Thailand’s Siriraj Institute of Clinical Research, of the Faculty of Medicine at Siriraj hospital, is recommending that people who have been inoculated with two AstraZeneca vaccine shots receive a third jab of a half dose of the Pfizer vaccine, which is as effective as a full dose in boosting antibody levels.

The recommendation is based on a study of cohorts who received the third jab after they were inoculated with two Sinovac vaccine shots or two AstraZeneca jabs.

According to the study, China’s Sinovac is good as a “priming” vaccine and, if followed by a third jab of either a full dose of AstraZeneca or half dose of Pfizer vaccine, will boost antibodies up to a level close to that generated by a full dose of Pfizer

A full third dose of the Pfizer vaccine generates antibodies measured at 2,377 BAU/ml (binding antibody unit) compared to 1,962 BAU/ml for half a dose of the Pfizer vaccine.

A third jab of AstraZeneca vaccine, after two Sinovac jabs, generates antibodies measured at 246 BAU/ml, compared to 129 BAU/ml with a third jab of Sinopharm vaccine, which is the lowest.

Antibody tests against the Delta variant shows the levels in cohorts who have received two Sinovac jabs, followed by a third jab of half a dose of Pfizer vaccine measured 499, compared to 411 with a full dose of Pfizer vaccine, 271 with an AstraZeneca booster shot and 49 for a Sinopharm booster jab.

Antibody levels in those receiving two AstraZeneca shots and a full third dose of Pfizer is 520 and 358 for a half dose of Pfizer booster shot. Pfizer was proven to produce higher antibodies than AstraZeneca and Sinopharm.

Undesirable side effects among the tested cohorts are mild or moderate, according to the study.