Patalon et al examined the odds of testing positive for COVID-19 after three shots of the Pfizer vaccine compared to two. Over 300,000 adults in Israel aged 40 or older were included in the retrospective case control study, using a test-negative and matched case-control design. During the study period from March 1, 2020, to October 4, 2021, 6.6% of those who received two doses tested positive and 1.8% of those who received three doses tested positive. Researchers suggest that the booster counteracts the waning immunity of the vaccine but indicate that they should continue monitoring this population over time.