Vaccine Trial Pause

April 1, 2021

“Temporary Pause”

One of the world’s largest vaccine trials aiming to combat COVID-19 halts after “an unexplained illness.”

What To Know

The Basics

  • Vaccine invented by Oxford University who partnered with a U.K.- based drug company, AstraZeneca.
  • Clinical trials ongoing worldwide: U.K., Brazil, South Africa & America. U.S. Phase III trial (final stage) began on August 31st for 30,000 Americans.
  • Vaccine initially showed promising immune response, “no safety concerns”, some flu-like side effects during first trials.

What Happened

We don’t know exactly but one participant developed an “unexplained illness.”

AstraZeneca: “In large clinical trials, illnesses will happen by chance and must be independently reviewed.”

New York Times report: participant developed inflammation of the spinal cord, often brought on by viral infections.

How Vaccines Work

  • Not all vaccines are the same.
  • AstraZeneca vaccine uses a viral “carrier” (a common cold virus in chimpanzees) to deliver a small amount of SARS-COV-2 genetic material, to produce an immune response.
  • Moderna & Pfizer vaccines use manufactured SARS-COV-2 genetic material to spark an immune response (mRNA vaccine). No mRNA vaccines have been approved for human use.


  • Dozens of potential vaccines are in clinical trials *worldwide* ~ 100+ preclinical development. In the U.S., three vaccines are in final stage trials.
  • More than one vaccine *may* eventually combat COVID-19.
  • We are witnessing a historic race for vaccine development, both for the speed of trials and the type of vaccine itself (an approved mRNA vaccine would be the first of its kind).

We have no way of knowing *if* the participant is reacting to the vaccine or separately ill. While we wait for more info, this once again puts focus on safety/efficacy in the race to a vaccine. This vaccine was thought to be a front-runner. TBD as to what happens next.

Announcement about the pause: CLICK HERE

NYT report on what happened to participant:

In depth understanding of different vaccines by JAMA

Good write-up by National Geographic on Moderna vaccine

by Jenna Lee,