Rationale: Pentavalent vaccines offer significant public health benefits by protecting against five major diseases with a single injection. However, concerns have been raised in various studies regarding potential associations between combined vaccines and conditions such as autism, febrile seizures, sudden unexpected death in infancy, demyelinating disorders, and neurodevelopmental disorders. Objective: This study aimed to evaluate the safety of pentavalent vaccines administered to infants aged between 6 and 14 weeks. Methods: A total of 423 infants, all aged 6 weeks and receiving their first pentavalent vaccine at selected healthcare facilities, were recruited for the study after obtaining informed consent from their mothers or caregivers. The infants were administered three doses of the vaccine at 6, 10, and 14 weeks. Mothers and caregivers were provided with diaries and thermometers to monitor and record any Adverse Events Following Immunization (AEFI) observed in their babies after each vaccine dose. Follow-up was conducted through telephone calls to ensure accurate monitoring and recording of any identified events. Results: The study identified various AEFIs in the infants following their routine immunizations. These included pain at the injection site, fever, swelling at the injection site, vomiting, refusal to feed, excessive crying, coughing, rash, stooling, restlessness, and severe local reactions. Fever was the most commonly reported systemic AEFI, with incidence rates decreasing from 66.98% after the first dose to 55.37% after the third dose. Pain and swelling at the injection site were the most frequently reported local AEFIs, with their incidence also decreasing from the first to the third doses. No statistically significant differences were observed in the occurrence of AEFIs across the three doses. Conclusions: The pentavalent vaccine was found to be safe for infants in the Federal Capital Territory (FCT), Nigeria, with the observed AEFIs being generally mild and decreasing in frequency with subsequent doses. Recommendations: Further studies should be conducted to monitor long-term safety and potential rare adverse effects of pentavalent vaccines. Additionally, public health education should emphasize the safety and benefits of pentavalent vaccines to increase vaccination rates and reduce vaccine hesitancy. Significance Statement: This study underscores the safety of pentavalent vaccines in infants, reinforcing their role in preventing multiple serious diseases through a single immunization schedule. The findings support the continued use and promotion of pentavalent vaccines in public health programs, particularly in regions with high infant mortality rates and limited healthcare resources.

The authors have declared no competing interest.

The authors affirm that no funding, whether financial or non-financial, was received for the execution of this study. This research was conducted independent of any external financial or material support. We uphold transparency in disclosing the absence of funding, underscoring the authors' commitment to conducting unbiased research driven solely by scientific inquiry.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Approval for the study was obtained from the Ethics Committee of the Federal Capital Territory Health Ethics Committee, with Protocol Approval Number: FHREC/2015/01/62/21-10-15. Administrative approvals were also secured from the Federal Capital Territory Development, FCT Primary Health Care Board, and Asokoro District Hospital, FCT. Prior to participation, all study participants were provided with oral explanations regarding the study's objectives and the assurance of anonymity. Informed consent was obtained from each participant before the commencement of the interview process. Confidentiality of all collected data was strictly maintained throughout the study duration, ensuring the privacy and integrity of participants' information.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present work are contained in the manuscript