Thai study shows antibody levels after 2 doses of Sinovac drop 50% every 40 days

Clinical research has shown that the antibody levels in a person fully inoculated with two doses of the Chinese-made Sinovac vaccine drops by half every 40 days, according to Dr. Anan Jongkaewwattana, a virology researcher at the National Centre for Engineering and Biotechnology (BIOTEC).

In his Facebook post on Monday, Dr. Anan said the clinical research was jointly conducted by BIOTEC and the clinical research centre of the Faculty of Medicine of Thammasat University to study the antibody level toward the RBD protein of the COVID-19 virus in 500 samples of people fully inoculated with Sinovac vaccine.

The research shows the following:

  • Their antibody levels have dropped by half over an average of 40 days.
  • The antibody levels of those fully inoculated after 60 days are lower than those fully inoculated for fewer than 60 days.
  • The antibody levels within 60 days of the second dose are on a par with the efficacy of the vaccines against the original coronavirus variant, which is between 60-70% by average, but among those who are fully inoculated after 60 days, the efficacy of the vaccine dropped to 50%.
  • There is no comparative information about the efficacy of the Sinovac vaccine after two doses in protection against the Alpha or Delta variants.
  • The level of antibodies tends to drop in line with the ages of those vaccinated, with the level of those who are over 40 lower than their peers who are under 40.

An actress, Ms. Nataphon Temeerak said, via Instagram, that she had her antibody levels checked twice, 14 days and 28 days after the second dose. She found out that the levels were 1286.4 AU/ml (arbitrary units/milliliter) and 1098.5 AU/ml respectively.

She said her posting about her different antibody readings is intended remind people of the importance of protecting ourselves, even after vaccination.

Another woman, Sathi Baiyok, wife of an actor, said her antibody level in the second test had dropped substantially from the result in the first test. Her tests were 20 days apart.


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