unassigned: Breakthrough infections post COVID-19 vaccination occur with the emerging variants of the SARS-CoV virus which might be either due to the newer variants escaping immune response or the waning of antibodies over time. However, there is lack of long term follow up evidence on the waning of immune response following inactivated COVID-19 vaccine. unassigned: A retrospective, observational study was conducted on serum samples of individuals who had received two doses of BBIBP-CorV vaccine. Individual's antibody responses were evaluated based on IgG anti-S and neutralizing antibodies measurements. Antibody samples were categorized into four groups, defined by the time interval from the individual's receipt of the BBIBP-CorV vaccine: < 30 days, 30-90 days, 91-180 days and >180 days. unassigned: A total of 6,668 serum samples from inactivated BBIBP-CorV vaccine recipients were analyzed for IgG anti-S and neutralizing antibodies. 571 (8.6%) samples were tested during the first 29 days interval post vaccination, 3642 (54.6%) were tested during 30-90 days interval, 2173 (32.6%) samples were tested during 91 to 180 days interval and 282(4.2%) were tested at >180 days interval post vaccination. We found that more than 50% of the individuals had antibody titers below the average cut-off range at the 91-180 days interval post vaccination. Older age (> 60 years), male gender, chronic kidney disease, hypertension, immunodeficiencies and increased interval post vaccination emerged as independent risk factors associated with lower immune response. unassigned: Inactivated BBIBP-CorV vaccine recipients, based on age, gender and associated comorbid conditions might need booster doses at an earlier interval than the currently followed six months interval.