Another Covid-19 lockdown is unlikely despite the Omicron coronavirus variant, Prime Minister Prayut Chan-o-cha said on Tuesday.

"Hopefully, this coming New Year will be a happy time. We haven't had that festive happiness for two years because of Covid-19,"  Prayut said, after the first case of the new strain was confirmed in the kingdom on Monday from a visiting American citizen.

"After all everything now depends on how effective the Covid Free Setting and other measures are. If there is no new spread of the virus, who would ever close or lock down [activities] again? No one would," the prime minister said.

"As of now, the requirement that international visitors undergo an RT-PCR Covid-19 test upon arriving in the kingdom is seen by health authorities as sufficient to detect the new variant and prevent it from spreading here," he added.

Public Health Minister Anutin Charnvirakul said the current vaccination campaign must be ramped up to keep people's immunity at a high level.

People who received their second vaccination shots in August are urged to get a booster.

All 17 people who came in close contact with Thailand's first Omicron case at the hotel of the American tourist have tested negative for Covid-19, the Department of Disease Control reported on Tuesday.