A medical worker collects a sample from a resident at a nucleic acid testing site in Zhenhai district in Ningbo, Zhejiang province on Dec 6, 2021. [Photo provided to China Daily]

Two COVID-19 cases reported by Ningbo's Haishu district on March 30 have the Omicron sub-variant BA.2, local authorities announced at a news conference that same day.

The two cases - a married couple - have been sent to a designated hospital for treatment.

One of their contacts tested positive during nucleic acid testing. The results for the rest turned out to be negative.

The city's Zhenhai district reported another case on the morning of March 31. Three days earlier, a truck driver in the district was identified as an asymptomatic carrier of COVID-19.

Xiangshan county and the county-level city of Yuyao administered by Ningbo also reported one case each on the morning of March 30.

Scientists have pointed out that while BA.2 is more transmissible than the original Omicron strain, it causes less severe disease.