# Intradermal Fractional ChAdOx1 nCoV-19 Booster Vaccine Induces Memory T Cells: A Follow-Up Study

## TL;DR

This study shows that a reduced intradermal dose of the ChAdOx1 nCoV-19 vaccine booster can effectively stimulate memory T cell responses.

## Contribution

The study demonstrates that fractional intradermal vaccination can elicit strong T cell responses comparable to intramuscular vaccination.

## Key findings

- Intradermal fractional ChAdOx1 nCoV-19 booster induces higher S1-specific CD8+ memory T cell responses than intramuscular vaccination.
- Effector T cell responses remain detectable three months post-vaccination.
- Memory T cell phenotypes from mRNA and viral vector vaccines show similar trends.

## Abstract

The administration of viral vector and mRNA vaccine booster effectively induces humoral and cellular immune responses. Effector T cell responses after fractional intradermal (ID) vaccination are comparable to those after intramuscular (IM) boosters. Here, we quantified T cell responses after booster vaccination. ChAdOx1 nCoV-19 vaccination induced higher numbers of S1-specific CD8+ memory T cells, consistent with the antibody responses. Effector memory T cell phenotypes elicited by mRNA vaccination showed a similar trend to those elicited by the viral vector vaccine booster. Three months post-vaccination, cytokine responses remained detectable, confirming effector T cell responses induced by both vaccines. The ID fractional dose of ChAdOx1 nCoV-19 elicited higher effector CD8+ T cell responses than IM vaccination. This study confirmed that an ID dose-reduction vaccination strategy effectively stimulates effector memory T cell responses. ID injection could be an improved approach for effective vaccination programs.