ABSTRACT
Importance: The SARS-CoV-2 variant Omicron escapes neutralizing antibody responses elicited after COVID-19 vaccination, while T-cell responses might be better conserved. It is crucial to assess how a third dose of vaccination modifies these responses, particularly for immunocompromised patients with readily impaired antibody responses. Objective: To determine T-cell responses to the Spike (S)-protein of Omicron in anti-CD20 treated patients before and after their third mRNA COVID-19 vaccination Design: Prospective observational monocentric study Setting: Conducted since March 2021 at the University Hospital Geneva Participants: Twenty adults with multiple sclerosis on anti-CD20 treatment (ocrelizumab) who received their third dose of mRNA COVID-19 vaccine 6 to 7 months after their second vaccination. Intervention: Blood sampling before and one month after the third vaccine dose Main outcomes and measures: Quantification of CD4 and CD8 (cytotoxic) T cells specific for SARS-CoV-2 S-protein of vaccine strain, Delta and Omicron variants , using activation marker induced assay (AIM) and comparing frequencies before and after the third vaccine dose. Results: S-specific CD4 and CD8 T-cell memory against all variants was maintained in around half of the patients six months after their second vaccination, albeit at lower frequencies against Delta and Omicron variants. A third dose enhanced the number of responders to all variants and significantly increased CD8 T-cell responses. The frequencies of T cells specific to Omicron and Delta remained lower than those specific to the vaccine strain after the boost. Conclusion and relevance: Vaccinated MS patients on anti-CD20 treatment show robust T-cell responses that recognize S from the circulating Delta and Omicron variants. Response rates increased after the third dose, demonstrating that a booster dose might improve cytotoxic T-cell mediated protection against severe disease in patients with low humoral response. The clinical relevance of the reduced frequencies of T cells specific to Omicron will need to be monitored in the future.