Study Finds T Cells Better Than Antibodies at Predicting COVID-19 Infection in Households
A prospective study of 484 vaccinated adults exposed to COVID-19 in their households found that antibody levels measured shortly after exposure did not predict who became infected. Instead, early differences in a subset of CD8+ T cells were associated with symptomatic infection. The findings suggest immune markers beyond antibodies may be important for evaluating vaccine performance.
The AB-PROTECT study followed 484 vaccinated adults across 339 Toronto-area households from late 2021 through early 2023, spanning delta and multiple omicron variants. Blood samples were collected within roughly three days of the index case's symptom onset, and nearly half of the 432 participants with complete follow-up developed infection within 28 days. The immune panel included spike-specific IgG and IgA, pseudovirus and live-virus neutralization against several variants, plus T-cell cytokine profiling.
Among 56 participants receiving cellular immune analysis, those who later developed symptomatic COVID-19 showed lower baseline levels of CD8+ T cells co-expressing GM-CSF and TNF. Antibody measurements shifted infection probability by no more than five percentage points across the full range of titers. The authors suggest future studies should measure immune markers before exposure in settings like hospitals or congregate care facilities.
This study could reshape how regulators evaluate vaccine performance, since neutralizing antibody titers currently serve as the primary bridge for efficacy data when variant-specific trials are impractical. If antibody levels near exposure do not distinguish who becomes infected, public health agencies may need to invest in standardized T-cell assays. Individuals and clinicians might also reconsider what "protection" means after vaccination, though the findings require replication in larger, pre-exposure cohorts before policy shifts.