Discussion
Simple, rapid and functional assays that accurately detect
SARS-CoV-2-specific T cell responses are essential for comparing T cell
immunity across multiple population cohorts and for long-term
assessments of vaccine efficacy. The results show that measuring plasma
IL-2 from SARS-CoV-2 peptide-stimulated whole blood accurately
distinguishes between COVID-19 convalescents and uninfected healthy
blood donors with a high degree of sensitivity and specificity. Plasma
IL-2 was also an excellent biomarker for distinguishing vaccinated and
unvaccinated individuals. Furthermore, T cell responses in previously
unexposed individuals developed early after just one vaccination, were
maintained after a second dose and were comparable to those with a
history of SARS-CoV-2 infection; a finding that was not observed using
ELISpot 18, 19. Positive IL-2 responses were still
detectable up to 78 days post-vaccination, in keeping with observations
that T cell responses are robust; in SARS-CoV-1 and MERS-CoV infected
individuals, T cell responses were still identifiable several years
after infection 20-22.
Reliable biomarkers are integral to assessing vaccine efficiency over
time. Unlike other immunoassays that detect antigen-specific T cell
responses such as ELISPOT and flow cytometry, this virus-specific IL-2
release assay is simple to perform and can be employed across multiple
laboratories for large scale epidemiological studies in conjunction with
testing for neutralising antibodies 23. The IL-2
release assay is a promising tool for use in multi-centre clinical
trials for the development of new vaccines or treatments against novel
SARS-CoV-2 variants, such as the delta variant, as they arise globally.
Further studies of this approach should aid decision making by health
policy makers, including vaccine booster requirements and monitoring of
SARS-CoV-2 infection in immune-supressed and other at-risk populations.