A fourth shot of the Pfizer COVID-19 vaccine increased protection against viral infection for only four to seven weeks, according to a massive study published Tuesday.
The study included 1.25 million people age 60 and over in Israel who received their fourth dose between January and March. Israel uses only the Pfizer vaccine.
People who got the fourth dose were half as likely to test positive for COVID-19 four weeks later when compared to people who only had three doses, according to the study.
