BEIJING (Reuters) – 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 the medRxiv website ahead of 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-subunit vaccines or mRNA vaccines based on newer variants of concern could be a good alternative for further boosting instead, according to the paper.

The invitro tests left unclear how the antibody responses may affect the three or four-dose vaccine’s effectiveness in lowering risk of Omicron-caused COVID disease or death.

The study showed that neutralising antibody level dropped by 53% 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 fourth 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 neutralizing 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 data on older people or minors. It also did not study the fourth dose’s impact on cellular immune response.

SOURCE: https://bit.ly/35i5GRa medRxiv, online February 21, 2022.

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