Vaccine efficacy and duration of protection

How concerned should we be about other new variants, such as Lambda and Mu?

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Virus variants with genetic changes that are predicted or known to affect virus characteristics and which suggest an emerging risk to global public health are defined by WHO as variants of Interest (VOI). As of September 2021, WHO has identified and is globally tracking 5 VOIs, of which the most recent additions are Lambda and Mu.

The Lambda (C.37) has been reported in more than 40 countries but most of the reports shared on the global GISAID database include fewer than ten sequences so far.

There are post-vaccination immunology studies (i.e. neutralizing antibody studies) but no vaccine effectiveness studies to date for this variant. The immunology studies include evaluations of Sinovac, Moderna, Pfizer-BioNTech and Janssen vaccines. It is too early to draw definitive conclusions, but there are some reductions in neutralization (less so for the mRNA vaccines than the others). However, the correlate of these reductions in the immune response and the clinical effectiveness is unknown.

WHO will continue to closely monitor the variant, but at present, it does not seem to be spreading rapidly. Even in Peru where it was first detected, reports from the Pan American Health Organization (PAHO) state that the Gamma variant is outcompeting the Lambda variant.

The Mu variant (B.1.621) was classified as a new variant of interest on 30 August 2021. The Mu variant has a constellation of mutations that indicate potential properties of immune escape. Preliminary data show a reduction in the neutralization capacity of convalescent and vaccinee sera, similar to that seen for the Beta variant. More studies are required to understand the phenotypic and clinical characteristics of this variant.  Since its first identification in Colombia in January 2021, there have been a few sporadic reports of cases of the Mu variant and some larger outbreaks have been reported in South America and in Europe. The epidemiology of the Mu variant, particularly with the co-circulation of the Delta variant, is being closely monitored.


  • WHO is closely monitoring newly emerging virus variants as well as identified variants of interest and variants of concern.
  • Lambda and Mu variants of interest show potential for reductions in neutralizing capacity, but it is too early to draw definitive conclusions.