The identification of the new Omicron variant has led to apprehensions about its possible impact on the currently available vaccines. Researchers are engaged in efforts to quantify the extent of immune evasion exhibited by the new variant.

 The first data came from researchers at the Africa Health Research Institute, South Africa, who conducted a study to examine if the omicron variant escaped the protection offered by two doses of the Pfizer-BioNTech mRNA vaccine. They also investigated if the variant required ACE2 receptor binding to enter host cells to cause infection. For the study, 14 plasma samples from 12 participants were tested, which included one group of six persons who had been vaccinated with the Pfizer vaccine but had no past history of Covid-19. The other group consisted of persons who had received the Pfizer vaccine and had also been infected with SARS-CoV-2 during the first wave of the pandemic, when the major circulating virus was the D614G variant and had detectable antibodies. The geometric mean titer (GMT) FRNT50 for the D614G strain was found to be 1321 indicating a robust antibody response and therefore strong neutralization capacity of antibodies against this ancestral variant. When these same samples were tested against the Omicron variant, the GMT FRNT50 was 32 for Omicron showing a 41-fold decline in the neutralization efficacy compared to the response with the earlier strains. However, five previously infected persons were still able to neutralize the Omicron variant suggesting that the immune escape was partial. A 3-fold decline in the antibodies induced by vaccination against the Beta variant was seen indicating that immune evasion with Omicron was “more extensive”.

In what may be positive news and probably we can breathe a sigh of relief here; the Omicron variant uses the same entry route - via the ACE2 receptor - as the other SARS-CoV-2 variants, to infect host cells. This implies that the new variant may not be as intractable as feared. In experimental laboratory studies using human cell lines, Omicron was found to infect cell lines that expressed the ACE2 receptors, but not those cells that were deficient in this receptor.¹ Though this study did not test the Omicron variant in persons who had received booster dose, the authors suggest that complete vaccination followed by booster dose could protect from severe disease caused by Omicron variant.

Echoing these results, a preliminary laboratory study from Pfizer suggested that a third dose of the vaccine could boost protection against the new variant. According to a press release issued by the company, antibodies induced by three doses of the Pfizer vaccine were able to neutralize the Omicron variant one month after the third dose.² Conversely, persons who had received two doses of the Pfizer vaccine had more than a 25-fold decrease in neutralization titers against the Omicron variant in comparison to the wild-type virus indicating that two doses of BNT162b2 may provide inadequate protection against Omicron. But, “since 80% of epitopes in the spike protein recognized by CD8+ T cells are not affected by the mutations in the Omicron variant, two doses may still induce protection against severe disease”.

Initial results from a preprint German study also showed marked reduction in neutralization ability of vaccine sera and monoclonal antibodies (casirivimab and imdevimab) against the Omicron variant compared to the Delta variant.³

These are early data and not yet peer-reviewed, but they do clearly show that two doses of the vaccine may not adequately protect against the new Omicron variant,although T cell response will help protect against severe disease. A booster dose would enhance the vaccine-induced antibody response. Larger studies are necessary to validate these preliminary results. 

References 

1. Cele S, et al. SARS-CoV-2 Omicron has extensive but incomplete escape of Pfizer BNT162b2 elicited neutralization and requires ACE2 for infection. https://www.ahri.org/wp-content/uploads/2021/12/MEDRXIV-2021-267417v1-Sigal.pdf.  
2. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-and-biontech-provide-update-omicron-variant, December 08, 2021
3. Wilhelm A, et al. Reduced neutralization of SARS-CoV-2 Omicron variant by vaccine sera and monoclonal antibodies. medRxiv, posted Dec. 8, 2021. doi: https://doi.org/10.1101/2021.12.07.21267432