Chinese researchers have found that recovered COVID-19 patients still harbor some immune cells that can fend off the virus at least two years after the initial infection, but updated vaccination is essential in boosting immunization against emerging variants.

The research, published by the medical journal The Lancet Microbe on Friday, recruited nearly 1,200 patients who were infected with the original novel coronavirus during the initial wave of the COVID-19 outbreak that hit hardest in Wuhan, Hubei province, and were discharged from Jan 7 to May 29, 2020.

Researchers analyzed blood samples from some of them during three follow-up visits carried out six months, a year and two years after they were released from hospital. None of them had been reinfected during that period.

Four components of the immune system were examined — immunoglobulin antibodies, neutralizing antibodies, and memory B cells and memory T cells — for their responses against the original strain plus the Delta and Omicron variants.

The study found that while immunity against the original novel coronavirus among these recovered patients had been retained for two years, immune responses against novel strains were less robust.

But in the meantime, recovered people still carried immunological memory — the ability of the immune system to respond more rapidly to reinfections — that is mainly reflected in memory T cells, and such immunity can also tackle new variants, according to the study.

"This study improves the understanding of the duration of SARS-CoV-2-specific immunity without boosting, which has implications for the design of vaccination regimens and programs," it said.

"Our data suggest that with the increasing emergence of variants, there is an urgent requirement to introduce an effective vaccine to boost the neutralizing antibody and overall T-cell responses to newly emerged SARS-CoV-2 variants," the study said.

The research team was led by Wang Jianwei, vice-president of the Chinese Academy of Medical Sciences and Peking Union Medical College, and Cao Bin, vice-president of China-Japan Friendship Hospital.

China is grappling with a surge in respiratory illness driven by the simultaneous spread of different pathogens, including the virus that causes the COVID-19 disease, according to China's health authorities.

In past months, the XBB strain — a subvariant of Omicron — and its lineages have dominated circulation in China, data released by the Chinese Center for Disease Control and Prevention shows.

At least six vaccines designed to tackle the XBB strain and some other variants have gained emergency market approval from China's top drug regulators, according to notices released by vaccine manufacturers.

Wang Huaqing, chief immunization planning expert at the China CDC, said during a recent news conference that China has prioritized those 60 and above, adults with chronic illnesses, people with weakened immunity and groups at high risk of contracting COVID-19 for the next step of the COVID-19 vaccination drive.