Vaccine boosters have proven effective in building immunity against the Omicron variant's BA.2 sublineage, according to the Department of Medical Sciences (DoMS).
The study was conducted on volunteers from Siriraj Hospital to monitor their immune response to the BA.1 and BA.2 sublineages of the Covid-19's Omicron variant, said the DoMS director-general Supakit Sirilak.
Dr Supakit said about 130 million courses of vaccines have been administered to people.
Of them, around 70% were either double-doses or double-doses plus a booster.
Last week, BA.2 accounted for 96% of all Covid-19 infections while BA.1 accounted for the rest.
In the next couple of weeks, BA.2 is expected to completely take over as it spreads faster than BA.1.
The previously dominant Delta strain of the virus has now disappeared.
Dr Supakit said the two-week study revealed double doses plus a booster shot of most mix-and-match formulas provided more effective immunity against BA.2 than BA.1.
The findings confirmed that BA.2 cannot evade an immune response after a full vaccination and a booster shot. This should allay many people's earlier fears, according to the director-general.
He added that two doses of the vaccine alone were not sufficient to protect against BA.2 as immunity gradually declines.
However, a booster given as part of the mix-and-match vaccine format -- such as two doses of Sinovac plus an AstraZeneca booster shot -- has proven to deliver a sustained immunity at high levels, even three months after having received the booster shot.
Dr Supakit said the immunity increase from two AstraZeneca doses plus a Pfizer booster shot was being studied.
According to research, double doses reduce fatalities up to fivefold while a double dose with a booster shot can achieve 31 times greater protection.
Dr Banlang Uppapong, the DoMS deputy director-general, said studies in the US, Italy, Switzerland and Argentina, which are yet to be published, showed two doses of vaccine did not significantly increase immunity against Omicron.
However, a booster shot in the cross-vaccine format can prompt immunity.
