Omicron variant of SARS-COV-2 in Shanghai: Clinical features and inactivated vaccine efficacy in 13,120

Background: Few reports concerning inactivated vaccine efficacy in elderly patients with Omicron infection. We aimed at demonstrating the clinical characteristics of elderly patients with mild disease and assessing the protective effect of the vaccine preliminarily. Methods: 13,120 mild patients who aged beyond 60 years old were included in this study totally, medical records were collected and analyzed. Results: Patients beyond 60 years had more chronic comorbidities, significantly lower ORF1ab and N gene CT values, and longer time of nucleic acid conversion than other age groups. Higher CT value of ORF1ab and N gene were found in older patients who received a booster dose of vaccine than in those who received two doses. The time of nucleic acid conversion was longest in unvaccinated old patients, with a decreasing trend from those who received two doses to those who received a booster doses. We also used random forest and logistic regression to screen for factors strongly associated with nucleic acid conversion and to predict the time of nucleic acid conversion. Conclusion: For mild patients with Omicron infection, patients aged>60 years had mild clinical symptoms, higher viral loads, and longer time of nucleic acid conversion, when compared with younger patients. The inactivated SARS-CoV-2 vaccine provided effective protection among adults with omicron variant infection, and the effectiveness of three doses of the vaccine was greater than that of two doses of the vaccine. Special attention should be given to elderly patients.