The department issued the warning after the Covid infection rate surged by 250% during the week to April 15 from the previous week.
Department director-general Tharet Krutnairawiwong said 435 patients were admitted to hospital for treatment of Covid-19 between April 9 and 15, about 62 cases per day.
Of the number, 30 had lung inflammation and 19 were put on ventilators, up 58% and 36% respectively, from the previous week.
Two Covid patients died during the week and both had received a Covid-19 vaccine booster shot more than three months before they got ill, Tharet said.
He warned people who have not received booster shots for three months to get one as soon as possible.
People returning from Songkran celebrations should stay away from elderly or sick relatives or sick for at least seven days to determine whether they have been infected or not, Tharet said.
The number of people contacting the virus during the Songkran celebrations rose because the government no longer requires people to wear masks and pass through a screening process before entering crowded venues, he said.
Tharet said a new Omicron sub variant, XBB1.16, has been found in 22 countries and six cases have been found in Thailand. The sub variant does not usually cause severe symptoms, but it is highly transmissible, he said.
Sopon Iamsirithaworn, deputy director-general of the department, said people with fever, coughs or runny noses should immediately take ATK rapid antigen tests so they can get treatment in time if they are infected with Covid-19.