Background. As a key component of existing public healthcare programs, vaccination is considered a very important medical intervention due to its cost effectiveness. Despite their effectiveness in lowering risk of diseases that in the past caused significant mortality and morbidity, vaccines carry with them some risk. In most cases the side effects are minor and self-limiting, but there have been reports of rare but serious adverse effects associated with vaccines. In Kenya, the Pharmacy and Poisons Board (PPB) together with the National Vaccines and Immunization Programme (NVIP) maintain surveillance to monitor vaccine safety. This is mostly passive surveillance through spontaneous reporting of Adverse Events Following Immunization (AEFIs).Evaluation of the data reported is important to come up with more precise and accurate methods of assessing and minimizing the risks associated with vaccines to ensure public trust in the immunization program. Objective. The main objective of this study was to analyze AEFI reported data at the PPB and two hospitals in Nairobi between January 2015 and December 2018. Methods. The study was descriptive and divided into two parts. The first part involved retrospective collection of AEFI reports at the Pharmacovigilance Department of the PPB, NVIP and selected hospitals in Nairobi. Data was extracted from AEFI reports, suspected adverse drug reaction reporting forms and electronic records. The second part involved interviews of key informants at each study site. Results: Of the 187 AEFIs reports analyzed, 93 (49.7%) were from females and 94 (50.3%) were from males. About 65 (35%) of the AEFIs occurred in persons aged between 10 to 15 years.

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