Good vaccine safety and reliability are essential to prevent infectious disease spread . A small but significant number of apparent adverse reactions to the new COVID-19 vaccines have been reported . Here, we aim to identify possible common causes for such adverse reactions with a view to enabling strategies that reduce patient risk by using patient data to classify and characterise patients those at risk of such reactions . We examined patient medical histories and data documenting post-vaccination effects and outcomes . The data analyses were conducted by different statistical approaches followed by a set of machine learning classification algorithms . In most cases, similar features were significantly associated with poor patient reactions . These included patient prior illnesses, admission to hospitals and SARS-CoV-2 reinfection . The analyses indicated that patient age, gender, allergic history, taking other medications, type-2 diabetes, hypertension and heart disease are the most significant pre-existing factors associated with risk of poor outcome and long duration of hospital treatments, pyrexia, headache, dyspnoea, chills, fatigue, various kind of pain and dizziness are the most significant clinical predictors . The machine learning classifiers using medical history were also able to predict patients most likely to have complication-free vaccination with an accuracy score above 85% . Our study identifies profiles of individuals that may need extra monitoring and care (e.g., vaccination at a location with access to comprehensive clinical support) to reduce negative outcomes through classification approaches . Important classifiers achieving these reactions notably included allergic susceptibility and incidence of heart disease or type-2 diabetes.