Although the newer variants of SARS-CoV-2 have weakened people’s antibody defenses, the second arm of the immune system (T cell immunity) has retained its functionality.
Most of the research has focused on antibody responses, esp. neutralizing antibodies - for two reasons.
1. A reduction in neutralizing-antibody levels correlates with a greater risk of symptomatic disease.
2. Antibodies are easier to study.
Antibody effectiveness appears to be fragile because a few mutations will cause a reduction in their neutralization capacity.
However, T cells appear to be more resilient. They retain their function as ‘killer’ cells and destroy virus-infected cells, thereby limiting the spread of infection and reducing the possibility of serious illness.
T-cells are resilient because their levels do not drop as quickly as antibodies. Also, T cells can recognize many more sites on the spike protein than can antibodies - this allows T cells to cross-recognize even the highly mutated variants.
Link: Read here
A few studies in this regard:
T cell immune response generated by SARS-CoV-2 vaccines is substantially preserved against the Omicron variant.
CD4+ T cells:
The proportion of CD4+ T cell epitopes potentially affected is 20%. However, the immunity loss mediated by CD4+ T cells could be slightly above 30% - the reason being that some of the affected epitopes are relatively more immunogenic.
CD8+ T cells:
The estimated loss for CD8+ T cells is approximately 20%.
The overall loss in protection:
The reduction observed in T cell immunity is significantly higher than that observed against other variants. If we combine T cell immunity loss with the substantial antibody neutralization loss, the overall protection provided by SARS-CoV-2 vaccines could be significantly lower.
Conclusion:
Though there is a 20-30% loss in T-cell immunity, authors expect the remaining 70 - 80% of on-target T cells induced by SARS-CoV-2 vaccines to reduce morbidity and mortality from Omicron infection.
Link: Read here
T cell responses induced by previous infection or vaccination remain robust against the Omicron variant.
Omicron’s extensive mutations have resulted in reduced susceptibility to neutralizing antibodies. However, most T cell responses, induced by vaccination or natural infection, have been able to cross-recognize the new variant.
Well-preserved T cell immunity to Omicron is likely to contribute to protection from severe COVID-19. The findings of this study supported the early clinical observations from South Africa.
Link: Read here
Divergent SARS CoV-2 Omicron-specific immune (T-cell and B-cell) responses in SARS-CoV-2 vaccine recipients
The study showed that vaccinated people retain T-cell immunity to the SARS-CoV-2 Omicron variant, potentially balancing the reduction observed in antibody neutralization against Omicron. Booster vaccinations may further restore Omicron cross-neutralization by antibodies.
Link: Read here
Highly Cross-Reactive Cellular Immunity to the SARS-CoV-2 Omicron Variant induced by COVID-19 vaccines
AstraZeneca/ Covishield or Pfizer vaccine recipients showed durable CD8+ and CD4+ T cell responses with demonstrable extensive cross-reactivity against both the Delta and Omicron variants. This study showed that current vaccines might provide considerable protection against severe disease due to the SARS-CoV-2 Omicron variant despite the substantial reduction observed in antibody-neutralization.
Link: Read here
Effectiveness of BNT162b2 Vaccine against Omicron Variant in South Africa
Another study showed that the Pfizer vaccine effectiveness is (albeit at a reduced level) maintained against the hospital admissions presumed to have been induced by Omicron, compared to previous variants. An additional COVID-19 vaccine booster dose may balance the reduction in vaccine effectiveness.
Link: Read here
Conclusion:
T-cell responses seem to have remained quite intact and the efficacy data emerging from South Africa could be due to T cells.
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