Existing vaccines are effective in dealing with the third wave pandemic caused by the Delta variant and booster shots are needed for future anti-epidemic strategy, Chinese medical experts said after Sinovac released the initial results of a Phase II clinical trial on booster shots of China-developed inactivated COVID-19 vaccines.
The study, conducted in East China's Jiangsu Province since May 2020, found that although neutralizing antibody levels declined six months after two Sinovac shots, a two-dose vaccination schedule generates good immune memory.
A third dose, given at an interval of six to eight months after the second dose, led to a strong boost in immune response, with geometric mean titers (GMTs) increasing to approximately 140.
The study also found that giving a third dose too early (28 days after the second dose) induced a much lower antibody level - only one-third compared with a third dose given six or more months after a second dose.
Optimizing the timing of a booster dose should take into account immunogenicity, vaccine efficacy/effectiveness, the local epidemic situation, infection risk, and vaccine supply, according to the study.
Provisions for booster shots in China would be highly probable, given the Sinovac clinical trial results, Zhuang Shilihe, a Guangzhou-based expert said.
Sheng Guoping, a doctor at Shulan Health in Hangzhou, East China's Zhejiang Province, told media that the level of antibodies would decline with time after vaccination, and accepting booster shots can help to ensure that there is always 60-70 percent of the population having antibodies in order to maintain herd immunity.
Sheng told media that groups of high infection risk can accept a test on antibody levels after vaccination and decide whether to accept booster shots based on the result. Those who accepted vaccines more than one year before can also consider accepting booster shots or even repeat the two-dose vaccination procedure, Sheng said.
Taking the 2022 Beijing Winter Olympics as an example, Zhuang said that it is necessary for people who would attend the event to receive booster shots as many of them may accept vaccines more than six months before, while crowds would increase infection risks.
