China will focus on nationwide vaccination of people and workers deemed at high risk of contracting COVID-19 over the winter and spring, as the cold weather can complicate prevention and control efforts, senior health officials said.

The high-risk groups include workers in customs, medicine and healthcare, the cold chain industry, public transportation, produce and seafood markets, and those traveling to medium－or high-risk areas for work or study, said Cui Gang, a senior official of the National Health Commission's Bureau of Disease Prevention and Control.

Zeng Yixin, vice-minister of the National Health Commission, said prioritizing these groups, which is only the first step toward nationwide mass inoculation, can help protect them against the virus, and aid China's effort to prevent a resurgence of the virus domestically and fend off risks from overseas.

"The temperature has dropped across various regions, making the prevention and control situation more serious and challenging," he said at a news briefing hosted by the State Council Information Office on Saturday.

As more vaccines get approved and mass produced, the elderly, people with underlying health conditions and those who are at higher risk of becoming severely ill patients after infection will be inoculated, he said. "We hope to build herd immunity protection through proactive vaccination, and effectively manage COVID-19 as soon as possible."

Zeng said various departments will meticulously check the quality of every batch of vaccine, ensure cold-chain transportation of the vaccine is safe, smooth and punctual, optimize inoculation locations and procedures, and closely monitor and handle any adverse effects.

Meanwhile, there will be greater scientific outreach to educate the public about vaccines, in order to promptly inoculate as many high-risk personnel as possible with their consent and understanding, he said.

Zheng Zhongwei, director of the National Health Commission's Development Center for Medical Science and Technology, said China has five vaccine candidates in phase-three clinical trials: three inactivated vaccines, an adenovirus vector vaccine and a recombinant protein vaccine.

Some of these candidates have enlisted enough recipients to meet the middle stage progress of the trials, he said. Vaccine developers are also keeping the National Medical Products Administration updated on their status.

Zheng said China will promptly release information regarding the status of its COVID-19 vaccine candidates.

"If the vaccine's (performance) statistics reach the required standards, the medical products administration will approve it for the market, and then our vaccine will be produced and rolled out more quickly, because we have already prepared for mass production," Zheng said.

Zeng, the vice-minister, said more research is needed to determine the duration of the vaccine's protection, and it is too early to conclude whether the Chinese vaccines can offer lifelong immunity or protection for five to 10 years like many other viral vaccines.

"However, there is little doubt that Chinese COVID-19 vaccines can offer protection for at least six months based on existing evidence," he said, adding that this standard is on par with the requirement by the World Health Organization.

Zeng said, optimistically speaking, it would be unlikely for the public to need to routinely take COVID-19 vaccines once every six months to one year, like flu shots, because the flu virus mutates very quickly and there may be a different strain every year causing an outbreak.

"Although we believe the COVID-19 vaccine is safe and effective, we advise the public to maintain personal hygiene and protection, including wearing masks, washing hands and practicing social distancing," he said.

This is because no vaccine can guarantee 100 percent protection, and people should not lower their guard, especially not before herd immunity has been established, he added.

Wang Huaqing, chief expert of vaccination planning at the Chinese Center for Disease Control and Prevention, said global genetic data shows that the novel coronavirus has not witnessed a major mutation that would hamper the efficacy of vaccines currently in development.

Zheng said more than 1 million doses of Chinese vaccines have been administered since July through the emergency use program, with no severe adverse reaction being reported.

In terms of overseas clinical trials for the three inactivated vaccines, Zheng said a total of 150,000 doses have been provided to 75,000 trial participants, with no severe reaction.

Jiao Yahui, an official at the health commission's Bureau of Inspection and Supervision, said the common side effects for the Chinese vaccines include headache, fever, redness and lumps at the injection area, as well as coughing, loss of appetite, vomiting and diarrhea.

However, these side effects typically occur shortly after inoculation, therefore Jiao advised vaccine recipients to monitor their condition for 30 minutes after being administered so medical workers at the vaccination location can quickly handle the issues should they occur.

Wang Xiaoyu and Zou Shuo contributed to this story.