Non-severe Omicron patients with no or stabilized underlying diseases are unlikely to develop into critical cases, according to latest research based on Shanghai's COVID-19 resurgence in March.

The research, led by Dr Zhang Wenhong, head of Shanghai's COVID-19 treatment team and director of the National Center for Infectious Diseases, signed up 33,816 local SARS-CoV-2 cases, aged 44.5 years on average, with non-severe symptoms at the beginning.

Among them, 22 patients, or 0.065 percent, developed severe or critical infections, all of whom were over 60 years old with underlying diseases, such as heart, brain or lung diseases, hypertension or diabetes. Many of them had not taken COVID-19 vaccines.

The severity rate dropped to zero among the non-risk group, or younger infections with no or stabilized underlying diseases, according to the study.

The study was conducted between March 22 and May 3 at some local COVID-19 designated hospitals, such as Huashan Hospital, Shanghai Sixth People's Hospital, Shanghai Ninth People's Hospital and Shanghai Fourth People's Hospital.

"Compared with (the) Delta (variant), Omicron's relative inability to damage the lungs may result in fewer cases of dangerous pneumonia and respiratory distress," one of the researchers in the study said.

However, the study showed that some initially non-severe Omicron patients could still develop pneumonia.

Patients with COVID-19 pneumonia were older and more likely to have co-morbidities, while young Omicron patients can also develop COVID-19 pneumonia, the researcher noted.

For instance, a recent local case of COVID-19 pneumonia caused by the Omicron variant was reported in a 19-year-old woman who had no obvious risk factors.

"The research indicates that we should focus on the vulnerable groups with accurate medical treatment, while providing basic medical support to non-severe patients to avoid putting too much burden on medical resources during the COVID-19 peak period," the researcher said.

The study also revealed the clinical features of non-severe Omicron patients, which include weeklong symptoms like cough with phlegm, lack of strength and fever.

Nucleic acid tests of infections with mild symptoms turn negative in six days on average, though that is affected by age, underlying diseases and vaccination.

Since it was first identified in November 2021, the Omicron variant of COVID-19 has become the dominant strain in the world. The transmission of Omicron BA.2 is nearly 30 percent higher than Omicron BA.1 transmission and is significantly higher than transmission of earlier non-Omicron variants.

Shanghai reported a total of 626,863 Omicron infections between March 1 and June 4. More than 95 percent of them had non-severe symptoms. Omicron BA.2 and BA.2.2 remain the dominant sub-variants in the city, according to the Shanghai Health Commission.