BACKGROUND: In general, COVID-19 vaccines are safe and effective, but minor adverse effects are common. However, adverse effects have not been measured in several countries including Greece. OBJECTIVE: To estimate the prevalence of adverse effects after the first COVID-19 booster dose, and to identify possible risk factors. MATERIAL AND METHODS: We conducted a cross-sectional study with a convenience sample in Greece during November 2022. We measured several adverse effects after the booster dose, such as fatigue, headaches, fever, chills, nausea, etc. We considered gender, age, chronic disease, self-assessment of health status, COVID-19 diagnóstico, and self-assessment of COVID-19 course as possible predictors of adverse effects. RESULTS: In our sample, 96% developed at least one adverse effect. Half of the participants (50.2%) developed one to five adverse effects, 35.9% developed six to ten adverse effects, and 9.5% developed 11 to 16 adverse effects. Mean number of adverse effects was 5.5. The most frequent adverse effects were pain at the injection site (84.3%), fatigue (70.8%), muscle pain (61%), swelling at the injection site (55.2%), headache (49.8%), fever (42.9%), and chills (41%). Females developed more adverse effects than males (p < 0.001). The prevalence of adverse effects of COVID-19 vaccines was statistically significant and positively associated with the severity of COVID-19 among COVID-recovered individuals (p < 0.05). Moreover, younger age was associated with increased adverse effects (p < 0.001). CONCLUSIONS: Almost all participants in our study developed minor adverse effects after the booster dose. Female gender, COVID-19 patients with worse clinical course, and younger individuals experienced more often adverse effects.