Here are a couple of thoughts on COVID-19 vaccine “booster shots”. I do not see a safety issue with an additional dose, but I have not yet seen data that convinces me that they are more protective against disease than the current vaccine schedules. I also think that is is much more important that unvaccinated people; those under 12, those in the USA who have not been vaccinated, and in the rest of the world are vaccinated first.
I do not expect a safety issue with boosters (or 3rd doses which is another way of looking at it), there have been clinical trials on immune-suppressed people and a 3rd dose of mRNA vaccines definitely boosts the amounts of antibodies that the person has. I have not seen data on whether the 3rd shot is actually protective against disease (or for that matter infection). There are quite a few vaccines that have multiple doses (for DTaP 5 doses are recommended), so three is not unusual.
My main concern is that, in most studies (with the possible exception of Israel, but there are some issues with those data that still need to be clarified, in my opinion), show that vaccination with 2 doses of the mRNA vaccines (and one dose of the J&J vaccine, but very few people talk about that one) protects very well against disease (COVID-19). It appeared that some of the mRNA vaccines (and to a lesser extent J&J) protect somewhat against infection by SARS-CoV-2 (not the same as disease) and that protection against infection may be waning, particularly relative to the delta variant. However, infection is not equivalent to disease, the latter of which the vaccines do an excellent job of protecting from (not 100%, but nothing is 100%).
I also have personal issues with "rich" countries distributing 3rd doses before many countries in the world do not even have one and (at least in some of those "rich" countries with relatively low vaccination rates) also people who have not even had one shot. I think that mandates for 1st doses would be much better than offering 3rd, but that is an opinion (although there is some modeling that indicates that getting more people even partially vaccinated would be much better than getting people 3rd shots). It is also pretty clear that as long as SARS-CoV-2 continues to circulate anywhere in the world, more variants will arise, hopefully none that can avoid vaccines, but who knows, so EVERYONE needs to be vaccinated.
Take home message, if you are immune-compromised (there is a very clear definition of this on the CDC website), get a 3rd shot. If not (and I fall in this category), I am not in any rush.