That means that at the moment there's just no reasonable way to decide whether it's safe to get a second dose or not. If there were studies that would show that almost nobody has a lasting head pain post vaccination, then one could conclude it's probably just a super weird coincidence, and take a risk with a second dose, to at least get the full protection against covid-19.
Without data the most prudent thing is to do no further harm to oneself, and live with increased risk of infection and all the restrictions and increased costs that are put on unvaccinated.
