A Comparative Study of Clinical Characteristics and COVID-19 Vaccine Effectiveness Against SARS-CoV-2 Variants: Wild-Type, Alpha, Delta, and Omicron in Beijing, China.

Background: To compare the clinical characteristics of symptoms and laboratory findings across SARS-CoV-2 variants (Wild-type, Alpha, Delta, Omicron) and assess the effectiveness of COVID-19 vaccines in preventing symptoms and laboratory abnormalities.

Methods: We conducted a retrospective cohort study of individuals with SARS-CoV-2 infection at Beijing Ditan Hospital, Capital Medical University. Patients were grouped by the SARS-CoV-2 variant (Wild-type, Alpha, Delta, Omicron) based on whole-genome sequencing. Thirteen symptoms and 22 laboratory indices were compared across variants, and Omicron patients were further analyzed by vaccination status with generalized estimating equations (GEE) model.

Results: One thousand four hundred and thirteen participants were included for the analysis as following: Wild-type group (N=322), Alpha group (N=67), Delta group (N=98), and Omicron group (N=926). Omicron patients showed the highest proportion (30.1%) of respiratory symptoms across groups. Patients displayed normal laboratory manifestation, except for inflammatory markers, coagulation function index and glucose. Meanwhile, the Omicron variant was featured by higher inflammatory biomarkers (serum amyloid A protein [SAA] and C-reactive protein [CRP]). In addition, Omicron patients with three or more vaccine doses had fewer symptoms and higher values of SAA and CRP compared to those with fewer than three doses. Results of GEE showed, when compared with ≤ 1 vaccine dose, red blood cell count, white blood cell count, neutrophil count, platelet count, haemoglobin, and C-reactive protein in patients with ≥ 3 doses of vaccine significantly increased; while aspartic transaminase, creatine kinase, blood urea nitrogen, activated partial thromboplastin time, prothrombin time and thrombin time dramatically decreased, respectively.

Conclusion: Omicron variant resulted in abnormal inflammatory response. Individuals with three or more vaccine doses are more likely to experience fewer symptoms and have stronger protection against the virus. This study highlights key differences in symptom onset and laboratory profiles across SARS-CoV-2 variants, reinforcing the importance of three vaccine doses in providing strong protection against the Omicron variant.