Immunogenicity and safety of an intradermal fractional third dose of ChAdOx1 nCoV-19/AZD1222 vaccine compared with those of a standard intramuscular third dose in volunteers who previously received two doses of CoronaVac: A randomized controlled trial

Introduction: The CoronaVac vaccine is widely used in Thailand to combat the coronavirus disease 2019 (COVID-19) pandemic. The limited immunogenicity of this vaccine is a concern, especially because of expanding delta variant outbreaks. A third boost may enhance antiviral immune responses.

Methods: This non-inferiority randomized controlled trial evaluated the immunogenicity and safety of an intradermal (ID) fractional third dose of AZD1222 vaccine compared with those of a standard intramuscular (IM) third dose. Participants were enrolled from August 9, 2021 to August 13, 2021 at Chulabhorn Hospital, Bangkok, Thailand. The eligibility criteria were age 18 years or older and prior two-dose Coronavac vaccination completed at least 2 months previously. Participants were randomly assigned to one of three groups by block randomization: (i) standard dose by IM administration (IM), (ii) 20% of the standard dose ID (ID1), or (iii) 40% of the standard dose ID (ID2). The primary endpoint was the geometric mean ratio of anti-receptor binding domain antibody in the ID1/ID2 vs. the IM groups 14 days post-vaccination.

Results: A total of 125 participants were randomized (IM, n = 41; ID1, n = 41; and ID2, n = 43). One participant was lost to follow-up by day 14 post-vaccination in the ID1 group. The geometric mean ratio (95% confidence interval) of anti-receptor binding domain antibody was 0.94 (0.80-1.09) in the ID1 group and 1.28 (0.95-1.74) in the ID2 group. Immunogenicity in both ID groups met the non-inferiority criteria. Local adverse events were more common in the ID groups than in the IM group but were mostly mild to moderate in severity.

Conclusion: An ID fractional third dose of AZD1222 was non-inferior to a standard IM third dose among individuals previously vaccinated with CoronaVac. Adverse events associated with the ID fractional third dose included mild to moderate local site reactions. This vaccination strategy may help conserve vaccine supply.