Among administering AZD1222, mild fatigue, fever, muscle ache, and headache were commonly observed in at least 87% of participants. Local and systematic reactions were highest on the first day upon vaccination, but most adverse reactions were mitigated by administering prophylactic paracetamol (acetaminophen). By the end of 4 weeks, serological antibodies were at their peaks in the ChAdOx1 group and were even higher in those receiving a second vaccine dose. A few individuals had significantly higher levels of serum neutralizing antibody than most AZD1222 group that could potentially imply they were previously asymptomatic carriers. These carriers could have developed a stable memory T-cell response in mitigating the effects of COVID-19 symptoms. Similar to AD16.COV2.S, adenovirus-based vaccines were capable of inducting strong cellular immune responses and ChAdOx1 was shown to increase spike protein-specific effector T-cell response as early as the first week with peaked response by the end of week 2. This response was sustained for up to 8 weeks, but second dose of administration did not confirm an increase in cellular responses. Further research would involve testing in older adults from 50 to 78 years old, as they tend to have more severe and fatal symptoms of COVID-19. Previous ChAdOx1-based vaccine against influenza had shown its effectiveness in inducing immunogenicity. Thus, expanding the clinical trial to older populations should still be relatively safe and effective.