I'm an older runner in my 50s. Enjoy running marathons (not racing at my age). Never had COVID, but after the vaccines I've been struggling for about a three weeks. Have a friend about the same age that had similar issues for about a month after the second shot.
Starting to get better, but it has thrown off my running quite a bit. Still would rather have the vaccine than getting COVID.
Probably won't affect younger runners like older ones.