The eastern coastal city of Qingdao, Shandong province has begun offering COVID-19 booster shots to people over 18 years old who have already been vaccinated amid rising evidence that shows an additional COVID-19 shot will improve immunity.

A medical worker inoculates a college student in Qingdao West Coast New Area on March 30. [Photo/VCG]

According to the Qingdao disease control and prevention center, the booster shots will be made available across the city for adults who have been fully vaccinated for at least six months.

Key groups will be prioritized during the initial stage of booster shots and will include people at higher risk of contracting the disease, such as workers in the cold chain, customs, transportation, and health services sectors, as well as those who plan to travel abroad.

In addition, essential workers in the public service sector, such as police officers, community workers, as well as senior care and sanitation staff, will also be given priority in receiving a booster shot.

Four types of vaccines will be deployed as booster shots. They include two Sinopharm inactivated vaccines, one created by the group's Beijing Institute of Biological Products and the other by its Wuhan Institute of Biological Products, one inactivated vaccine made by Sinovac Biotech, as well as an adenovirus vector vaccine from CanSino Biologics.

People intending to receive an additional booster shot will be given the same vaccine made by the same company. In cases where supplies of a particular vaccine run short, other vaccines of the same type but are created by a different company can be used as an alternative, local authorities said.

Booster shots are being adopted by an increasing number of countries as evidence shows that the efficacy of COVID-19 vaccines wane after six or more months, health experts have said.

Wang Huaqing, chief expert at the Chinese Center for Disease Control and Prevention, said that clinical trial results released by vaccine developers have shown a rapid rebound in the amount of neutralizing antibodies in humans following a booster shot.

"In addition to increased antibody levels, the spectrum of antibodies is also wider, suggesting that a booster shot will likely elicit stronger protection against variants," said Wang.