Viral load dynamics in asymptomatic and symptomatic patients during Omicron BA.2 outbreak in Shanghai, China, 2022: a longitudinal cohort study

Abstract

BackgroundsThe SARS-CoV-2 virus has caused global outbreaks, including the recent Omicron BA.2 wave in Shanghai in spring 2022. While, the viral load dynamic of Omicron infections with different clinical severity was still unclear.MethodsA prospective cohort was conducted on 48,830 hospitalized COVID-19 patients in three hospitals in Shanghai, China, from 23 March and 15 May 2022. Regular nucleic acid testing was performed and the Cycle threshold (Ct) value tested by RT-PCR was used as a proxy of viral load. The viral load dynamic by different clinical severity since initial detection and the risk factors were analyzed.ResultsThe study included 31% asymptomatic cases, 68% mild-moderate cases, 1% severe cases, 1.29% critical and fatal cases. 57% of patients tested positive upon admission, average Ct value remained stable with peak viral concentrations occurring at 4 days (median Ct value of 27.5), followed by a decrease with a viral shedding time (VST) of 6.1 days (IQR, 4.0-8.8 days). Omicron viral load varied by age and clinical severity, but peak Ct values occurred at similar times. Unvaccinated status, age over 60, and comorbidities were associated with higher peak viral concentrations and longer shedding durations. Asymptomatic cases had a 40% contagious probability within 6 days of detection. Mild-moderate cases and severe cases had a 27%, >50% probability of infectiousness post-symptom resolution, respectively.ConclusionThe initial Ct value could forecast severe consequences. Unvaccinated older people with specific comorbidities are the high-risk groups associated with high viral load and long shedding duration.