OpenSAFELY study shows the effectiveness of Pfizer COVID booster doses

By Neha MathurReviewed by Danielle Ellis, B.Sc.Jun 7 2022

In a recent study posted to the medRxiv* pre-print server, researchers assessed the effectiveness of the Pfizer–BioNTech BNT162b2 coronavirus disease 2019 (COVID-19) vaccine in the seven million United Kingdom (UK) adults who were eligible to receive booster shots between 16 September and 16 December 2021.

Study: Effectiveness of BNT162b2 booster doses in England: an observational study in OpenSAFELY-TPP. Image Credit: Jo Panuwat D / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Background

Under its national COVID-19 vaccination program, the UK initially prioritized its high-risk population to receive booster shots in September 2021 before being made available for its general adult population. However, after the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) Omicron, booster availability was reduced to three months on 8 December 2021. Earlier, boosters were available after six months from when an individual received their second COVID-19 vaccine dose.

About the study

In the present study, researchers estimated the effectiveness of booster shots of the BNT162b2 vaccine in adults who had received two vaccine doses between 16 September and 16 December 2021. The team used data from the OpenSAFELY-TPP database for the study analysis. The database had information on 40% of English primary care practices linked to National coronavirus surveillance, hospital, and death registry data. The study follow-up continued for 10 weeks or 31 December 2021, where the researchers compared each booster recipient with a matched unboosted control subject based on booster priority and prior vaccination status.

The team allocated a matched control to each study participant based on the National Health Service (NHS) region, study entry date, vaccine brand, and date of second vaccine dose. A Cox regression model adjusted for these factors to estimate hazard ratios (HRs), where 1-HR expressed as a percentage indicated the BNT162b2 booster effectiveness. The HR estimates compared boosted with unboosted people, overall and separately, for days 1 to 28 and 29 to 70. Additionally, the team estimated HRs for days one to seven, eight to 14, 15 to 28, 29 to 42, and 43 to 70. The study had several outcomes: a positive-SARS-CoV-2 test, COVID-19-related death, COVID-19 hospitalization, and death unrelated to COVID-19.

Study findings

The study population comprised 6,990,219 individuals matched as controls and then rematched in the booster group. As expected, the matching factors and the proportion of people with pre-existing morbidities were similar between the study groups. Although the standardized mean difference between the two groups was consistently under 0.1, the control group subjects had more learning disabilities, lower SARS-CoV-2 testing frequency, and higher rates of severe mental illness and deprivation. The screening of 23,151,145 person-weeks of follow-up yielded 123,378 positive SARS-CoV-2 tests, 3,672 COVID-19 hospitalizations, 588 COVID-19 deaths, and 6,990 non-COVID-19 deaths.

The boosted individuals showed an aberrant increase in the aggregate incidence of positive SARS-CoV-2 tests around seven days after receiving booster shots; however, they did not show a similar pattern for other severe COVID-19 outcomes. Moreover, the differences in the aggregate incidence of positive SARS-CoV-2 tests between the study groups were apparent in the first few days.

Accordingly, the 10-week risk of positive SARS-CoV-2 test in the boosted vs. unboosted group was 47.3 and 84.0 per 1000, respectively, corresponding to a risk difference of 36.8 per 1000. The risk difference for COVID-19 hospitalization, COVID-19 death, and non-COVID-19 death were 3.6, 1, and 8.1, respectively.

The (overall) booster effectiveness for days one to seven were 50.7%, 80.1%, 88.5%, and 80.3% for positive SARS-CoV-2 test, COVID-19 hospitalization, COVID-19 death, and non-COVID-19 death. Additionally, the booster effectiveness against all COVID-19 outcomes was generally lower during days one to 28 than between days 29 to 70. Within shorter periods, the booster effectiveness against positive SARS-CoV-2 tests appeared to wane from around six weeks after booster vaccination, 68.2% vs. 45.3% between days 15 to 28 and days 43 to 70.

Conclusions

Overall, a booster shot of BNT162b2 reduced the rates of positive SARS-CoV-2 tests by ~50% during the first 10 weeks after the booster administration. Additionally, the booster shots decreased the rates of COVID-19 hospitalization, COVID-19 death, and non-COVID-19 death. More importantly, the booster effectiveness remained similar against all severe COVID-19 outcomes regardless of the brand of vaccine used for primary vaccination and prior infection.

Conversely, the estimated booster effectiveness was lower in those below 65 years and clinically vulnerable populations. Since the current study showed evidence of waning booster effectiveness against positive SARS-CoV-2 tests, the authors emphasized the need for monitoring booster vaccine effectiveness over time.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources