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

Objectives: 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.

Method: 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.

Results: 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.

Conclusion: A booster dose of ChAdOx1/AZD1222 given intradermally at one-fifth dose did not show a significant difference compared to the standard intramuscular injection.