“This is, at this point, probably more improv than science,” Robert Wachter, chair of the department of medicine for the University of California at San Francisco, told the Times regarding the pre-emptive rush for second boosters.

Safety concerns about the fourth dose are mostly centered on exposing people to unnecessary risks that already come with vaccine doses. Side effects in the new Israeli study were similar to those seen previously, with the most common complaints being pain at the injection site, fever and headaches.

Experts say that concerns about additional boosters causing “fatigue” to the immune system are largely unwarranted. However, the timing of the shots could be critical, with boosters given too early possibly being less effective than those given later.