The COVID-19 vaccine pioneered at Oxford University and manufactured worldwide by pharmaceutical giant AstroZeneca has been one of the three leading candidates to deliver a safe and effective vaccine to the public by year’s end. Regulators in the U.K. have indicated that they could allow use of the vaccine by healthcare workers and others most at risk from COVID-19 in advance of certification by the E.U. In the United States, the vaccine was considered the leading candidate for an “October surprise,” allowing Trump to announce a vaccine available in some limited quantity in advance of the election.
But on Tuesday, AstroZeneca announced a temporary halt to the Phase 3 trials after at least one patient has developed what’s being called “a potentially unexplained illness in one of the trials.” Though intentions are to eventually have the vaccine tested by over 30,000 volunteers in sites across the United States, it’s believed this represents a serious adverse reaction by one of the early volunteers in a U.K. Phase 3 trial that began in August.
This sort of action is not unusual. Because vaccines are given to hundreds of millions, or even billions, of patients, even a very low percentage of adverse reactions represent a potential disaster. And because vaccines are given to healthy people, rather than those who are ill, they understandably need to demonstrate a level of safety much greater than drugs given to patients in extremis. This temporary halt by no means indicates that the Oxford/AstroZeneca vaccine is a failure. It could well be back on track in short order. This is actually a very reassuring sign that, even with all the pressures in place, researchers are attempting to stick to the rules meant to deliver a vaccine that is both very safe and effective.
AstroZeneca has made it clear that the halt to further vaccination was made at their discretion and wasn’t forced on them by regulators. It’s likely the trial will resume after a short period to review the status of the volunteer in question.
The description of the incident as an “unexplained illness” may be an indication that this is simply a volunteer falling ill with some disease unrelated to the trial. With thousands of people enrolled in a large Phase 3 trial, it’s inevitable that such events happen, and holds like this are not unusual. In fact, Phase 3 trials are often placed on a temporary hold even when nothing at all has happened, just to allow researchers time to review what they’ve learned before proceeding.
Published data from the Phase 2 trials is available in the British medical journal The Lancet. Researchers found no serious adverse reactions among 534 recipients of the vaccine, though many volunteers did report lesser symptoms including pain at the site of injection, chills, muscle ache, and headaches—all things that are commonly reported by people receiving other vaccines, such as the annual flu vaccine. In many cases, these complaints were similar in level to volunteers receiving a placebo.
Where vaccines under test by Moderna and Pfizer/BioNTech utilize messenger RNA and techniques that are new to human vaccines, the Oxford/AstroZeneca vaccine is more traditional in nature. It uses the shell of adenovirus that naturally infects chimpanzees as a vector to deliver fragments of the SARS-CoV-2 virus intended to generate an immune response. All patients in the Phase 2 study were found to have a high level of neutralizing antibodies following two doses of the vaccine.