Study population and setting
Administration of a third (booster) dose of the Pfizer BNT162b2 vaccine was approved on July 30, 2021 in Israel for individuals age 60 years and older who had received their second dose at least five months prior. This retrospective analysis extracted data from July 30 to August 31, 2021 from the Israeli Ministry of Health database on September 2, 2021. The goal of the study was to examine the effect of the booster dose on the rate of COVID-19 infections and severe illness in this population. Data was pulled only for individuals in the 60 years and older group who had their second dose at least five months earlier, which yielded information from 1,137,804 individuals. The rate of COVID-19 infection and severe illness was compared between those who received a booster at least 12 days earlier and those who did not (non-booster). COVID-19 illness was confirmed by PCR, and severe illness was defined as having a resting respiratory rate of more than 30 breaths per minute, oxygen saturation of less than 94% while breathing ambient air, or a ratio of partial pressure of arterial oxygen to fraction of inspired oxygen of less than 300. For the non-booster group, days at risk began 12 days after beginning of study (August 10, 2021) and ended at either time of occurrence of a study outcome, the end of study the period, or at the time of receipt of a booster dose. Finally, the rate of infection 4-6 days after receiving a booster was compared to the rate of infection at least 12 days after receiving a booster. A Poisson regression was used to calculate rates after adjusting for possible confounding factors, including age (60-69, 70-79, 80+), sex, demographic group (general Jewish, Arab, ultra-Orthodox Jewish), and date of second vaccination in half month intervals.
Summary of Main Findings
From this retrospective cohort, there were 4,439 cases of infection out of 5,193,825 person-days at risk in the non-booster group compared to 934 cases out of 10,603,410 person-days at risk in the booster group. This amounted to a lower rate of infection in the booster group compared to non-booster group by factor of 11.3 (95% CI, 10.4 to 12.3) at least 12 days after receiving a booster. There were 294 cases of severe illness out of 4,574,439 person-days at risk in the non-booster group compare to 29 cases out of 6,265,361 person-days at risk in the booster group, which translates to a 19.5 fold lower rate of severe illness (95% CI, 12.9 to 29.5) in the booster group. Finally, the rate of confirmed infection at least 12 days after booster was lower than the rate of infection from 4-6 days after receiving a booster by a factor of 5.4 (95% CI, 4.8 to 6.1).
The authors chose the cutoff of 12 days post-booster dose carefully, allowing for 7 days of immunity to build up plus 5 days of delay in detection of infection by PCR test. The date of second vaccine dose was included in the regression in order to account for waning effects of earlier vaccination, and the fact that high-risk groups received their vaccines earlier.
Rates of infection in the 4-6 days post-booster time period could be underestimated due to booster recipients possibly undergoing less frequent PCR testing and behaving more carefully in the days following their booster dose. Next, the definition for severe illness may not be completely accurate. An oxygen saturation of 95% could be normal for some individuals, such as the elderly or people with COPD, so a saturation below 94% would not be severe in these cases. Additionally, deaths were not mentioned in this analysis. Finally, the rate ratio analysis is slightly unclear to the reader since the number of people in the booster and nonbooster groups was dynamic, and therefore one cannot calculate rates by dividing number of instances by number of people, in addition to the fact that the authors adjusted for specific factors within their model.
This is one of the first reports on the effectiveness of receiving a booster (third) dose of the Pfizer BNT162b2 vaccine in reducing rates of COVID-19 infection and severe illness in adults ages 60 years and older.
This review was posted on: 8 October 2021