If SARS disappeared in 2003, why not COVID-19?

Q: “How do we explain the sudden disappearance of SARS in 2003 and why does this not happen with COVID-19?” Is there a notable difference between these two strains which explains this? ” asks Guillaume Lemieux of Quebec.
A : We often hear that the Severe Acute Respiratory Syndrome (SARS) that infected 8,000 people in 2002-2003, nearly 800 of whom died, is a close relative of COVID-19 – and rightly so, as the have shown genetic analyzes. So indeed: how the heck was it possible to overcome SARS (no case since 2003) with localized quarantine measures while COVID-19 spreads despite the current quasi-planetary paralysis?

The fact is that despite their relationship, these two viruses have very different characteristics. First, SARS tended to infect the lower respiratory tract while COVID-19 remained at the top, making it more easily transmitted. In this regard, the famous “R0” (the average number of people that a patient will infect) of COVID-19 seems for the moment to be in the same waters as that of SARS, ie between 2 and 3 new infections by affected person. However, it is a safe bet that the figures currently available underestimate the contagiousness of COVID-19, warns Dr. Guy Boivin, holder of the Canada Research Chair in emerging viruses at Laval University.

“Official” data only takes into account “laboratory-confirmed” cases, but we know that a large number of cases went unnoticed because in many people, the coronavirus gave only a “little cold” , therefore, to say. A recent study in Science concluded that in the Chinese city of Wuhan before it was quarantined on January 23, only 1 in 7 cases was detected. Other unpublished work , therefore to be considered with caution, puts the figure of 1 in 4 in the Chinese province of Hubei (where Wuhan is located, where the epidemic started) on February 11.

It will take large serological studies, which detect antibodies in those who have been exposed to COVID-19, to find out exactly how many people have had it, says Dr. Boivin. (Which will also decrease death rates, but that’s another story.)

What we know for the moment suggests that COVID-19 is more transmissible than SARS, confirms his colleague from UQAC, virologist Tarek Bouhali: “The problem with this new virus is that it attaches to the upper respiratory tract, which is why it is so infectious [… while] SARS goes deeper into the lungs. ”

In addition, as the latter was much more virulent, there were far fewer cases that were likely to go unnoticed. “Most [SARS] patients develop pneumonia,” reads the US public health website , and around 10% die. By comparison, at least 80% of people who get the coronavirus have only mild symptoms, or no symptoms at all.

Not to mention, adds Dr. Boivin, that “for SARS, there was no viral excretion before the onset of symptoms, whereas there seems to be for COVID-19, which complicates the measurement of public health”. Casually, this is potentially a lot of people who can be vectors of the disease without knowing it – much more than in the case of SARS, in any case.

This is why one of these two viruses could have been completely contained while the other is still running, and possibly for a long time …


COVID-19 raises a lot of questions. In order to respond to as many people as possible, science journalists have decided to join forces. The media members of the National Cooperative of Independent Information ( Le Soleil, Le Droit, La Tribune, Le Nouvelliste, Le Quotidien and La Voix de l’Est ), Québec Science and the Déclic Center team up to answer your questions. . You have some? Write to us . This project is made possible thanks to a contribution from the Chief Scientist of Quebec , who invites you to follow him on Facebook , Twitter and Instagram .

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