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Interim Analysis of ChAdOx1 nCoV-19 Vaccine Against SARS-CoV-2
Quick Takes
- The ChAdOx1 nCoV-19 vaccine is safe and has an efficacy ranging between 50%-90% in preventing symptomatic COVID-19 depending on the dosing regimen.
- Most participants were 18-55 years age. Efficacy in the older, higher-risk population which would benefit the most is unclear.
- While press releases from other coronavirus vaccine developers claim >90% efficacy, the impact on public health will depend on the ability to distribute and administer the vaccine at scale.
Study Questions:
How safe and effective is the ChAdOx1 nCoV-19 vaccine (AZD1222) for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)?
Methods:
The ChAdOx1 nCoV-19 vaccine (AZD1222) was developed at Oxford University and consists of a replication-deficient chimpanzee adenoviral vector ChAdOx1, containing the SARS-CoV-2 structural surface glycoprotein antigen (spike protein; nCoV-19) gene. This study provides an interim pooled analysis of four ongoing randomized trials of the ChAdOx1 nCoV-19 vaccine done across three countries (United Kingdom [UK], Brazil, and South Africa), including 11,636 participants who were randomized 1:1 to receive the vaccine or a control product. Participants in the intervention group received two doses of the vaccine at least 4 weeks apart. Participants were swabbed if they experienced symptoms. In the UK trial, asymptomatic participants provided self-administered nose and throat swabs for testing. The primary outcome was virologically confirmed, symptomatic coronavirus disease 2019 (COVID-19), defined as a SARS-CoV-2–positive swab combined with at least one qualifying symptom (fever ≥37.8°C, cough, shortness of breath, or anosmia or ageusia). Participants who had a positive test at baseline or within 14 days of the second vaccine dose were excluded.
Results:
A total of 11,636 participants met the inclusion criteria for the primary analysis, 5,807 of whom received two doses of ChAdOx1 nCoV-19 and 5,829 of whom received two doses of control product. Median follow-up was 2 months (1.3-2.3). Most patients were 18-55 years of age. Those ≥56 years contributed to 12.2% of the cohort. Women consisted of 60.5% of the cohort. There were 131 cases of symptomatic COVID-19: 30 (0.5%) cases among 5,807 participants in the vaccine arm and 101 (1.7%) cases among 5,829 participants in the control group, resulting in an overall vaccine efficacy of 70.4% (95.8% confidence interval, 54.8–80.0). Efficacy was interestingly higher in those who received a low dose as their first dose: 90.0% (67.4–97.0%) vs. 53-65% for standard dose recipients. Efficacy after a single dose was 64.1%. Ten participants were hospitalized due to COVID-19, all of whom were in the control group. Serious adverse events occurred in 168 participants, 79 of whom received ChAdOx1 nCoV-19 and 89 of whom received the control product.
Conclusions:
The ChAdOx1 nCoV-19 vaccine is safe and has an efficacy ranging between 50%-90% in preventing symptomatic COVID-19, depending on the dosing regimen.
Perspective:
Currently, 48 vaccines are under clinical study, of which 11 are under evaluation in phase 3 clinical efficacy studies. This is the first published study showing effectiveness of a viral-vectored coronavirus vaccine in preventing symptomatic COVID-19 using a pooled analysis strategy. The pooled analysis strategy has its caveats, as procedures varied across the different trials, including follow-up and ascertainment of the primary outcome. Subgroup analysis, however, suggests findings were generally consistent, albeit with a wide confidence interval. Overall efficacy of the vaccine was 70%, similar to the efficacy of the flu vaccine. The US Food and Drug Administration indicated they would license vaccines with ≥50% efficacy. The efficacy reported is in a generally low-risk population, as most of the participants were <55 years old. Several questions remain: What is the effectiveness in a higher-risk and older population? Can the vaccine prevent asymptomatic transmission? What is the optimal dosing regimen? While press releases from other coronavirus vaccine developers claim >90% efficacy, the impact on public health will depend on the ability to distribute and administer the vaccine at scale.
Clinical Topics: COVID-19 Hub, Prevention
Keywords: Ageusia, Coronavirus, Cough, COVID-19, Fever, Influenza Vaccines, Primary Prevention, Public Health, severe acute respiratory syndrome coronavirus 2, Spike Glycoprotein, Coronavirus, Treatment Outcome, Vaccination
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