Trial objectives, participants and supervision
In this multicenter, double-blind, randomized, placebo-controlled trial conducted in South Africa, we evaluated the safety and efficacy of two standard doses of the ChAdOx1 nCoV-19 vaccine, administered 21 to 35 days apart, compared to saline solution ( 0.9% sodium chloride) placebo. Adults 18 to less than 65 years of age, without chronic or well-controlled medical conditions, were eligible to participate. Included among the participants were 70 HIV-negative people enrolled in group 1, in whom intensive safety and immunogenicity studies were planned. The main exclusion criteria were positivity for the human immunodeficiency virus (HIV) in the screening (for the efficacy cohort), previous or current Covid-19 confirmed in the laboratory, a history of anaphylaxis in relation to vaccination and morbid obesity (index of body mass [BMI, the weight in kilograms divided by the square of the height in meters], ≥40). Detailed inclusion and exclusion criteria are provided in the Supplementary Appendix, available with the full text of this article at NEJM.org. The ChAdOx1 nCoV-19 vaccine was developed at the University of Oxford, which was responsible for conducting and supervising the study (see Supplementary Appendix).
The authors had full access to the study data, confirmed the accuracy and completeness of the reported data and guaranteed the study’s fidelity to the protocol (available at NEJM.org). An independent data and security monitoring committee has reviewed the effectiveness and unblinded security data. A local study safety doctor reviewed all serious adverse events as they occurred. The trial was monitored by an external clinical research organization, which ensured adherence to the protocol.
The study was reviewed and approved by the South African Health Products Regulatory Authority and the ethics committees at the University of Witwatersrand, Cape Town, Stellenbosch and OxTREC before the study began. All participants were fully informed about the study’s procedures and possible risks, and all signed written informed consent documents prior to enrollment in the study.
Testing Procedures
Trial participants were randomly assigned to receive a dose of 0.33 to 0.5 ml (depending on the batch) of the ChAdOx1 nCoV-19 vaccine or placebo by intramuscular injection on the day of randomization and a second injection 21 to 35 days later. The injections were administered in the deltoid muscle of the non-dominant arm, and the participants were observed for 30 minutes after the injection for acute reactions. The injections were prepared and administered by the site staff who were aware of the assignments of the participants in the trial group, but were not involved in any other trial procedures. Study participants and all other study teams remain oblivious to the study group’s duties. Details of the test procedures are provided in the protocol (pages 68–73). Follow-up is in progress.
Safety
The safety analysis assessed the occurrence of local and systemic reactogenicity requested within the first 7 days after an injection, unsolicited adverse events within 28 days after an injection, baseline changes in laboratory safety measures and serious adverse events. More details on the methods used to assess safety and reactogenicity are provided in the Supplementary Appendix. Adverse event data as of January 15, 2021 are included in this report.
SARS-CoV-2 Test, Complete Genome Sequencing and Genome Assembly
The use of a nucleic acid amplification test for SARS-CoV-2 infection included sampling on scheduled routine visits (detailed in the protocol) and on non-routine visits when participants had any symptoms suggestive of Covid-19 disease. Participants were informed at the time of randomization about which clinical symptoms should trigger a visit to investigate possible SARS-CoV-2 infection (Table S1 in the Supplementary Appendix). In addition, short messages were sent to the participants every 2 weeks as a reminder to present for investigation if they had symptoms. The details of the nucleic acid amplification tests, complete genome sequencing and phylogenetic analysis are described in the Supplementary Appendix.
Neutralization Tests
The SARS-CoV-2 serostate at randomization was assessed using a nucleoprotein (N) IgG assay, as described elsewhere.8 For antibody neutralization studies, pseudovirus neutralization assays (see the Methods section in the Supplementary Appendix) were performed at Monogram Biosciences, to prototype the virus in serum samples obtained 2 weeks after the second dose of the vaccine in 107 vaccine recipients ChAdOx1 nCoV-19 randomly selected that were seronegative for IgG N protein at the time of enrollment.
To assess the neutralization activity of antibodies induced by the B.1.351 vaccine, serum samples from participants in group 1 who had SARS-CoV-2 negative serological status at enrollment and assay titles for assorted pseudovirus neutralization assays for the pico D614G original 14 days after the second injection were tested with pseudovirus and live virus neutralization assays for activity against variant B.1.351.14.21 The testing of the neutralizing antibody activity against the original virus and variant B.1.351 was carried out before the assignment of the test group was revealed. The pseudovirus assays for neutralization activity against the original D614G peak, a triple RBD mutant (containing only K417N, E484K and N501Y) and peak B.1.351 were performed at the National Institute of Communicable Diseases (South Africa).14 The live virus neutralization assay test was carried out by a microneutralization focus assay in Vero E6 cells at the African Institute for Health Research, South Africa.14.21 Details of the pseudovirus and live virus neutralization assays have been published and are briefly described in the Supplementary Appendix.14.21
Efficacy objectives
The primary endpoint was efficacy against the symptomatic symptomatic nucleic acid amplification test – Covid-19 starting more than 14 days after the second injection in participants who were seronegative at randomization. Confirmed symptomatic Covid-19 and the classification of mild, moderate and severe disease were pre-specified and are defined in tables S1 and S2. Covid-19 cases were evaluated by at least two physicians who were independent of the study and were unaware of the study group’s duties. Disagreeing assessments were discussed between the two reviewers. The effectiveness of the vaccine against variant B.1.351 was a pre-specified secondary objective.
Other secondary efficacy objectives included efficacy against Covid-19 in the general population (including participants who were seropositive at randomization), specific efficacy for the seropositive base group, and efficacy against Covid-19 starting more than 14 or more than 21 days after first dose. Further details of secondary efficacy analyzes are included in the Supplementary Appendix. In addition, a post hoc analysis was carried out for general and seronegative populations, to assess the efficacy of the vaccine against the disease that occurs more than 14 days after the first injection, with restricted endpoint cases until October 31, 2020, as a proxy for non-B .1,351 Covid-19 variant. Variant B.1.351 has only started to be identified in the areas where the test sites (Johannesburg and Tshwane in Gauteng, and Metro do Cabo in the Western Cape Province) were based from mid-November 2020 (Fig. S1).15
Statistical analysis
Participants who received at least one dose of the ChAdOx1 nCoV-19 vaccine or placebo and returned the completed diary cards by day 7 after the first injection were included in the safety analysis of reactogenicity. The occurrence of each sign and symptom of local and systemic reactogenicity requested for 7 days after vaccination, adverse events and serious adverse events until January 15, 2021, are presented according to the test group.
The primary efficacy analysis was oriented towards the end point for the mild, moderate or severe Covid-19 compound and required 42 cases to detect a vaccine efficacy of at least 60% (with a lower limit of 0% for the confidence of 95% range), with 80% power. The vaccine’s effectiveness was calculated as 1 minus the relative risk, and 95% confidence intervals calculated using the exact Clopper-Pearson method are reported. Only participants in the population per protocol (all participants who received two doses of vaccine or placebo and were grouped according to the injection they received, regardless of their planned group assignment) who were seronegative for SARS-CoV-2 at the time of enrollment were included in the primary efficacy analysis. A sensitivity analysis was performed that included seronegative participants in the modified intention-to-treat population (all participants who received two doses and were grouped by their planned assignment, regardless of the injection they received). The confidence intervals reported in this article have not been adjusted for multiple comparisons.