Shanghai has reported a surging number of new local COVID-19 cases since March 1 and a large majority of them are asymptomatic patients.Professor Hu Bijie, director of the infectious diseases department at Zhongshan Hospital and a member of Shanghai's COVID-19 treatment experts' team, explains the reasons behind the asymptomatic cases and offers prevention tips for local expatriates.1. Why are a large proportion of COVID-19 cases in Shanghai asymptomatic?Hu: Firstly, the characteristics of the dominant Omicron variant, which has weak virulence and is highly contagious, make many of the infected people asymptomatic. The original coronavirus often led to lower respiratory infection with pneumonia. Oxygen machines were needed for some patients.