The Pfizer Covid vaccine caused allergic reactions. For patients that are deemed to be at an increased risk of adverse side effects following a risk assessment (e.g. because of diabetes, heart disease), should the vaccine be given in 3 stages instead of 2, where the first stage is a preliminary very small amount to test for adverse reactions, and then the latter 2 stages are as originally planned?
Consider this a very tentative answer, employing analogy rather than any hard data on the Pfizer vaccine.
In general it can be fairly difficult to know exactly what component in a vaccine caused the allergic reaction. So, an option is indeed to test with the actual vaccine. An AAAAI Q&A on the matter points to this study that experimented with the influenza and measles vaccines:
the results of the skin prick test (undiluted vaccine) and the intradermal skin test (1:10 diluted vaccine) indicated that the latter was more useful in both cases of measles (54 patients) and influenza vaccine (69 patients) Overall, the skin test using 1:10 diluted vaccine was the more suitable for predicting an immediate-type reaction to measles and influenza vaccinations. Patients having negative 1:10 skin tests can be expected to show no adverse reactions to the remaining injections and even the positive subjects will complete the course of vaccine doses by the stepwise method.
Frankly, I find the abstract (and the rest of the paper) fairly confusing in their terminology. I think they use "skin test" interchangeably with intradermal [skin] test, and in opposition to [skin] prick test, i.e. I think they ultimately recommend an intradermal 1:10 v/v diluted solution as the best method in terms of balancing risk of a severe reaction with actually detecting one at all.
Actually, they have this protocol illustrated in the paper, which does a prick test first, followed by an intradermal only if the prick test was negative.
But the conclusions of the paper are just about as confusing as the abstract. It's not terribly clear to me if they recommend following this more elaborate protocol, or just doing the intradermal test.