This observational study explored the effectiveness of a third dose of mRNA COVD-19 vaccine as a booster dose against the delta variant, using data from Clalit Health Services in Israel. The study looked at individuals who had received the third vaccine dose between July 30,2020 and September 23,2021 and compared them to individuals who did not receive the dose. Only individuals who did not have a prior SARS-CoV-2 infection were included. Thus, both the third dose group and the control group had 728,321 participants. The results showed that the effectiveness of the third vaccine dose against admission to the hospital was 93% when compared to only two doses. Similarly, the third dose also had a 92% effectiveness against severe disease, a 81% effectiveness against COVID-19 related death, and 88% effectiveness against documented SARS-CoV-2 infection. These findings suggest that a third dose of the COVID-19 vaccine more effective against COVID-19-related outcomes than only receiving two doses.