Scientists stressed the need to inoculate the vulnerable while urging vigilance against the newly discovered Omicron subvariants, and called for more research to understand their quick transmissibility and severity.

Shanghai reported seven new COVID-19 patient deaths on Wednesday, following three reported on Monday and seven on Tuesday. Fifteen of these were age 60 or older, and all of them suffered from underlying diseases.

Only two were vaccinated, according to the Shanghai Municipal Health Commission.

Of the people infected in the current wave of the COVID-19 outbreak in Shanghai, 19 percent are older than 60, Sun Xiaodong, deputy head of the Shanghai Center for Disease Control and Prevention, said on Wednesday.

Shanghai has about 5.8 million people who are 60 or older. About 40 percent of them have not been fully vaccinated and 60 percent have not received a booster shot. "In general, the vaccination rate (of the elder group) is rather low," Sun said at a daily news briefing.

Li Zhiqiang, vice-president of Zhongnan Hospital of Wuhan University in Hubei province, told People's Daily that the deaths serve as a warning that the epidemic is still serious and it is imperative to vaccinate seniors.

A study published last week by the University of Hong Kong analyzed the latest outbreak in Hong Kong. It shows the Omicron BA.2 variant's short incubation period and high transmissibility make it especially dangerous for vulnerable populations, especially unvaccinated seniors.

According to Hong Kong public health authority data, the overall case fatality rate of the epidemic was 0.74 percent, but soared to 2.97 percent for the unvaccinated.

For people who received one shot of a vaccine, the fatality rate was 0.97 percent, while it was 0.16 percent for those who had two doses and 0.04 percent for those with three doses.

While previous studies have indicated the Omicron variant may cause milder disease, the Hong Kong study found the intrinsic severity of BA.2 may not be much lower than the ancestral strain in terms of fatality risk for unvaccinated patients.

"Our findings highlighted the importance of achieving a high coverage of vaccination especially in older adults, and the need to reassess public health and social measures in control of epidemics in response to a more transmissible SARS-CoV-2 variant in the future," the study said. The paper is now awaiting peer review.

New mutations detected

Zhong Nanshan, one of China's leading respiratory disease experts, said in an article published in early April by the journal National Science Review that the Omicron variant has a higher mortality risk than seasonal influenza, and that allowing it to spread unabated would likely result in social instability and the emergence of other novel variants with greater infectivity.

Last week, the World Health Organization said it has started tracking two newly detected Omicron subvariants: BA.4 and BA.5, which have been circulating at low levels in several countries in Africa and Europe, including South Africa, Botswana, Germany and Denmark.

Both strains exhibited additional mutations on their spike protein, the part of the virus used to invade human cells, as well as other mutations elsewhere.

Laboratory studies said these mutations are associated with "potential immune escape characteristics," the WHO said.

Preliminary data indicate these two new strains are not more contagious or deadly than the original BA.1 Omicron variant, but more research is needed to understand their impact on immune escape potential, it added. The Omicron subvariant BA.2 is currently the dominant strain in the world.

Tulio de Oliveira, a biology researcher in South Africa, told the journal Nature that samples of the new strains have been sent to researchers around the world, including those at the United States' National Institutes of Health and the Chinese Center for Disease Control and Prevention.

The United Kingdom's Health Security Agency said another recently discovered Omicron subvariant called XE may be the most contagious strain known to date. XE is a recombinant strain that combines the characteristics of the BA.1 and BA.2 variants.

The latest update from the UK health agency showed cases of the XE strain had nearly doubled in Britain, going from 637 cases on March 25 to 1,125 cases as of April 5.The agency said that recombinant variants are not an unusual occurrence, especially when there are several variants in circulation.

"The best way to stop variants developing or spreading is to keep pushing down infection rates and the transmission of the virus in our community," it said.