Vaccination is the most promising approach for ending or containing the SARS-CoV2 pandemic. However, serious post-COVID vaccine reactions including immunocytopenia (ITP) syndrome has been increasingly reported. Several factors cause increased risks including multiple doses, age-dependent heterogeneity in immune-responses, platelet cross-reactions with microbial components, and Long-COVID syndrome. Thus, in the absence of a widely available specific therapeutics, vigilance is important while more studies are imperative. Using a structured questionnaire sent to different regions in Saudi Arabia, we conducted a comprehensive investigation on the frequency, rates, disease patterns, and patient demographics of post-COVID-19 side effects on febrile patients after three major vaccines. Results indicated the majority administered Pfizer BioNtech vaccine (81%, n=809); followed by AstraZeneca (16%, n=155); and Moderna (3%, n=34). In overall 998 participants, 74% (n=737) had no serious symptoms; however, 26.2% (n=261) revealed typical syndrome. In a focused group of 722, shortness of breath (20%), bruises or bleeding (18%), inattention (18%), GIT symptoms (17.6%), skin irritation (8.6%), and anosmia and ageusia (8%) were the most prominent. The onset time was mostly in 1-3 days in 49% (n=128), followed by 4-7 days in 21.8% (n=57), 8-14 days in 16.5% (n=43), and more than a month in 12.6% (n=33). The onsets occurred mostly after the first, second, or both doses 9%, 10%, and 7%, respectively. The frequency of symptoms was significantly higher among after Moderna ® vaccine (P-value = 0.00006) and it was significantly lower in participants who received Pfizer (P-value 0.00231). We did not find significant difference in symptoms related to differences in regions. Similarly, the region, age, gender, education, and nationality had no influence in the dose and onset timings. The findings of this study have significant clinical implications in disease management strategies, preventive measures, and vaccine development. Future vertical studies would reveal more insights into the mechanisms of post-COVID vaccine syndrome.