Antibody levels in people fully vaccinated with the Sinovac vaccine decline by half every 40 days, according to findings from a joint study between Thammasat University's faculty of medicine and the National Centre for Genetic Engineering and Biotechnology (Biotec).
The findings were revealed by Anan Jongkaewwattana, director of Veterinary Health Innovation and Management Research Group of Biotec.
Mr Anan 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.
Meanwhile, Chalermchai Boonyaleepun, deputy chairman of the Senate committee on public health, posted on Blockdit that vaccines have done their job in lowering Covid-19 infections and fatalities in healthcare workers despite reports that some of them who had received the vaccines still caught the virus.
He did not mention the vaccine types but most medical workers received two doses of Sinovac.
Citing information from the Department of Disease Control, he said of the 700,000 who were fully vaccinated, 707 became infected, a ratio of 101:100,000. Of the 21,000 not vaccinated, 173 were infected, a ratio of 823:100,000. The rate among medical workers not vaccinated is eight times higher, he said.
Seven medical workers have died from Covid-19, two of whom were vaccinated. The death rate in the vaccinated group was 0.28% while in the non-vaccinated group it was 2.89%.