Thailand is in a tropical area, which isn't exactly in the northern or southern hemisphere. As a result, we have both northern and southern versions of the flu shot available. The ones currently available are the southern strain versions, which may overlap with the northern strain versions from last year. However, the flu strains in Thailand bear more resemblance to those in the southern hemisphere than the northern. Therefore, if you're getting the flu shot for the first time, I suggest opting for the southern strain.
If you plan to stay in Thailand indefinitely, or at least for several years, I recommend transitioning to the southern strain. This means getting the southern strain flu shot as soon as possible this year and repeating it next year. However, if you move frequently or spend most of your time in northern countries, I suggest waiting for the northern flu shot later this year.