Background: Immunization which has been proven to be the most cost-effective intervention in promoting child health and as such one of the success stories in public health. They are safe and effective globally, but are usually not without adverse events occasionally.

Methods: A cross-sectional descriptive study was carried out over one-year, among children aged 6weeks to 18months attending the Paediatric outpatient clinic in Rivers State University Teaching Hospital (RSUTH). A sample size of 370 children who met the inclusion criteria were recruited.

Results: Of 370 mother-baby pair, 231(62.4%) mothers were aged 30-39years and attained tertiary education 250(67.6%). Most babies were aged 0-5months (58.1%) with M:F ratio of 1.06:1. Most of the mothers 158(78.6%) were told of possible Adverse events following immunization (AEFI) by the health workers prior to vaccination. AEFI was reported in 201(54.3%) children, majority were generalized reactions 170(84.6%) and none had rare adverse events. Most occurred at the 2nd immunization visit 158(78.6%), 1-11hours following vaccination (74.6%). The commonest AEFI were fever 169(84.1%) and swelling at the injection site 92(45.8%). Only 6(3.0%) mothers reported the AEFI incidence and 4(2.0%) were admitted in the hospital for fever following vaccination. No child with AEFI had their immunization schedule disrupted.

Conclusion: There was a high incidence of AEFI in children attending the Paediatric outpatient clinic, with poor level of reporting of such incidence. Education of the public about AEFI, in addition to strengthening the country’s vaccination surveillance system, will improve its detection, reporting and monitoring, which are critical in managing vaccine reactions.