Last week I managed to (just) avoid falling out with my fully vaxxed mother when she gloated that I couldn't travel on holiday because I hadn't been vaccinated.
This subject came about because she is due to go abroad on the 28th Dec.
Anyway, this morning she called me to wish me merry xmas and told me her husband had tested positive for covid so they are now self-isolating. I managed to not gloat, but I'm wondering if I could have indulged just this once :)
Right now I'd go for a quick comment (not snarky, but framed as a question about the effectiveness of the vaccine) and cast more doubt into the efficacy of the vaccine by bringing up stories similar to hers that you'll find on the web.
This way, it'll seem like you have been looking for a possible explanation of what has happened (even though you knew the answer all along).
When enough doubt has been cast, you can discuss it further. Who knows, maybe she'll even admit that she has been wrong.
Even if not and you choose to gloat later, it's better to do it when everyone is calm and not worried about getting sick.