What You Need to Know About the AstraZeneca COVID-19 Vaccine Trial Pause

vaccine vials
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Key Takeaways

  • AstraZeneca paused Phase 3 clinical trials of COVID-19 vaccines worldwide.
  • One clinical trial participant in the U.K. reportedly experienced a spinal inflammatory disorder.
  • A committee of independent experts is investigating the illness and will determine when Phase 3 clinical trials can resume.

UPDATE: As of October 23, AstraZeneca has resumed vaccine trials in the U.S. The company's clinical trials first resumed in the U.K. on September 12.

Pharmaceutical company AstraZeneca paused late-stage human clinical trials of COVID-19 vaccines around the world on September 9 after a participant in the U.K. became ill.

The patient may have developed spinal cord inflammation called transverse myelitis, but details have not been officially released. It is unknown whether the person who became ill received the vaccine or a placebo, although STAT has reported they received the vaccine.

Now, a committee of independent experts commissioned by AstraZeneca is reviewing safety data surrounding the event, which is protocol for large clinical trials when an unexpected or unexplained illness occurs in a participant.

“We will be guided by this committee as to when the trials could restart, so that we can continue our work at the earliest opportunity to provide this vaccine broadly, equitably, and at no profit during this pandemic,” Pascal Soriot, Chief Executive Officer of AstraZeneca, said in a statement.

The world is watching and waiting for a COVID-19 vaccine, so hearing there is a pause may be disheartening. The good news is that it means researchers are following the scientific process rather than succumbing to political pressure to quickly produce a vaccine.

What This Means For You

Although clinical trials are on hold, they are not terminated. Hope is not lost that a COVID-19 vaccine will become available in the future. Researchers from nine pharmaceutical companies are working together and following protocols to ensure that a safe and effective COVID-19 vaccine will ultimately be approved, manufactured, and available to the public. 

What Does a Pause Mean?

The pause in the Phase 3 clinical trials does not mean that there will never be a vaccine against SARS-CoV-2—the virus that causes COVID-19—and that we’ll never be able to leave our homes. The pause ensures that a standard review process is taking place to safeguard the integrity of all nine of the Phase 3 clinical trials currently underway for a COVID-19 vaccine.

During clinical trials, mild side effects are expected, such as pain at the injection site, headache, and fatigue. Each of these side effects can be a sign that the immune system is responding and creating antibodies. But symptoms like headache and fatigue occur with many illnesses, and may be coincidental occurrences in people participating in clinical trials. Those mild side effects are not cause for alarm.

When unexpected side effects or more serious illness occurs in a Phase 3 participant, scientists temporarily pause a clinical trial until they investigate the situation.

While the AstraZeneca trial is paused, eight other companies are in the middle of Phase 3 clinical trials, according to the New York Times coronavirus vaccine tracker:

  • BioNTech
  • GlaxoSmithKline
  • Johnson & Johnson
  • Merck
  • Moderna
  • Novavax
  • Pfizer
  • Sanofi

What Is the Process for Developing a New Vaccine?

The process for creating a new vaccine involves six stages, according to the Centers for Disease Control and Prevention (CDC). The first two stages involve exploration and a pre-clinical stage, during which scientists do lab work and begin studying the effects of a newly-created vaccine on cells and animals. The third stage is clinical development, which has three phases:

  1. Phase 1: During a Phase 1 study, scientists try to determine the strongest vaccine dose that causes the fewest side effects. This helps determine the proper dose to administer to people. A few dozen people participate in Phase 1 studies. 
  2. Phase 2. In a Phase 2 study, the vaccine is tested among hundreds of people who have similar characteristics, such as age, gender, and medical conditions, to determine how effective the vaccine is in certain types of people. If Phase 2 studies are successful, people will produce antibodies, and the clinical trial moves into Phase 3.
  3. Phase 3. The purpose of Phase 3 clinical trials is to evaluate safety of a vaccine. Thousands of people participate in Phase 3 trials and scientists continue to monitor the effectiveness of the vaccine, as well as side effects that people experience.   

Next Steps

Once Phase 3 clinical trials restart, they will continue until there is enough evidence of vaccine safety and effectiveness. Then, the fourth step in the vaccine development process can take place: the vaccine goes to the Food and Drug Administration (FDA) for regulatory review and approval.

When the vaccine receives approval, the fifth step can proceed: the vaccine starts to be manufactured and distributed. Testing doesn’t end there, though. Quality control tests are performed throughout the manufacturing process, and manufacturing facilities are inspected by the FDA. Studies to monitor side effects (also called adverse events) in vaccinated people take place after the vaccine is approved, licensed, and administered to the public.

For now, we wait as AstraZeneca’s committee of independent experts do their review. We continue to wash our hands, wear our masks, and stay six feet apart to do what we can to prevent or slow the spread of SARS-CoV-2.

How Will COVID-19 Vaccines Work?

There are several variations of COVID-19 vaccines in Phase 3 testing, but all of them need to transmit a tiny bit of the SARS-CoV-2 virus into your body. The tiny bit of genetic material is not enough to make you sick, but just enough so your immune system can recognize it as foreign material and create antibodies against it. That response primes your immune system so that if you come into contact with SARS-CoV-2 in the future, your body can attack it right away, preventing a full-blown illness.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Phillips N, Cyranoski D, Mallapaty S. A leading coronavirus vaccine trial is on hold: scientists react. Nature. 9 September 2020. doi:10.1038/d41586-020-02594-w

  2. Centers for Disease Control and Prevention. Vaccine Testing and the Approval Process. May 1, 2014.

By Donna Siegfried
Donna Siegfried is an award-winning journalist with over 30 years' experience. She is a member of the American Medical Writer’s Association.