According to a study, the share of people who get this new coronavirus, but who have no symptoms would represent 30 % of the cases.
June 12, 2020
Updated on June 13, 2020 6h19
The invisible epidemic
“Since the beginning of this pandemic, there is almost no model that has just been,” observes Nathalie Grandvaux, senior researcher in virology at the University of Montreal. This is because those models are based on what we know of the other viruses, but it appears increasingly clear that the virus-there does not behave in the same way. We do not know really not a lot of other viruses in which the transmission by asymptomatic persons is important.”
If there is one thing that the pandemic COVID-19 revealed these last few months, this is how it is important to know the proportion of patients who have no symptoms, or at least not yet (the “presymptomatic”, see the box below), but that can still transmit the disease. And at what point, also, it is difficult to measure…
A study led by american researcher Eric Topol and published in early June in the Annals of Internal Medicine has reviewed all the data which is used to encrypt the part of the people who catch this new coronavirus, but which show no symptoms. At the bottom word, ” conclude the authors, this would represent 30 % of the cases, but more likely around 40 to 45 %.
But now, it is a simple average that hides a huge variability : among the 16 samples included in the article, the rate of asymptomatic ranged between 6 % and… not less than 96 %! And the COVID-19 is far from the only disease for which this rate is difficult to measure : even for a virus also current, former and studied, such as the influenza, the proportion of patients without symptoms is still highly debated in science. Researchers have made the tour of the scientific results on this issue in 2015, in the medical journal Epidemiology, and have found results “scattered” a little bit everywhere in between 15% and 75%.
So what is it that makes this measure (in appearance yet very simple) so difficult? “It comes in good part from the fact that we look at different groups, responds to Dr. Guy Boivin, a researcher in infectious diseases at the research Center of the CHUQ. It is the immune response that dictates the presence of symptoms and that, that is going to change from one group to another. The highest rate of asymptomatic for the COVID-19 are in groups in which the immune response is relatively low, such as young children, who have not yet fully developed their immune systems, and pregnant women, whose immune system reacts less strong to not reject the baby.”
It should be noted in this regard that a study conducted in a clinic of obstetrics, New York at the turn of April has found that out of 33 pregnant women admitted for their delivery, and test COVID-19 was positive, no less than 29 (88 %) were asymptomatic. Similarly, the lowest proportion of asymptomatic raised in the article of the Annals of Internal Medicine (6 %) was among the residents of a home for the elderly, and the highest (96 %), in a prison.
There are also questions of definition that can vary the results. For some, it takes a complete absence of symptoms to be considered to be asymptomatic, while others include very mild cases, or even all those who do not have a fever.
But above all, there simply is no perfect method. A person may not have symptoms because it is in the beginning of the disease, and therefore appear asymptomatic in a study so that it is in fact “présymptomatique” — the tests detect the virus in her, but she just has not yet had time to have symptoms. To avoid this confusion, it is necessary to make “longitudinal studies” that follow the patients over time, but this is not always possible. Fortunately, it seems for now that most of the ticas seem to stay that way : in the longitudinal studies analyzed by Eric Topol, between 0 and 10 % only of the patients are of the “pre” that end up possibly by a show of coughing, fever, etc
The same thing applies to the various ways to establish if the patient has had the disease. The tests that detect the virus itself (often in its genetic material) only have a window of two to three weeks to determine whether someone has caught it, says Dr. Boivin. After this stage, the virus is not present and cannot be detected directly. Conversely, the so-called test “serological” do not detect the virus itself, but antibodies that are associated (and which testify to an infection in the past). “But you never know exactly when was this infection-there, it can go to far, and it is sure that the ability to remember if you had symptoms or not may be more biased with the serological tests with PCR assays [that detect the genetic material of the virus]”, he adds.
In the case of the COVID-19, there are still very few works that are based on serological tests, but it has been involved in studies on influenza. And, warns Ms. Grandvaux, it will be necessary to take account of the fact that antibodies to other coronaviruses (present in humans for a long time and that does not give colds) seem to respond with the COVID-19, up to a certain point, which could interfere with the results.
Anyway, insists Ms. Grandvaux, “we need this information here. I understand that during the peak of the first wave, we did not have the resources to test people beyond those who exhibited symptoms. But now [the number of tests in Quebec peaked at 14,000 per day, it is less than 10 000/day since 6 June], perhaps we could begin to test the asymptomatic to know what proportion are a. And to find out if their number decreases as the symptomatic. This is fundamental if we want to know where one is, at the fair”.
At first glance, the question seems simple : it has symptoms of the COVID-19, or you don’t. End of the story. But in reality, it is a little more complicated than that. Here is a small glossary to find.
Symptomatic : a person in which a microbe causes symptoms. In the case of the COVID-19, this are often fever, cough, loss of taste, or of smell, etc
Asymptomatic : a person who catches a germ and that ultimately develop antibodies, but who shows no symptoms.
Présymptomatique : person who has caught a microbe and which do not yet have symptoms. Often ranked among asymptomatic, but wrongly : it is only a matter of time before the symptoms appear, whereas in the “real” asymptomatic, the symptoms never appear.
Paucisymptomatique : person who does not feel any symptoms, but who has that anyway. It’s just that they are so low (the prefix pauci also means “little”) there is a need for clinical tests to identify it.