About half the population has now acquired immunity against Covid-19 but this remains well short of the government's 80% target, according to the Department of Disease Control (DDC).
Dr Chakkarat Pitayowonganon, director of its epidemiology division, said around 10% of the country's 67.5-million people are estimated to have contracted the virus so far.
When combined with those who have received two doses of vaccine plus a booster shot, those with immunity against the novel coronavirus should account for about 50% of the population.
Dr Chakkarat said boosters must be prioritised, especially for vulnerable groups such as the elderly and people with serious underlying illnesses.
During last week's Songkran holiday, many people of working age were out enjoying activities in public places. They ran a low risk of developing severe symptoms if infected because most of them were fully vaccinated and had received one or two booster jabs, he said.
But there is mounting concern that they might have passed the virus on to vulnerable people during Songkran visits to family and friends, Dr Chakkarat noted.
Around 55% of those in vulnerable groups have not been inoculated at all, according to the director.
Dr Chakkarat said many people returned to their home provinces for Songkran carrying the virus, which is raising fears that post-festival caseloads could skyrocket.
The country recorded 129 Covid-19 fatalities over the previous 24 hours, up five from the last count and a new record for the current wave of the epidemic, the Public Health Ministry announced on Tuesday morning.
New cases declined for a fourth day, down 103 to 16,891, but that did not include 12,945 positive results from antigen tests, which would raise the total to 29,836.
Dr Chakkarat advised people who come into close contact with vulnerable people to wear face masks, sleep in separate rooms, practice social distancing when eating and make sure elderly relations are vaccinated.
Dr Chakkarat said the initial post-Songkran infection rate has fallen slightly but this may be due to fewer RT-PCR and antigen tests having been carried out.
Experts say daily caseloads could rise to 50,000 to 100,000 following the festival.