– 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 indeed underestimate the real number of people affected. The survey by the supervisory ministry, carried out between March and August, on a sample of 85,000 blood donors, shows that 0.7% of them 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
As in every epidemiological study, difficulties appear. Which ones do you see in this one?
– The difficulty, which is considerable, is that a large part of infected subjects do not contain IgG antibodies in their blood at the time of the blood sample. The latter are in fact really detectable by serological tests only from the 14th day following contamination, and gradually decrease to remain present for an average of 40 days. We also know that these IgG antibodies do not always appear, or disappear quickly in many infected people, said to be asymptomatic, who do not develop the disease. We cannot therefore be satisfied with the results of this study.
– What do you think needs to be done to achieve more precise results?
– The IgG antibody must be modeled. From the calculation of the probabilities, it is necessary to evaluate the proportion of the number of people likely to be deprived of IgG at the time of a sample. Possibly, it is necessary to cross-reference with the published figures of the other test for the detection of the virus, the PCR. It is only from this preliminary work that we can obtain a rough estimate of the level of spread of the infection among the Moroccan population.
– Is Morocco the only country to encounter such difficulties?
– It is true that this calculation is not easy, and 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 “estimated”, in an opinion published on July 27, the share of the population concerned at 4.4%. This from a body of evidence, without any in-depth study having been able to corroborate this assertion. In another study from the British Medical Journal, joining other studies, we show that we underestimate the proportion of the population already facing the Coronavirus. According to the authors, this is primarily because current antibody tests cannot identify people who had mild infections. And then because another antibody, IgA, would seem “more relevant in Covid-19, especially in asymptomatic people”.
– So IgA remains the right antibody to detect in this kind of study?
– The advantage of IgA antibodies 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 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 total, in any case, the population affected by Covid-19 would actually be much larger than estimated in most countries, including Morocco.