A protein-based COVID-19 vaccine made by Sinopharm, when given as a booster after two doses of an earlier shot from the Chinese firm, elicited a stronger antibody response against the Omicron variant than a third dose of the original, a study showed.

The study, which was published on Tuesday and had not been peer reviewed, came amid concerns over the effectiveness of Sinopharm's BBIBP-CorV shot, one of the two leading COVID-19 vaccines exported by China, against the Omicron variant.

An earlier study showed a BBIBP-CorV booster had weaker neutralization against Omicron than against the original coronavirus strain. 

Sinopharm's NVSI-06-07 protein-based vaccine, approved for emergency use as a booster in the United Arab Emirates in December last year, adopts a different technology than the BBIBP-CorV shot that contains an inactivated form of the coronavirus.

Among 192 healthy adults vaccinated with two BBIBP-CorV doses for six months or longer, the neutralizing antibody level against Omicron in those later given a NVSI-06-07 booster was "significantly higher" than that in those who received a BBIBP-CorV third dose, researchers said in a paper.

Read more: UAE approves new Sinopharm vaccine as booster

The antibody-based results are different from the efficacy readings about how well the NVSI-06-07 booster after BBIBP-CorV vaccination would protect people from Omicron-caused disease.

The authors of the paper, including researchers from Sinopharm's units and Sheikh Khalifa Medical City in Abu Dhabi, cautioned that it remained unclear for how long the NVSI-06-07 booster's effect would last.