ChAdOx1-nCoV-19 vaccine maintains efficacy despite a longer prime-boost interval
1. Overall vaccine efficacy at least 14-days after the second dose was 66.7%.
2. After the initial 21-day exclusion period, there were no hospital admissions for COVID-19 in the ChAdOx1-nCoV-19 group.
Evidence Rating Level: 1 (Excellent)
Study Rundown: At the time of this publication, the COVID-19 pandemic continues to burden healthcare systems across the globe and the demand for effective vaccines remains high. The ChAdOx1-nCoV-19 vaccine, perhaps better known colloquially as the AstraZeneca vaccine, is a chimpanzee adenoviral vectored vaccine with a full-length SARS-CoV-2 spike insert. With the high demand for vaccines, distribution and optimal dosing intervals have become a major area of discussion. This study was a pooled analysis of four randomized controlled trials in the UK, Brazil, and South Africa that evaluated the efficacy of the ChAdOx1-nCoV-19 in adults. The primary outcome was virologically confirmed symptomatic COVID-19 more than 14 days after the second vaccine dose. Study results demonstrate that overall vaccine efficacy at least 14-days after the second dose was 66.7%. In addition, after at least 21 days had elapsed after receiving the first vaccine dose, there were no hospital admissions for COVID-19. In participants who received two standard doses, vaccine efficacy was found to be improved in those with a longer-prime boost interval (≥12 weeks) compared to those with a short interval (<6 weeks). However, these were exploratory analyses unable to test for significance. Overall, this study pooled data from four different randomized controlled trials and provided insight on the effect of dosing intervals. Future studies may look to evaluate whether the vaccine may reduce transmission between individuals.
Click to read the study in The Lancet
In-Depth [randomized controlled trial]: Between Apr 23 and Dec 6, 2020, 17 178 participants (8597 receiving ChAdOx1 nCoV-19 and 8581 receiving control vaccine) were recruited and vaccinated across four randomized controlled trials in the UK, Brazil, and South Africa. The included studies were a phase 1/2 study in the UK (COV001), a phase 2/3 study in the UK (COV002), a phase 3 study in Brazil (COV003), and a phase 1/2 study in South Africa (COV005). Eligible participants were adults aged ≥18 years, SARS-CoV-2 N protein seronegative at baseline, and had at least 14 days of follow-up after the second dose. The primary outcome of virologically confirmed symptomatic COVID-19 more than 14 days after the second vaccine dose was defined as a nucleic acid amplification test (NAAT)-positive swab combined with at least one qualifying symptom (fever ≥37.8℃, cough, shortness of breath, or anosmia or ageusia). Overall, there were 332 cases of primary symptomatic COVID-19 occurring more than 14 days after the second dose. The overall vaccine efficacy was found to be 66.7% (95% CI, 57.4-74.0) with 84 of 8597 (1.0%) participants in the vaccine group and 248 of 8581 (2.9%) in the control group testing positive for the primary outcome. From 22 days after receiving the first dose, there were zero hospital admissions for COVID-19 in the vaccine group compared to 15 in the control group. Overall vaccine efficacy was higher in participants who received a second standard dose more than 12 weeks after the first (81.3%, 95% CI 60.3-91.2) compared to those who received their second dose within 6 weeks of the first (55.1%, 95% CI 33.0-69.9). Overall, this pooled analysis shows that the ChAdOx1-nCoV-19 vaccine is efficacious at reducing primary symptomatic COVID-19 and associated hospital admissions.
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