Vaccine booster shots are recommended for higher immune response against the Omicron COVID-19 variant for people having already received two jabs of any vaccine makes, according to Thailand’s Siriraj Institute of Clinical Research (SICRES) and the Department of Medical Sciences (DMSC).

DMSC Director-General Dr. Supakit Sirilak, said that the antibodies in the blood system generated by vaccines are effective against the Delta variant, but that reduces substantially against the Omicron variant, due to its ability to evade the immune system.

Simply put, he said two doses of whatever vaccines are not sufficient to deal with the Omicron variant, hence the need of a booster shot.

Meanwhile, a new study conducted by the SICRES confirmed its previous study published last week that a booster of the Pfizer vaccine after two doses of Sinovac vaccine produce the highest level of immune response against both the Delta and Omicron variants. However, in the research findings released yesterday (Sunday), it was found that the best response against the latter was in fact generated by half a dose of the Pfizer booster, instead of a full dose, measured at 551 GMT (Geometric Mean Titer).

The institute used the plaque reduction neutralization test (PRNT) technique. According to the vaccination protocol, the third doses are administered 12 weeks after the second inoculation.

Tests using the PRNT technique on samples inoculated with two Sinovac doses and a booster dose of AstraZeneca or Pfizer show the following:

• With booster dose of AstraZeneca, immune response was measured at 170 GMT
• With half dose Pfizer booster, immune response was measured at 551 GMT
• With a full dose booster of Pfizer, immune response reaches 543 GMT

With two AstraZeneca doses plus a booster shot of AstraZeneca or Pfizer, the results are as follows:

• 3 GMT for AstraZeneca booster dose
• 232 GMT for a half dose Pfizer booster
• 521 GMT for a full dose Pfizer booster

With mixed Sinovac and AstraZeneca doses and a third booster dose of the Pfizer vaccine, the test results show a 204 GMT immune response for a half dose of Pfizer and 150 GMT for full dose.