https://www.cdc.gov/media/releases/2021/p1021-covid-booster.html
Walensky has approved Moderna and J&J boosters, and also mix and match boosters!
This is all by self-attestation.
Moderna's booster is a half dose. They are using the same vials as the full dose, just drawing half the amount.
Mix and match boosters really open things up here. I think people should be mindful at this stage of the specific risks associated with all three vaccines. The mRNA vaccines have a rare risk of myocarditis that is generally observed with young men. The J&J vaccine has a rare risk of TTS that is generally observed with young women. Consider a different vaccine for the booster if you fall into either subgroup.
All homologous boosters have good preliminary data, so don't feel you need to mix and match if you don't want to.
The side effects of the booster doses have generally been like the second dose. So set your expectations accordingly. I'll note it is perfectly safe to take pain relievers as needed after the coronavirus vaccine. Swollen armpit lymph nodes on the side of the injection have been seen at higher rates with the third dose, so don't be alarmed if that happens.
The booster will provide protection after about a week. Thus far the data is excellent on booster effectiveness. We are seeing the vaccine effectiveness return to high pre-Delta variant levels after boosters.
In addition consider getting a Flu shot with your booster shot. We are just entering the traditional Flu season. Now is the perfect time to get vaccinated. Flu season may come back with a vengeance.