“Memory” T Cells from Prior COVID-19 Vaccination or Infection Recognize Omicron

“Memory” CD4+ and CD8+ T cells may explain why Omicron causes less severe disease than previous COVID-19 variants.

The highly infectious Omicron variant rapidly became the dominant COVID-19 strain. In addition to being more infectious, Omicron (B.1.1.529) is causing unprecedented breakthrough infections among people who are fully vaccinated or have previous immunity.

Notably, however, Omicron is causing less severe disease than prior variants, such as Delta. A new study, led by researchers at the Karolinska Institutet, found this may result from “memory” T cells developed during mRNA vaccination or prior COVID-19 infection.

The investigators collaborated with the Karolinska University Hospital in Sweden to collect blood samples from 40 vaccinated persons, 48 persons who had recovered from a mild or severe COVID-19 infection, and 48 persons who had not been vaccinated nor infected.

Samples from the vaccinated group were taken 6 months after their second dose of either the Pfizer-BioNTech or Moderna COVID-19 vaccines. The previously infected group gave samples 9 months after their confirmed infection in spring 2020, and the healthy non-vaccinated participants gave samples at the end of 2020.

The investigators identified CD4+ and CD8+ T cells specific to the COVID-19 spike protein, induced by mRNA vaccination or previously contracting COVID-19. Pairwise comparisons across groups revealed the spike-reactive CD4+ and CD8+ T cells displayed similar phenotypic traits, functional attributes, and memory distributions to both the ancestral COVID-19 strain as well as the Omicron variant.

T cells in both the previously infected and vaccinated groups recognized the Omicron spike protein, but the best response was observed among the mRNA vaccinated individuals.

The investigators concluded that the SARS-CoV-2 spike-specific CD4+ and CD8+ “memory” T cell responses, particularly as a result of mRNA vaccination, remained intact against Omicron.

“These results suggest that booster immunization may provide benefits that extend beyond the induction of neutralizing antibodies to enhance protection against recurrent episodes of severe COVID-19,” said Marcus Buggert, the study’s lead author. ““We now want to understand why the response differs from one individual to the next and if a third vaccine dose can augment the T cell response to omicron even more.”