The first Covid-19 strain, to delta and lambda, and now we have a new Covid-19 variant to worry about. It is the new ‘mu’ variant. According to the World Health Organization (WHO), it is monitoring the new variant which has been confirmed in at least 39 countries.

Mu, also known by scientists as B.1.621 — was added to the WHO’s list of variants “of interest” on Aug. 30, the international health organization said in its weekly Covid epidemiological report published on Tuesday.

According to the report, mu contains genetic mutations that indicate natural immunity. What this means is that the current vaccines or monoclonal antibody treatments may not work well against it as it does against the original virus. However, more studies are still needed to determine whether it is more contagious, more deadly or more resistant to current vaccines and treatments.

“Preliminary data presented to the Virus Evolution Working Group show a reduction in neutralization capacity of convalescent and vaccine sera similar to that seen for the Beta variant, but this needs to be confirmed by further studies,” WHO wrote in its report.

The agency is already monitoring four variants ‘of concern’, including delta, which was first detected in India; alpha, first detected in the U.K.; beta, first detected in South Africa, and gamma, first detected in Brazil. It is keeping a close eye on the lambda variant, which was first discovered in Peru – that have caused outbreaks in multiple countries and contain genetic changes that could make them potentially more deadly than other strains. Variants of concern means a mutated strain that is either more contagious, more deadly or more resistant to current vaccines and treatments.

The new variant, mu, was first identified in Colombia but has since been confirmed in at least 39 countries, according to the WHO. Although the global prevalence of the variant among sequenced cases has declined and is currently below 0.1 percent, its prevalence in Colombia and Ecuador has consistently increased, the agency warned.

“The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes,” WHO said.