BEIJING: Antibodies triggered by a third dose of Sinopharm's COVID-19 shot given to those who completed its primary two-dose regimen dropped sharply after six months, and a fourth shot did not significantly boost them against Omicron, a Chinese study showed.
The study, published on Monday (Feb 21) before peer review, said repeated immunisation using inactivated vaccines such as the Sinopharm shot as a fourth booster may not be ideal to further increase antibody response against Omicron.
Certain protein-based vaccines or mRNA vaccines based on the genetic information of variants of concern could be a good alternative for further boost instead, according to the paper.
It remained unclear how the antibody readings may affect the three- or four-dose vaccine's effectiveness in lowering risk of Omicron-caused COVID-19 disease or death.
The study showed that neutralising antibody level dropped by 53 per cent against Omicron around 26 weeks after the third dose of Sinopharm's BBIBP-CorV, versus levels seen two weeks after the third dose, researchers said.
The booster shot was given to 38 healthcare workers in China who had completed the two-dose regimen of the vaccine five months earlier.
The study said a fourth dose, given six months after the third, did not significantly lift the neutralising antibody level against Omicron, although it still recalled the level to around the peak after a third dose.
The study, conducted by researchers at a hospital affiliated with Sun Yat-sen University, did not include readings among older people or minors. It did not study the fourth dose's impact on cellular immune response, which is a different part of the human immune system than the antibody-based response.