Adverse events following COVID-19 vaccination in Kwara State, North-central Nigeria

**Background:** The COVID-19 pandemic, declared by WHO in March 2020, has caused significant global morbidity and mortality. Vaccination remains the mainstay of control, but vaccine hesitancy driven by fear of adverse events threatens uptake. In Nigeria, skepticism about COVID-19 vaccine safety is high. The AstraZeneca viral vector vaccine was the only vaccine available in Kwara State during the study period. This study aimed to analyze adverse events following AstraZeneca COVID-19 vaccination in five major immunization centers in Kwara State, North-central Nigeria, to provide local safety data.

**Methods:** This was a retrospective descriptive study of adverse events reported from five immunization centers (Primary Health Care Center Idofian, Civil Service Hospital, University of Ilorin immunization center, and University of Ilorin Teaching Hospital immunization centers in Oke-oyi and Amilegbe) from March to July 2021. All individuals receiving the AstraZeneca vaccine were asked to report adverse reactions. Those who reported adverse events were monitored by twice-daily phone calls and home visits and were followed for 3 months after vaccination. Data collected included gender, age, vaccine dose, dates of vaccination and presentation, use of antipyretics, interventions, and outcomes. Local adverse events included injection-site pain, itching, redness, swelling, and induration. Systemic adverse events included fever, headache, generalized body weakness/pain, malaise, muscle ache, joint pain, fatigue, dizziness, chills/rigor, vomiting, diarrhea, abdominal pain, and changed mental status. Severity was graded as mild, moderate, or severe according to WHO criteria as modified by Bae et al. Outcomes were classified as full resolution, resolution with sequelae, or fatality. Statistical analysis used SPSS version 26 with Chi-square tests; p<0.05 was considered significant.

**Key Results:** Of 5,816 vaccinated individuals, 95 reported adverse events, yielding an incidence of 1.6%. The mean age was 41.6±13.7 years, with the largest age group being 30-39 years (24.2%). Females predominated (51.6%). Most adverse events occurred after the first vaccine dose (90.5%). Onset was within the first day of vaccination in 58.9% of cases, with 5.3% occurring within 6 hours. Almost all adverse events (95.8%) were systemic with or without local reaction; 35.8% were both local and systemic, and 4.4% were local only. Severity was mild in 81.1%, moderate in 16.8%, and severe in 2.1% (two cases). One severe case was serious and life-threatening, involving anaphylactic shock, hyperpyrexia, severe headache, loss of taste, chest discomfort, difficulty breathing, and inability to communicate in a 30-year-old woman; she was resuscitated, admitted, and discharged after 48 hours with full recovery and no sequelae at 3 months. No fatalities were reported. Most adverse events (80%) required no therapeutic intervention; 2.1% required hospital admission. Full recovery occurred in 98.9% of cases. Time to outcome was 2 days in 45.3% of cases. The most common local reaction was injection-site pain (100% of those with local reactions). The most common systemic reactions were headache (51.6%), fever (49.5%), generalized body weakness (40.7%), and generalized body pain (23.1%). No significant associations were found between adverse event type or severity and age, gender, vaccine dose, or time to onset (p>0.05 for all comparisons).

**Clinical Implications:** This study demonstrates a low incidence (1.6%) of adverse events following AstraZeneca COVID-19 vaccination in a general population in North-central Nigeria, with the vast majority being mild-to-moderate, self-limiting, and resolving without intervention. No deaths, blood clots, pulmonary embolism, deep vein thrombosis, focal neurological deficits, or cardiac events were observed within 3 months of follow-up. These findings contrast sharply with higher adverse event incidences reported in healthcare worker populations (79.4-93.5%), suggesting that adverse event rates in the general population may be substantially lower than previously reported. The low incidence may also reflect underreporting, which the authors acknowledge as a limitation due to poor adverse event reporting culture in Nigeria. The absence of serious adverse events and fatalities provides important safety reassurance that could help address vaccine hesitancy in Nigeria, where only 2.6% of the population was fully vaccinated as of February 1, 2022. The study supports continued health education efforts to alleviate vaccine safety concerns and promote uptake.