Comparison of COVID-19 Prevalence and Clinical Presentation During Pre- and Post-Vaccination Eras in Iran

Introduction Since the emergence of COVID-19, SARS-CoV-2 variants have exhibited distinct clinical presentations, influenced by viral evolution and the impact of vaccination. This study compared the prevalence of COVID-19 and symptom profiles during pre- and post-vaccination periods in Iran, focusing on changes driven by variants ( e.g. , Delta, Omicron) and nationwide immunization efforts. Method A cross-sectional study was conducted on 7,051 individuals systematically sampled from 85,262 patients referred to a COVID-19 diagnostic center in northeast Iran (September 2020–March 2022). The data included demographics, symptoms (such as fever, cough, and dyspnea), comorbidities, and PCR results. The pre-vaccination period (waves 3–5) was compared to the post-vaccination period (wave 6, dominated by Omicron) using statistical analyses that included chi-square tests, logistic regression, and the Bonferroni correction. Results SARS-CoV-2 prevalence was 44.6% (95% CI: 43.3–45.8%), declining from 47.2% (pre-vaccination) to 30.4% (post-vaccination; p < 0.001). Symptomatic individuals had higher infection rates (55.5% vs . 28.2%; p < 0.001). Post-vaccination, respiratory symptoms (fever, cough, dyspnea) increased 1.3–1.8-fold among PCR+ cases ( p < 0.001), while anosmia/ageusia remained stable (p = 0.879). Logistic regression identified male sex, symptom presence (except sore throat), and epidemic wave as significant predictors of PCR positivity. Discussions The Omicron wave, despite high vaccine coverage, showed a reduction in overall cases but an increase in respiratory symptoms, suggesting that vaccine-mediated attenuation of severity occurred without curbing transmissibility. Findings align with global reports of Omicron’s upper respiratory tropism but contrast with studies noting reduced fever. Conclusion COVID-19 clinical presentations in Iran shifted post-vaccination, with Omicron associated with milder but more respiratory symptoms. Public health strategies should adapt testing protocols to prioritize respiratory symptoms and emphasize variant-specific boosters.