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The Moroccan population affected by Covid-19 is underestimated

The Ministry of Health published on September 17 the first study on the extent of the spread of the virus in the Moroccan population (seroprevalence). The results, useful in themselves, were unfortunately given “raw” without carrying out a mathematical extrapolation, the only one capable of providing a realistic “estimate”. Current serological tests in fact underestimate the number of people affected. The ministry’s investigation, carried out between March and August on a sample of 85,000 blood donors, shows that 0.7% of them would have been in contact with the virus. It focused on research in these donors of biological substances responsible by our body to fight against Covid-19 called antibodies, and more precisely on a particular category of these: IgG.

The impossible quest for IgG antibodies
The difficulty (and it is considerable!) Is that a large part of contaminated subjects do not have IgG antibodies in their blood when they are taken. The latter are in fact really detectable by serological tests only from the 14th day following the contamination and gradually decrease to remain on average 40 days still present. We also know that these IgG antibodies do not always appear or disappear quickly in many infected people who do not visibly develop the disease (they are said to be asymptomatic) or who have a mild form. We cannot therefore be satisfied with this announced result of 0.7% of the population in contact with the virus, based on the sole presence of the IgG antibody. It must be the subject of a modeling, in particular from the calculation of the probabilities to assess the proportion of the number of people likely to be deprived of IgG at the time of a sample and possibly from a crossing with the published figures from the other test for the virus, PCR. It is only from this preliminary work that we can obtain an even approximate estimate of the level of spread of the infection among the Moroccan population. It is true that this calculation is not easy and the developed countries are also struggling to obtain satisfactory figures on this subject from only current serological tests. Thus, in France, the Scientific Council of this country had roughly “estimated”, in an opinion published on July 27, the share of the population concerned at 4.4%, this from a body of evidence, without qu ‘no in-depth study has been able to support this claim.

We don’t have the right antibody for epidemiological studies!
A study published on September 3 in the British Medical Journal tends to show (joining other studies) that we underestimate the proportion of the population already confronted with the coronavirus. Firstly because, according to the authors, “current antibody tests do not identify people who had mild infections”. And then because another antibody, IgA, would seem “more relevant in the fight against Covid-19, in particular in asymptomatic people”. The advantage of these latter antibodies, IgA, lies in the fact that they are detectable in serum, saliva or breast milk, that they are also present earlier and persist longer than IgG. A Luxembourg study, published in the journal MedRxiv, confirms these assertions by showing that 11% of the 1,862 people sampled have IgA antibodies against the coronavirus, against only 1.9% for IgG antibodies. If we transpose this last result to the case of Morocco, the share of the affected population would thus currently be 4.2% (a school hypothesis only, to be validated by an exhaustive study!). In all, in any case, the population affected by Covid-19 would actually be much larger than estimated in most countries, including Morocco. This is not necessarily bad news in the sense that a certain level of collective immunity (required threshold from which an infected subject no longer transmits the pathogen because he encounters too many protected subjects), even transient, would make it possible to slow down the epidemic. Especially when we know, in infectiology, that this protection also relies on the memory of past infection held by certain immune cells (T deslymphocytes) and generally ensuring a faster immune defense reaction in the event of reinfection. Even if this threshold is also difficult to estimate currently for Covid-19: between 60% and 70% according to the World Health Organization, from 20 to 40% according to other studies … In addition, the targeted surveys of serological surveillance announced by the Ministry appear very useful with regard to health professionals and prisoners in penitentiary establishments. A little less on the other hand with regard to people with chronic diseases and those over 60 years of age: the results of these last 2 types of study, whatever they may be, in fact have no impact on protective behavior. maximum that these two populations at risk must continue to adopt. On the other hand, there is a lack of an evaluation of the seroprevalence in the large metropolises and first of all Casablanca, in order to adapt public health policies accordingly. We know that this is where we risk finding very high figures, such as New York, Paris, Rio de Janeiro … By extending them, if necessary, in certain areas of these cities. : we have recently seen in Bombay, India, a serological prevalence of 51% to 58% in disadvantaged neighborhoods, against 11% to 17% in the rest of the city!

By Dr. Khadija Moussayer
Specialist in internal medicine and geriatrics, president of the Moroccan Association of Autoimmune Diseases
and systemic (AMMAIS), former head of department at the Hospital de Kénitra, former intern at the Hospitals of Paris (Pitié Salpêtrière – Charles Foix Hospital)


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