(Bloomberg) -- One of Thailand’s main Covid-19 vaccine regimes generates a lower immune response than inoculation combinations that include an mRNA-based dose, according to a study by the Siriraj Institute of Clinical Research.

Preliminary results showed that a Sinovac Biotech Ltd. vaccine as a first shot followed by an AstraZeneca Plc jab -- a pairing widely used in Thailand -- elicited a weaker immune response than a two-dose regime in which Pfizer Inc.’s vaccine was administered as the second shot four weeks after an initial jab of either Sinovac or Astra, the study said. 

Thailand was the first nation to start administering the Sinovac-then-Astra combo, with the goal of increasing protection against the more contagious delta variant and addressing vaccine shortages. 

In one of the early studies that prompted Thailand to adopt the two-brand regime, results showed that a first dose of the China-made vaccine followed by an Astra shot three to four weeks later could elicit an immune response eight times stronger than two doses of Sinovac. 

The Bangkok-based Siriraj Institute’s research found no severe adverse events following immunization using any of the regimes. Results from its study of anti-receptor binding domain two weeks after the second dose are as follows: 

  • Astra-Pfizer produces the highest anti-RBD levels on average at 2,259.9 binding antibody units per milliliter
  • Sinovac-Pfizer 2,181.8 BAU/ml
  • Sinovac-Astra 1,049.7 BAU/ml
  • Two doses of Astra 278.5 BAU/ml
  • Astra-Sinovac 172.1 BAU/ml
  • Two doses of Sinovac 164.4 BAU/ml

Thailand’s current vaccine pairings are Sinovac-Astra, Astra-Pfizer, and two doses of Astra or Pfizer shots, according to the Health Ministry. People who have received two doses of Sinovac earlier this year are eligible to receive a booster shot using Astra or Pfizer.

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