Gregory Glenn
Analyst · Cantor Fitzgerald
Thanks, Stan. Turning to Slide 4. In the first quarter, we made excellent progress in advancing 2373 through our various clinical trials. As Stan mentioned, we announced both interim data and final analyses for our South Africa Phase IIb and U.K. Phase III trials, rapidly completed our enrollment of our PREVENT-19 Phase III trial and initiated a number of other studies building on the excellent safety and efficacy data generated to date. In the next few slides, I will give an overview of these key clinical developments from the quarter as well as discuss our upcoming clinical milestones.
Beginning with our South Africa Phase IIb trial on Slide 5. In the complete analysis from this trial, which included over 4,400 participants, 2373 achieved its primary endpoint, demonstrated overall efficacy of 48.6% and efficacy of 55.4% amongst HIV negative trial participants. We also confirm II/III efficacies against the B.1.351 variant first identified in South Africa, demonstrating approximately 51% efficacy against mild, moderate or severe disease against this variant in HIV negative participants. Importantly, there were no severe cases in the vaccine group and 5 severe cases, including 2 deaths, in the placebo group. Four out of 5 were due to the variant of concern, B.1.351. Last week, full results from our South Africa Phase IIb study were published in the New England Journal.
Now moving to Slide 6. You can see a summary of results from the final analysis of our U.K. Phase III trial, which included over 15,000 participants. In this trial, 2373 achieved its primary efficacy endpoint, demonstrating overall efficacy of 89.7%. The final analysis also confirmed 2373's high efficacy against multiple variants of COVID-19, demonstrating 96.4% efficacy against the original COVID-19 and 86.3% efficacy against the B.1.1.7 variant, first identified in the U.K.
The final analysis also showed that 14 days after 1 dose, vaccine efficacy was 83.4%. In participants 65 years of age and older, we demonstrated a vaccine efficacy of 89%. And finally, over 40% of the study participants in this trial had medical comorbidities that placed them at high-risk for severe COVID disease. Among this group, 2373 demonstrated a 90.9% efficacy. We view this as particularly important, illustrating 2373's ability to protect a portion of the population at high-risk for COVID-19 disease. Our results from this trial have been submitted for publication in a peer-reviewed journal and posted to net archives.
In summary, on Slide 7, 2373 has shown excellent efficacy in trials conducted in settings where both the original and variant strains were circulating. Importantly, in both trials, 2373 demonstrated 100% protection against severe disease and it was well tolerated. Adverse events were balanced between the vaccine and placebo group. At this time, we have also initiated crossover arms in both trials, which will help further inform our booster strategy.
Now turning to our PREVENT-19 Phase III trial in Slide 8. In the U.S. and Mexico, where we are conducting this trial, we announced that in February, we had completed enrollment for this trial -- this Phase III trial. The notably diverse study population included 30,000 participants from a wide range of demographic backgrounds as well as individuals who are at high-risk of COVID-19 due to factors such as medical comorbidities or age.
At the end of April, we also implemented a blinding crossover allowing all subjects in our PREVENT-19 trial to receive active vaccine. The successful conduct of the trial will allow unblinding for a final analysis as opposed to an interim analysis, based on the events we've accrued prior to crossover as the trial is well powered to describe efficacy. Stan will discuss the timing of announcements in more detail later in the call. The updated protocol has been posted to our website today.
Moving to Slide 9. PREVENT-19, which have been enrolling U.S. participates in December of last year, was also conducted in the context of a rapid emergent of variant strains. By the end of April, at the close of the endpoint accrual period, we've seen the rapid rise of B.1.1.7 strain. This variant now accounts for over 60% of the U.S. cases. But as I mentioned, in our U.K. Phase III study, we showed that 2373 was 86% effective against the B.1.1.7 variant and 96% effective against the original strain of COVID-19.
And now turning to Slide 10. We also initiated a pediatric extension of PREVENT-19 in April. On this slide, you can see the study design, which includes 3,000 adolescent participants, 12 to 17 years old, across 75 trial sites in the U.S. 2/3 of the participants will receive 2373 and 1/3 will receive placebo in order to evaluate the efficacy, safety and immunogenicity of 2373. We see this as an important step towards expanding the reach of our vaccine and making progress towards ending the pandemic. We expect to implement a blinded crossover of 6 months after the initial set of vaccinations.
In parallel with advancing 2373 through our various efficacy trials, we saw significant progress in 2 other areas of our clinical program in the first quarter. Our variant strain, COVID-19 variant vaccine and combination vaccine candidates. I would like to briefly touch on our recent developments of these 2 programs.
Last week, we shared preliminary data at the World Vaccine Congress from a preclinical study of our B.1.351 candidate, which you can see on Slide 12. As a part of this work, primates who originally received a 2-dose regimen of 2373 last year were boosted 1 year later with our B.1.351 candidate, containing Matrix-M adjuvant. Data from this study showed within 7 days of receiving the B.1.351 booster, animals exhibited strong functional immune response. Immunity debt, as we showed at the World Vaccine Congress, can block 1.351 spike from binding to ACE2. We'll have more to say on this work in the coming weeks.
Finally, turning to Slide 13. We also dedicated significant efforts during the quarter to exploring various combination vaccines, including a combined NanoFlu 2373 vaccine candidate, all combined with Matrix-M adjuvant. We recently completed a preclinical study of this combination vaccine to assess its immunogenicity and protective efficacy in animal models, and the data are summarized in the manuscript posted to bio archive.
Briefly, we found that the combination NanoFlu 2373 vaccine induced strong functional antibodies in ferrets with hemagglutinin inhibition antibodies and ACE2 receptor inhibiting antibodies that were comparable between immunization with both a combination vaccine and respective component vaccines. In these animals, the combination vaccine induced high levels of anti-spike and neutralizing antibodies.
When immunized hamsters were challenged with SARS-CoV, the quadrivalent and 2373 combination vaccine showed protection which was not diminished compared to the 2373 alone. No viral replication was detected 4 days after the COVID challenge and the lung showed no changes. These results demonstrate that a novel combination of flu COVID vaccine has the potential for a transformative impact on both diseases. We expect to bring this candidate into clinical trials later this year.
I would also like to highlight on Slide 14, ongoing clinical trials conducted by our partners, reflecting collaboration around the world for development of our COVID vaccine. These include both a Phase I/II trial conducted by Takeda in Japan as well as a Phase II/III clinical trial conducted by the Serum Institute of India to evaluate the safety and immunogenicity of 2373.
These trials were initiated in the first quarter. Enrollment is complete in the Takeda trial and enrollment is about to begin in the Phase III portion of the Serum trial. We look forward to sharing additional updates as these trials progress and expect these clinical trials led by our partners will play a meaningful role in promoting global access to our vaccine.
Next, I would like to discuss the combo study, which was initiated in April. This is a U.K. government-sponsored study being conducted by the University of Oxford that will assess 2 dose priming and boost regimens of COVID vaccines, mixing the AZ, Pfizer, Moderna and Nova vaccines in various combinations, given 8 weeks apart. The study will evaluate the safety and immunogenicity with results expected in the third quarter.
We also wish to highlight an exciting collaboration regarding an important advance in malaria prevention. The University of Oxford's malaria vaccine candidate R21, which includes our Matrix-M adjuvant, has been advanced through a Phase IIb clinical trial conducted in a study population of 450 children, ages 5 to 17 months. In April this year, we announced a publication of the data from this trial in Preprints with The Lancet, where R21 demonstrated 77% efficacy.
These landmark results underscore the potential for this vaccine to serve as a tool to help control malaria globally. Clinically -- clinical development of this vaccine candidate continues with a Phase III licensure trial underway in 4 countries in Africa to evaluate the safety and efficacy of R21 in 4,800 participants, ages 5 to 36 months.
We look to the commercial potential of this vaccine and we supply Matrix-M adjuvant to the component to Serum Institute of India, who will manufacture the R21 vaccine. In efforts to ensure widespread distribution, Serum Institute has granted rights to use -- has been granted rights to use Matrix-M adjuvant in the vaccine in endemic regions and will pay Novavax royalties on its market sales of the vaccine. Novavax will have rights to sell and distribute the vaccine to travelers in military vaccine markets.
With that, I'd like to turn it over to John to discuss our key supply developments for the quarter.