Relative vaccine effectiveness of ChAdOx1/AZD1222 vaccines as booster dose via intradermal injection with a one-fifth dose compared with the intramuscular injection in the prevention of SAR-CoV-2 infections in Phuket: A retrospective cohort study.

• A cohort study of 138,264 cases was conducted in an isolated island population of 414,471. • Intradermal injection with one-fifth of the standard dosage showed no difference in vaccine efficacy. • Males who received the intradermal injection had a higher survival rate. • Booster-dosed males have a higher survival rate for COVID-19 infection than females. This study assessed the real-world relative vaccine effectiveness of the ChAdOx1/AZD1222 vaccine given intradermally at one-fifth dose compared to the standard intramuscular injection, following the completion of 2 doses of CoronaVac, due to limited vaccine availability in Thailand during the Coronavirus disease 2019 (COVID-19) pandemic. This retrospective cohort study used 138,264 records from Vachira Phuket Hospital, Phuket, Thailand. The records were divided into 2 groups: 49,387 recipients received one-fifth doses via intradermal injections, and 88,877 recipients received standard-dose intramuscular injections from September 14 to October 3, 2021, with follow-up until December 31, 2021. Relative vaccine effectiveness for the cohorts was estimated using Cox regression, adjusting for demographic and clinical risk factors. The adjusted hazard ratio between the intradermal and intramuscular groups was 0.88 (95% Confidence Interval 0.76-1.02, P = 0.09), indicating a nonsignificant protective factor for the intradermal group. Further stratified analysis revealed no significant difference between the 2 groups. The 21 and 28-day postvaccination periods minimized the possibility of confounding due to differences in the cohorts' timeframes. A booster dose of ChAdOx1/AZD1222 given intradermally at one-fifth dose did not show a significant difference compared to the standard intramuscular injection. [ABSTRACT FROM AUTHOR]