AstraZeneca starts 30K-subject U.S. phase 3 COVID-19 vaccine trial

USA flag in New York
AstraZeneca's U.S. study of its COVID-19 vaccine candidate is the centerpiece of a planned 50,000-subject phase 3 program. (CC BY-SA 2.0/Tomás Fano)

AstraZeneca has begun a U.S. phase 3 clinical trial of its COVID-19 vaccine candidate AZD1222. The Biomedical Advanced Research and Development Authority-funded study will enroll 30,000 adults at sites in the U.S. and some other countries in which the virus is prevalent. 

AZD1222 began phase 2/3 testing in Brazil, South Africa and the U.K. earlier in the summer, putting AstraZeneca toward the front of the race to bring a COVID-19 vaccine to market. Those clinical trials are set to enroll around 17,000 people, potentially giving AstraZeneca a big enough sample to show whether AZD1222 works, but won’t shed any light on how people in the U.S. react to the vaccine.

The new phase 3 trial will provide AstraZeneca with those data and, in doing so, could boost its effort to bring the chimpanzee adenovirus-vectored vaccine to market in the U.S. The FDA accepts foreign clinical trial data, but filings for approval typically include results from U.S. study centers.

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AstraZeneca is now working to start enrolling adult volunteers at 80 U.S. sites. According to the listing on ClinicalTrials.gov, 62 of the sites are already in place. AstraZeneca is also set to add sites in Chile and Peru as part of an effort to include ex-U.S. locations based on the predicted transmission rates of the virus. The seven-day rolling average of new cases per million people in Peru is near the country’s record level of around 250. The number in Chile is down from the peak but has plateaued around 90. 

Trial sites in the U.S. and elsewhere will randomize participants on a two-to-one basis to receive two doses of AZD1222 or placebo four weeks apart. AstraZeneca is using a vaccine against meningococcal bacteria as a control in its other studies to make it harder for participants to tell whether they received AZD1222, but has opted to use saline in the U.S. trial. 

The study will assess the effect of AZD1222 on the numbers of people who contract COVID-19 more than two weeks after receiving their second dose. AstraZeneca is also evaluating the safety and tolerability of the vaccine and its effect on secondary endpoints such as the number of people who fall severely or critically ill after being infected with the coronavirus.

As in other COVID-19 trials, AstraZeneca’s use of an event-driven primary endpoint means the time to the release of data will depend on the level of viral transmission in the areas where participants live. That makes it impossible to conclusively say when AstraZeneca will have data from the U.S. trial, but there are signs elements of the study that it has some control over will move quickly.

In a separate statement, UW Health and the University of Wisconsin School of Medicine and Public Health, two sites involved in the study, said they plan to enroll 1,600 participants in eight weeks. A similar rate of enrollment at other sites could enable AstraZeneca to hit its recruitment target in the coming months.

The U.S. study is the centerpiece of a planned 50,000-subject phase 3 program. In addition to active clinical trials in Brazil, South Africa, the U.K. and U.S, AstraZeneca plans to initiate studies in Japan and Russia. 

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