BEIJING -- China has been expanding vaccine inoculation, booster shots in particular, to combat COVID-19 and build an immune barrier as the coronavirus continues to spread worldwide.

A series of Chinese cities including Xi'an, Guangzhou and Beijing has launched a campaign to administer COVID-19 booster shots for those aged 18 and above who had completed their full vaccination at least six months ago.

Since Oct 25, the Xinfadi wholesale market in Beijing, which was previously hit by COVID-19, has launched booster dose vaccination for over 1,000 staff and more than 8,000 vendors.

"I finished my full vaccination in April, and now six months have passed, so I came for a booster dose, which is an assurance for me," said Duan Hongmei, a greengrocer at Xinfadi.

In Beijing's Daxing district, there are 23 temporary vaccination sites and 13 mobile vaccination vehicles, providing convenience for residents. By the afternoon of Oct 31, more than 500,000 people have been inoculated with booster shots in Daxing.

Wang Huaqing, an expert with the Chinese Center for Disease Control and Prevention, said a booster shot following full vaccination is necessary, as the neutralizing antibodies in recipients are declining and the protective effect is weakening as time goes on.

To strike a balance between economic development and efforts to combat the Delta variant, it is necessary to accelerate the pace of vaccination and build an immune barrier in the population.

Over 1.07 billion people in China had completed their vaccination against COVID-19 by the end of October, and more than 2.2 billion vaccine jabs had been administered, data from the National Health Commission showed.

Experts say that China needs 80 percent or more of the population vaccinated in the country to build an immune barrier.

A growing number of Chinese provincial-level regions including Beijing, Hubei and Liaoning have expanded their COVID-19 vaccination programs to cover children from the age of 3, rather than limiting vaccination to those aged 12 and above, as was previously the case.

"We used to consider children as a less vulnerable group in the face of COVID-19, but as the epidemic is still spreading, more children are found to be infected," Wang said.

Infectious diseases are easy to spread in schools and kindergartens which are densely populated. Launching a vaccination program for children aged between 3 and 11 is an important part of building an immune barrier for the whole population as well as a key measure to curb the COVID-19, according to the Beijing vaccination organization and coordination working group.

Many districts in Beijing started the COVID-19 vaccination for children aged between 3 and 11 on Oct 28. "The feeling is almost the same as the inoculation of other vaccines," said Liu Xinyu, a 10-year-old student from the Anding township central primary school in Daxing district.

Wang Yanying, a staff of the vaccination office in Daxing district, said they have organized teachers to follow up on the condition of children after the vaccination, and many vaccination sites have been equipped with pediatricians, medicines for children, ambulance, and a sound mechanism to deal with emergencies, in an endeavor to ensure the safety of children.