Effectiveness of Naturally Acquired and Vaccine-Induced Immune Responses to SARS-CoV-2 Mu Variant

Edmilson F. de Oliveira-Filho; Bladimiro Rincon-Orozco; Natalia Jones-Cifuentes, Brigitte Peña-López; Barbara Mühlemann; Christian Drosten; Andres Moreira-Soto; Jan Felix Drexler


Emerging Infectious Diseases. 2022;28(8):1708-1712. 

In This Article

Abstract and Introduction


SARS-CoV-2 Mu variant emerged in Colombia in 2021 and spread globally. In 49 serum samples from vaccinees and COVID-19 survivors in Colombia, neutralization was significantly lower (p<0.0001) for Mu than a parental strain and variants of concern. Only the Omicron variant of concern demonstrated higher immune evasion.


Diverse SARS-CoV-2 variants have arisen during the pandemic. As of May 4, 2022, there had been 2 recognized variants of concern (VOC), Delta and Omicron, in addition to earlier emerging VOCs Alpha, Beta, and Gamma and strains previously categorized as variants of interest (VOI). Many VOIs have been understudied in terms of pathogenesis, transmissibility, and potential for immune escape. Delta and Omicron illustrate how variants emerging in tropical settings can spread globally.

Mu was first reported as a VOI in early January 2021 in northern Colombia. While outcompeting other locally circulating variants, Mu spread to additional countries, such as Ecuador, United States, Mexico, and Spain; as of early 2022, it was still circulating at low levels in Colombia.[1] Mu caused 70% of all COVID-19 cases in Colombia during May–July 2021 (Figure 1), a period which also accounted for the highest number of deaths in Colombia during the pandemic, suggesting substantial pathogenicity of Mu.[1] Mu was later outcompeted by Delta and Omicron, and the number of Mu-related cases gradually decreased through the end of 2021 (Figure 1).

Figure 1.

Incidence of SARS-CoV-2 and circulation of variants, by month, Colombia, 2021. Data on variant circulation was obtained from GISAID (https://www.gisaid.org) and data on the number of cases in Colombia from the Our World in Data database (https://www.ourworldindata.org).

Recent studies relying on data from spike-based pseudovirus testing suggested substantially lower neutralization of Mu compared with the parental B.1 virus in antiserum samples from persons in Japan and China who had received either the BNT162b2 (Pfizer-BioNTech, https://www.pfizer.com) or Sinovac (https://www.sinovac.com) vaccines or recovered from COVID-19.[2,3] Because of inherent limitations in pseudovirus-based systems for reproducing response variations based on natural infection,[4] regional differences of immune responses,[5] and different vaccines used in Colombia, we comparatively characterized the neutralization of Mu and VOCs using fully infectious viruses and serum samples from persons in Colombia. The study was approved by the Ethics Committee of the Universidad Industrial de Santander (protocol 4110) and by the Ethics Committee of the Charité-Universitätsmedizin Berlin (protocol EA2/031/22). All participants provided written informed consent.