Immunogenicity and reactogenicity of mRNA COVID-19 vaccine booster administered by intradermal or intramuscular route in Thai Older adults.

ABSTRACT

INTRODUCTION: Intradermal (ID) vaccination may alleviate COVID-19 vaccine shortages and vaccine hesitancy. METHODS: Persons aged ≥65 years who were vaccinated with 2-dose ChAdOx1 12-24 weeks earlier were randomized to receive a booster vaccination by either ID (20-mcg mRNA1273 or 10-mcg BNT162b2) or intramuscular (IM) (100-mcg mRNA1273 or 30-mcg BNT162b2) route. Anti-receptor binding domain (anti-RBD) IgG, neutralizing antibody (NAb), and IFNγ-producing cells were measured at 2-4 weeks following vaccination. RESULTS: Of 210 participants enrolled, 70.5% were female and median age was 77.5 years (interquartile range 71-84). Following booster dose, both ID vaccination induced 37% lower levels of anti-RBD IgG than IM vaccination of the same vaccine. NAb titers against ancestral and omicron BA.1 was highest following IM mRNA-1273 (geometric mean 1,718 and 617), followed by ID mRNA-1273 (1,212 and 318), IM BNT162b2 (713 and 230), and ID BNT162b2 (587 and 148), respectively. Spike-specific IFNγ responses were similar or higher in the ID groups when compared with IM groups. ID route tended to have lower systemic AEs, although more local AEs reported in ID mRNA-1273 group. CONCLUSIONS: Fractional ID vaccination induced lower humoral but comparable cellular immunity compared to IM and may be an alternative option for older people.