For a study, researchers sought to understand that self-reported COVID-19 vaccination during the COVID-19 pandemic may benefit quick assessments of vaccine effectiveness (VE) when source material, such as immunization information systems (IIS), was challenging to obtain. They examined how VE estimations were affected by the discrepancy between self-reported COVID-19 vaccination status and source data. Adults over 18 who were admitted to 18 U.S. hospitals between March and June 2021 were included, along with COVID-19 cases and SARS-CoV-2 negative controls. A few questions were put to the recipients of the COVID-19 vaccine. Abstractors examined IIS, medical records, and other sources for information on vaccines. Self-report and documentation of immunization status were compared using unweighted kappa with 95% CIs, and the percentage of agreement was assessed. The effectiveness of VE in avoiding COVID-19 hospitalization after complete immunization (2 doses of mRNA generated ≥14 days before disease onset) was then separately assessed. About 594 out of 2,520 participants could not be assigned to a vaccination group due to insufficient self-reported vaccination data; these patients tended to have more severe illnesses. With a kappa of 0.9127 (95% CI: 0.9109-0.9145), 1,924 patients who had access to both self-report and source documentation demonstrated 95.0% (95% CI: 93.9-95.9%) agreement. VE was 85% (95% CI: 81-89%) based just on source documentation data, while it was 86% (95% CI: 81-90% ) based only on self-report data. The inability to self-report whether or not they had received the COVID-19 vaccine was experienced by one-fourth of hospitalized patients. There was good agreement between patient self-report data and source-confirmed status. Self-report may be a reliable source of information regarding COVID-19 immunization for quick VE evaluation and public health responses.