Antibody levels in people fully vaccinated with the Sinovac vaccine decline by half every 40 days, according to a study.

The study was by Thammasat University's faculty of medicine and the National Centre for Genetic Engineering and Biotechnology (Biotec).

Anan Jongkaewwattana of Biotec wrote on Facebook that their study of 500 people, who received two doses of Sinovac, indicated that the level of antibodies drops by 50% every 40 days. The level of antibodies in people who received a second jab more than 60 days after the first was on average lower than that of those who got the second dose in less than 60 days, he said.

Mr Anan said the vaccine potency within 60 days of the second shot is between 60%-70% against the original strain. The potency against the original strain declines to about 50% in those receiving the second shot for over 60 days.

However, no data is available about the potency of two doses of Sinovac against variants, especially the highly contagious Alpha and Delta strains.

The overall level of immunisation is likely to drop in older people, he said, adding those aged over 40 showed lower antibody levels than those younger.