unassigned: In early 2021, the Ministry of Public Health of Thailand announced heterologous regimens for COVID-19 vaccines using CoronaVac as the first dose followed by ChAdOx1 nCoV-19 at 3 weeks apart. Priority was given to individuals above 60 years old and those who had seven underlying conditions, including obesity. The vaccine regimen was evaluated for safety and immunogenicity in overweight populations in Chiang Mai, Thailand. unassigned: Participants who had a COVID-19 vaccination appointment for the heterologous prime-boost regimen were enrolled. Before each immunization and on day 28 following the second dosage, blood samples were taken, and were examined for anti-spike and neutralizing antibodies by using an indirect ELISA and virus neutralization assays. Safety profile of the vaccine regimen was assessed via a self-recorded diary of adverse events after each vaccination. unassigned: No serious adverse events related to vaccination were reported during study period and the majority of adverse reactions were fatigue and pain at the injection site. The levels of anti-spike IgG were 26.3, 56.4 and 1752.1 BAU/mL at baseline, 21 days after first dose and 28 days after second dose, respectively. At 4 weeks after complete vaccination, the median inhibition rates of neutralizing antibody determined by surrogate neutralization assay against wild type, Delta and Omicron variants were 95.2, 85.0 and 3.8, respectively. Moreover, the NT50 level against wild type and Delta variants determined by pseudotyped virus neutralization assay were 133.3 and 41.7, respectively. The neutralizing activity against Omicron variant was almost lower than cutoff level for detection. unassigned: The heterologous CoronaVac-ChAdOx1vaccination was safe, well-tolerated and able to induce humoral immunity against wild-type and Delta variants but not against the Omicron variant in overweight population.