The Mu variant of SARS-CoV-2 is gaining more and more attention, even if its progression is nowhere near as rapid as that of the Delta variant. Experts from the Laboratoire de santé publique du Québec (LSPQ) are closely monitoring this variant which has taken hold in Quebec.

Appeared in Colombia in January 2021, the Mu variant (B.1.621) was designated “variant of interest” by the World Health Organization (WHO) on August 30. Since last May, the LSPQ has detected around 100 cases in Quebec. “So it doesn’t look at all like a progression like that of the Delta variant. However, among the variants that are neither of concern nor of interest, it is the one with the highest number of cases detected each week. These are numbers which are low for the moment, but which should not be neglected, ”specifies Sandrine Moreira, responsible for the coordination of genomics and bioinformatics at the LSPQ.

For the moment, it is therefore not a so-called “worrying” variant such as the Alpha discovered in the United Kingdom, the Beta detected in South Africa, the Gamma spotted in Brazil and the Delta appeared in India, for which have been proven to possess a characteristic that makes them more dangerous. Delta, for example, is more contagious and more virulent (it can cause a more serious infection), while Beta shows some resistance to vaccines.

Mu has been called a “variant of interest” because it is suspected that it could be more contagious and, more importantly, that it could escape certain vaccines. “This suspicion stems from the fact that in Colombia it seems to have been responsible for a good number of infections. A significant proportion of Mu variants was observed [39 % des échantillons ayant été séquencés], but on a very small number of samples. For the moment, we do not have data allowing to conclude to a greater transmissibility ”, affirms Mme Moreira.

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But there’s also the fact that the variant has been detected in at least 42 countries to date, including Chile, where the Mu variant has been identified in 40% of samples sequenced in the past month, and the United States. , particularly in Florida, California and Texas. “Since, in Quebec, we have a lot of snowbirds returning from the southern United States, there might be some connection with what we are seeing here. These are just assumptions. This possible increased transmissibility needs to be confirmed, ”concludes Moreira.

Mu, a “brother” of Beta

Experts are also keeping an eye on the Mu variant because it has the same two genetic mutations as the South African beta variant against which the effectiveness of the AstraZeneca vaccine is less.

“In the phylogenetic tree of variants, Mu has a common ancestor with the Beta variant from South Africa, it is found on a [branche] sister. However, it has been shown that the Beta variant has two mutations in position N501Y and E484K of the S protein which have an impact on the neutralization of the virus by the antibodies. [générés par le vaccin, ces derniers n’arrivent pas à neutraliser le virus aussi bien]. Can we transpose this observation to the Mu variant because it has the same two mutations? No, maybe not. It has not been demonstrated, but this is what we fear. In any case, we are vigilant ”, underlines Mme Moreira.

A study published on September 6 – but which has not yet been peer reviewed – indeed indicates that the Mu variant is 7.6 times more resistant to the neutralizing power of the serum of doubly vaccinated people (with the Pfizer vaccine) than the original strain .

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Andrés Finzi, who in turn is currently evaluating the efficacy of vaccines against the Mu variant, also expects to see “very likely a loss of sterilizing protection of vaccines against Mu”, that is, that is, the loss of protection that would prevent re-infections with mild symptoms.

“But the cellular response should always be there to prevent serious forms of infection,” says the researcher in immunopathology at the CHUM. Because as he keeps saying, the immunity induced by the vaccine is not limited to the presence of neutralizing antibodies, it also includes so-called memory cells which may respond less quickly to the aggressor, but which provide protection against hospitalizations and severe forms.

One thing is certain, “we have less to fear with the Mu than with the Delta, which is really problematic because of its greater transmissibility and greater virulence,” says Mme Moreira.

She also points out that, unlike what is happening “elsewhere in the world, or even elsewhere in Canada, where it is overwhelmingly majority”, the Delta variant is responsible for only 60 to 70% of the infections that occur. are now declaring themselves in the province. “Here in Quebec, oddly, it is not increasing as quickly as we might have feared. […] This may be a consequence of vaccination and compliance with public health measures. Or maybe it’s an effect of another variant that would take the place a bit. It is therefore for this reason that we are monitoring the Mu variant ”, points out Moreira.

We have less to fear with the Mu than with the Delta, which is really problematic because of its greater transmissibility and greater virulence.

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