Fully vaccinated people, including the elderly, are less likely to develop moderate or severe cases of COVID, and getting a booster shot further lowers the risk, a new study published by Chinese researchers on Thursday found.

The study is based on 27,000 cases recorded in Jilin city in the northeastern province of Jilin between March 3 and April 12. Jilin used domestically developed, inactivated COVID-19 vaccines for its immunization campaign.

Results show that unvaccinated or partially vaccinated people were about 1.8 times more likely to suffer moderate infections, nearly 9.5 times more likely to become severely ill and about 2.9 times more likely to become critically ill than fully vaccinated groups.

Compared to those who have received a booster shot, the unvaccinated or partially vaccinated were at even higher risk of developing mild or severe infections, according to the study.

The same trend was also observed among people aged 60 and above, adding to compelling evidence of the value of vaccinations and booster shots.

The study, led by Dong Xiaoping, chief virologist at the Chinese Center for Disease Control and Prevention, was published in China CDC Weekly, the national public health bulletin and academic platform established by the China CDC.

"Preliminary protective data on COVID-19 here is strong enough to indicate the public significance of the strategy of full vaccination and boosters, particularly among the older population, which is at markedly higher risk of critical and fatal outcomes," the study said.

It added that repeated booster shots, either using the same vaccine as initial injections or different vaccines, may be an effective measure for dealing with the epidemic in the future.