As far as shots go - nobody "has to" get anything to enter Belize, nor to re-enter the US from Belize.
Tetnus shots protect you against lockjaw, which you get from stepping on rusty nails, etc. Belize probably has fewer rusty nails than the average US backyard - "but" you are more likely to be bare-footed here than at home. It is a good idea to have that shot updated every 7-10 years regardless of where you live or travel. Tetnus shots are also readily available in San Pedro, should you want or need one.
Hep A can be contracted virtually anywhere, and the vacination is given many times in a general program of preventative medicine.
When you go traveling you tend to check up on your vacinations and it's a great time to update the things you need for day-to-day life anywhere.
Travel in Belize has a few of major health risks - sunburn, hangovers and severe heartache when you "have to" leave. Come down, relax and enjoy!!!!!