A booster shot of COVID-19 vaccines could be the answer to increasing and sustaining immunity levels and also to blunting the spread of some emerging variants, experts said.

Given that leading domestic vaccine developers are testing booster shots in clinical trials, experts stressed that only after the trials are completed and results analyzed can the role and need for booster shots be determined. Monitoring of real-world transmission of the virus after mass immunization is also needed to shed light on future vaccination strategies.

A booster shot is usually administered after people have finished regular vaccination procedures, but their bodies still cannot mount sufficient immune response to fend off the virus.

"It is unavoidable that some people are not sensitive to vaccines, and the level of antibodies triggered by a regular number of shots do not reach a satisfactory level in them," said Feng Duojia, president of the China Association for Vaccines.

"Booster shots have been used in the past when delivering vaccines against measles, hepatitis, and the combination dose for diphtheria, tetanus and whooping cough, with the aim of further production of antibodies," he said.

"For instance, mass immunization with a two-dose vaccine candidate with an efficacy rate of 70 percent means that about 30 percent of vaccinated people are likely to remain vulnerable to the virus. Then a booster, a third shot targeting this small segment, is likely to be the deciding factor to produce an adequate response in them," he said. "The method has been deployed and has proved to work in the past."

Booster shots are also expected to play a role in prolonging protection from the virus.

Yin Weidong, CEO of Sinovac Biotech, said in a recent interview that the company is testing a booster shot for its inactivated COVID-19 vaccine approved for public use in China.

"If protection effects from the two-shot regime diminish, taking a third shot is one way to go," he told China Central Television.

Zhang Yuntao, vice-president of Sinopharm's China National Biotech Group, which has developed two inactivated vaccines, also said that preliminary results from human trials have shown that booster shots can effectively improve "persistence of antibodies" and "resistance to mutations."

Feng, of the vaccines association, said one strength of inactivated vaccines is that giving an original booster shot will be effective in strengthening protection against some new variants, without tweaking the product's components and dosage. He added that significant mutations will still require redesigning or adjusting of the vaccines in use.

The prospect of using booster shots to keep up protection levels has also sparked discussions of the possibility of annual COVID-19 vaccinations, just as flu vaccines are given each year.

"The public should be mentally prepared for the possible scenario that the novel coronavirus could be constantly circulating for years," he said.

Regular cycles

Anthony Fauci, director of the United States' National Institute of Allergy and Infectious Diseases, said in an interview with MSNBC that regular cycles of vaccination aimed at continuing to boost people's immunity may be needed, either with the original vaccine or a newly developed one for a new strain.

At the moment, it is not recommended that the general public undergo tests to measure their antibodies after inoculation, said Feng Luzhao, a public health professor from the Chinese Academy of Medical Sciences and Peking Union Medical College.

"The level of antibodies is just one barometer for evaluating how robust protection is. We need rigorous results from clinical results to devise plans to roll out booster shots," he said.

Feng added that a common practice is for local disease control centers to gauge the transmission rate of the virus in a region to see if a vaccine has worked as expected. "If not, a booster shot will then be taken into consideration," he said.