Real-world data shows that the COVID-19 vaccines are highly effective in preventing clinical disease and even more effective in preventing severe outcomes associated with COVID-19, such as hospitalization and death. However, new evidence suggests that the effectiveness of the vaccines decreases over time. To combat this issue, booster doses have now been implemented in the UK.
It has been reported recently that vaccine effectiveness (VE) against symptomatic COVID-19 peaks in the early weeks following the second dose and falls by 20+ weeks against the Delta variant for the BNT162b2 and ChAdOx1-S vaccines.
In most groups, VE against severe COVID-19-related outcomes remains high to 20+ weeks following vaccination, but in older adults and those with comorbidities displayed greater waning, compared to younger, healthy adults.
On the 14th of September 2021, booster COVID-19 vaccines were introduced in the UK. From the evidence gathered in the COV-BOOST trial, that showed that the mRNA vaccines could elicit a strong booster effect with low reactogenicity, regardless of the primary vaccine type, the UK Joint Committee on Vaccination and Immunisation (JCVI) recommended either a half dose of mRNA-1273 or a full dose of BNT162b2 vaccine to be administered as a booster dose, no earlier than six months following the completion of the primary vaccine course.
In a new study, researchers from several British institutes estimated the effectiveness of booster vaccination against symptomatic COVID-19 in adults aged 50 years and older.
A preprint version of this study, which is yet to undergo peer review, is available on the medRxiv* server.
This study utilized a test-negative case-control design which estimated VE of a booster dose of the BNT162b2 vaccine against symptomatic COVID-19 confirmed via polymerase chain reaction (PCR).
The authors compared the vaccination status of adults over 50 years of age who had a PCR confirmed symptomatic SARS-CoV-2 infection with individuals who complained of COVID-19 symptoms but had a negative PCR result.
There was a total of 271,747 eligible tests from individuals 50 years or over. The tests were performed within ten days of symptom development and had linked to the National Immunisation Management system, with a 95% match rate.
From this total, 13,569 had not received any dose of vaccine, 149,434 received ChAdOx1-S 140 days post a second dose, 84,506 received BNT162b2 140 days post a double dose. Thus, of the participants who had received a BNT162b2 booster dose, 6,716 had initially received the ChAdOx1-S vaccine, and 17,521 had initially received the BNT162b2 vaccine.
The overall VE of a booster dose in the proportion of cases and controls can be detected at approximately day seven and then stabilizes around day eleven. VE of 87.4% was observed with a BNT162b2 booster relative to those that had received two doses of the ChAdOx1-S vaccine initially and 84.4% in those who received two doses of the BNT162b2 vaccine initially.
The authors also performed secondary analysis, focusing on the two-to-six-day period following the booster dose as the baseline. These results were similar to the primary results with a relative VE 14 days following a booster dose of 85.5% for the ChAdOx1-S vaccine and 82.6% for the BNT162b2 vaccine. When the analysis was performed using unvaccinated individuals as the baseline, administration of a booster dose was associated with an absolute VE of 93.1% when ChAdOx1-S was the primary vaccine and 94.0% when BNT162b2 was the primary vaccine.
The results from this study suggest that utilizing booster vaccines may offer high levels of protection against symptomatic COVID-19, at least for a short period of time. However, due to the recent implementation of the booster vaccination program in the UK, further follow-up research is required to evaluate how the protection changes over time. In individuals who received the BNT162b2 vaccine as the primary course compared to those who received the ChAdOx1-s vaccine as the primary course, there was a slightly lower relative VE estimate of the booster vaccine, which may likely be due to the different baseline with higher VE following two doses of BNT162b2 when compared to ChAdOx1-S.
This study provides evidence gained from the real-world setting of significantly increased protection associated with a booster vaccine against symptomatic COVID-19 in individuals aged over 50 years, irrespective of which primary course of vaccine they were given. Furthermore, these results show that via a booster vaccine, high levels of immunity can be achieved among older adults who are at an increased risk of severe COVID-19.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Effectiveness of BNT162b2 (Comirnaty, Pfizer-BioNTech) COVID-19 booster vaccine against covid-19 related symptoms in England: test negative case-control study, Nick Andrews, Julia Stowe, Freja Kirsebom, Charlotte Gower, Mary Ramsay, & Jamie Lopez Berna, medRxiv, 2021.11.15. https://doi.org/10.1101/2021.11.15.21266341, https://www.medrxiv.org/content/10.1101/2021.11.15.21266341v1
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