Health experts have urged people who have not been fully vaccinated against COVID-19 to get vaccinated, especially the elderly and those with weakened immune systems, so as to protect themselves and maintain herd immunity levels.

Zheng Hui, a researcher at the Chinese Center for Disease Control and Prevention's national immunization program, said that following the wave of COVID-19 that peaked in late December and subsided in late February, China's population has accumulated "good hybrid immunity", that is, immunity provided by a combination of infection and vaccination.

"How will the virus mutate in the future, and with the hybrid immunity in place, when will the next wave of epidemic arrive and peak remain uncertain," she said. "But currently and in the short term, our vaccination strategy is to close up immunity gaps across the population."

Two categories of people are deemed as harboring less potent immunity against the virus, according to Zheng — those who have not been infected and have not completed recommended inoculation procedures, as well as recovered COVID-19 patients who had not been fully vaccinated before infection.

"Even though getting infected will induce a certain level of antibodies, human bodies will need antigenic stimulation at least three times to generate a relatively high level of protective effects," she said. Antigenic stimulation can be achieved either through infection or vaccination.

During an interview with state broadcaster China Central Television, Yin Zundong, head of the China CDC's national immunization program, said that people who have not received sufficient antigenic stimulation are highly suggested to obtain vaccines. The policy is also outlined in a notice released by the State Council's Joint Prevention and Mechanism earlier this month.

Yin added that hybrid immunity has been shown to be more potent and long-lasting than immunity solely induced by infection or vaccination.

"The risk of a second infection in the short term, especially within three months, is very low," he said. "So for vaccinated people who have recovered from a recent infection, there is no need to get a new dose in the short term."

Yin added that COVID-19 immunity will wane in the long term, but the decrease mainly means protection against getting infected or developing mild symptoms will drop, rather than efficacy against severe diseases or deaths.