The aim of this study was to examine the effectiveness and potential adverse reactions when providing a different Pfizer-BioNTech booster shot to 235 volunteers who had previously received the BBIBP-CorV Sinopharm primary vaccination series. Between February and December 2022, a questionnaire-based cross-sectional study was conducted in Duhok, located within the Kurdistan Region of Iraq. The individuals included in the study were adults aged 18 and above, who had received a Pfizer-BioNTech booster shot following the completion of a two-dose vaccination regimen with Sinopharm (BBIBP-CorV). The findings revealed that among those vaccinated with BBIBP-CorV, there were breakthrough infections at a rate of 4.26%, and no significant correlation was identified between post-vaccination infections and factors such as demographics or medical history. Furthermore, individuals who had a Pfizer booster dose experienced breakthrough infections at a rate of 5.73%, and similarly, no link was discovered between this rate and demographic or medical factors. Additionally, the study uncovered that participants commonly experienced side effects, primarily consisting of mild effects at the injection site. The study implies that both the Sinopharm and Pfizer vaccines demonstrate satisfactory safety profiles. It also suggests that giving a heterologous booster dose to individuals who have finished their primary vaccination with the BBIBP-CorV vaccine offers a significant level of protection against infection.