say the risk of coming into contact with infected people or surfaces. For some, it will mean staying at home longer. For others, adopt more stringent hygiene practices. In Hubei Province, China, a version extreme this reduction in exposure - including mandatory quarantine, rapid diagnosis and isolation of patients, closure of workplaces and schools - could have worked: the spread of the epidemic seems to have slowed there.

We must therefore prepare for what our daily life will change over the next 12 months. Holidays may have to be canceled, our social interactions will be different. Every morning, as soon as we wake up, we should have in mind the issue of risk management. Indeed, the Covid-19 epidemic is not taking place halfway around the world, it is not a simple cold or a small flu, and it will not go away on its own. She seems pretty well on the way to accompany us for a while.The Conversation

Maciej F. Boni, Associate Professor of Biology, Pennsylvania State University

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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