People who have not been vaccinated against Covid-19 are being urged to register for inoculation, as the latest "variant of concern" spreads from country to country.
It has not yet been detected in Thailand.
Centre for Covid-19 Situation Administration spokesman Taweesilp Visanuyothin said on Wednesday that existing vaccines were likely to limit the severity of symptoms produced by the new variant.
He said the Department of Disease Control reported that the symptoms from Omicron were little different from those of other variants.
There were reports that patients had muscle ache and fatigue, but did not lose their sense of smell and taste.
"Omicron's symptoms are not severe but the variant is likely to spread faster than other variants because it is quickly replacing the Delta variant in South Africa," he said.
There was not enough information to analyse the reproduction and severity of Omicron or vaccine effectiveness against it, but experts believed vaccines could prevent serious symptoms. Even so, the new variant had already caused death, Dr Taweesilp said.
He urged unvaccinated people to quickly get inoculated against Covid-19, saying only about 50% of people in the Northeast had been vaccinated to date.
Dr Taweesilp said it appeared Omicron could significantly avoid antibodies and there was a high chance of reinfection.
Omicron patients had now been found in more European countries - Portugal, Austria, Sweden and Spain - in addition to the United Kingdom, Italy, Germany, the Netherlands, Belgium, Denmark, Switzerland, Turkey and the Czech Republic.
In the Asian continent, Japan was the latest country with Omicron infection, in addition to Israel, Hong Kong and Singapore.
The new variant was first detected in South Africa and since then reported in Botswana, Mozambique, Zimbabwe, Namibia, Malawi, Eswatini and Lesotho in southern Africa, as well as in Canada and Australia.
"Most infected people were from the eight African countries," Dr Taweesilp said.
Between Nov 15 and 27, 330 people from the eight high-risk African countries remained in Thailand and 23 of them were traced, he said.
Of the 330 people, 252 entered Thailand through the Sandbox programme. Dr Taweesilp urged them to report to their nearest public hospital for free RT-PCR tests.
During the same period, 453 people from other low-risk African countries stayed in Thailand and 18 of them were traced, he said.
The World Health Organization said on Wednesday that travel bans would not stop the variant spreading, and could do more harm than good.