Cases of people catching the JN.1 sub-variant of the Omicron Covid-19 variant rose last week, resulting in an increase in hospitalisations and more than 10 deaths.
Most hospitalised patients were elderly and did not receive a vaccine or boosters.
The Department of Disease Control (DDC) said there were 718 new reported Covid-19 cases, a daily average of 102 new cases, and 11 deaths in the week ending Jan 20.
The week earlier, there were, on average, 93 new cases daily. The DDC said there were four deaths due to Covid-19 that week.
Out of the 718 cases last week, 209 patients suffered severe pneumonia, with 149 requiring endotracheal intubation.
The DDC said all of the deceased last week belonged to the "608 at-risk" group of elderly people with a chronic disease. Six of those who died were unvaccinated, while the others received only two vaccine doses.
According to information from the Department of Medical Sciences, the Covid-19 variant currently spreading in Thailand is Omicron's JN.1 sub-variant.
Symptoms of JN.1 include coughing, a sore throat, aches and pains, headache and a runny nose.
Health officials say there are no signs indicating that JN.1 is more severe than the original Omicron variant.
On Dec 5 last year, the World Health Organization (WHO) recommended the elderly with a chronic disease or dysfunctional immune system get a Covid-19 vaccine every six to 12 months to reduce the risk of severe illness and death.
The DDC has stressed that patients in the at-risk 608 group with a cold and a positive antigen-test-kit (ATK) test result must wear a face mask and see a doctor for treatment as soon as possible.
Those ill with the virus can contact the DDC for more information by calling 1422.