Figure 1. Selection of Persons in the Four-Dose Group and the Matching Control Group.

Of 258,994 eligible vaccinated persons, 210,265 (81.2%) were successfully matched and included in at least one of the study groups as follows: 182,122 participated as members of the four-dose group and 72,505 participated as members of the control group, with an overlap of 44,362 persons who were initially matched as members of the control group and then were rerecruited as members of the four-dose group after receiving a fourth dose, along with a new matched control. CHS denotes Clalit Health Services.