Israel’s Sheba Medical Center has begun the first study in the world of the efficacy of administering a fourth dose of the coronavirus vaccine. The 6,000 participants include 150 medical personnel who had a 3rd dose in August. They will receive the Pfizer vaccine.
https://www.timesofisrael.com/israeli-hospital-to-begin-study-on-efficacy-of-4th-covid-vaccine-dose/