Mu and C.1.2 variant spike proteins partially resistant to neutralization by vaccine-elicited antibodies

Mutations within severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have resulted in different strains. Some have been classified by the World Health Organization (WHO) as variants of concern. These include Alpha, Beta, Gamma, and Delta, and variants of interest that include Lambda and newly classified Mu. There are also two unclassified variants, the C.1.2 variant, and the Delta+N501S variant. Due to the Mu and C.1.2 variants possessing mutations located in the receptor-binding domain of the spike protein, these variants may contribute to increased transmissibility, resistance to neutralizing antibodies, and therapeutic monoclonal antibodies.

Study: Neutralization of Mu and C.1.2 SARS-CoV-2 Variants by Vaccine-elicited Antibodies in Individuals With and Without Previous History of Infection. Image Credit: Fit Ztudio/ Shutterstock

A team of researchers from the Grossman School of Medicine, USA, measured the infectivity of SARS-CoV-2 variants C.1.2, Mu, and Delta+501S spike proteins and determined the susceptibility to neutralizing antibodies gained through previous infection or via vaccination. Also, the authors tested if these variants were susceptible to neutralization from monoclonal antibodies.

A preprint version of this study, which is yet to undergo peer review, is available on the bioRxiv* server

The study

The authors determined the susceptibility of the viruses with variant spike proteins to neutralizing antibodies by evaluating the neutralizing titers of serum antibodies produced from the mRNA-1273 and BNT162b2 mRNA vaccines and the Ad26.COV2.S adenoviral vector-based vaccine. Antibodies gained through prior infection neutralized D614G spike with a mean titer of 334. At the same time, neutralization of Mu, Beta, Delta, and Delta+501S was decreased by 7-9-fold, and the C.1.2 variant appeared to be more resistant with a 9-fold decrease.

Compared to antibodies from a previous infection, antibodies from the BNT162b2 vaccine neutralized virus with the D614G spike with a mean titer of 862, a 2.6-fold increase. BNT162b2 neutralizing titers against Beta, Delta, and Delta+N501S decreased by 4.8-, 3.4-, and 3.4-fold. More resistance was displayed with the Mu and C.1.2 variants, decreasing by 6.8 and 7.9-fold, respectively.

The C.1.2 variant was also the most resistant to mRNA-1273 vaccinated sera with an 11.2-fold decreased titer. Sera from individuals vaccinated with Ad26.COV2.S produced neutralizing antibody titers that neutralized D614G with an average titer of 245. The C.1.2 variant titers fell into a range below 50, the minimum detectable by the assay used by the authors. The highest neutralizing titer against all the variants was from the sera of individuals with a previous infection before receiving the BNT162b2 vaccine.

The authors established an ACE2 avidity assay to measure the ACE2 binding avidity of the variant spikes. Analysis of the assay results showed an increase in ACE2 binding avidity for the Beta, Delta, Delta+501S, and Mu spikes, indicated by a decrease in the required concentration to achieve 50% saturation of the spike proteins. In contrast, the affinity to which C.1.2 bound ACE2 decreased, this variant required a 1.4-fold increase in ACE2 concentration for 50% binding in comparison with D614G and a 2.4-fold decrease when compared to the Beta variant.

Using a virion binding assay, the authors confirmed these results by incubating pseudotyped virions with sACE2 and ACE2.293T cells. The results from this assay showed that virions with Beta, Delta, Delta+N501S, D614G, and Mu spikes bound ACE2 similarly, while C.1.2 binding was decreased. From these results, it could be suggested that the C.1.2 variant spike protein binds ACE2 with lower affinity when compared to the other variant spike proteins

The Regeneron monoclonal antibodies utilized in this study maintained their ability to neutralize C.1.2, Delta, Delta+N501S, and Mu. The monoclonal antibody REGN10933 lost a 50-fold titer against the Beta variant spike protein, but against the other variants, it maintained its neutralizing activities. The REGN10987 monoclonal antibody maintained its neutralizing abilities against all the variants, and so did the combination of both monoclonal antibodies.


The Mu and the Delta variant are almost identical, so it could be suggested that this variant does present any additional danger to society. The C.1.2 variant spread should be closely monitored due to the large number of mutations and relative neutralization resistance. However, because of the location where this variant was detected, it has restricted geographic distribution potential.

The high antibody titers displayed in experienced patients against all of the variants is promising because it shows that individuals have the capacity to mount a broadly neutralizing response which may be vital for potential future variants. It could be suggested that these findings show booster vaccinations may result in a similarly broad antibody response.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information

Journal reference:
  • Tada, T. et al. (2021) “Neutralization of Mu and C.1.2 SARS-CoV-2 Variants by Vaccine-elicited Antibodies in Individuals With and Without Previous History of Infection”. bioRxiv. doi:

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Gemma Wilson

Gemma is a journalism graduate with keen interest in covering business news – specifically startups. She has as a keen eye for technologies and has predicted quite a few successful startups over the last couple of years.

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