It might be obvious to many in the medical field, but it had me wondering when reading about the new trials for the Oxford-AstraZeneca vaccine that is currently being tested on a group of people.

When you give a group of 10,000 people the vaccine or placebo, do they actually get exposed to the virus or is everything just based on statistics, i. e. assuming that some of the participants will eventually become exposed to the virus by chance?

The fact that a forced exposure would leave both (especially the participants with the placebo) at risk, leaves me to believe that it is not done this way, but then wouldn't a group of 10,000 be far too small?

2 Answers 2


You are right, no ethical committee would allow anyone to expose study subjects on purpose to a potentially life-threatening infectious agent. So you will have to use statistics to figure out if the vaccine has reduced the likelihood that someone got infected.

The group size depends. If for example, you used healthcare professionals as subjects the likelihood that they will be exposed to COVID is high so you get an answer much faster. But in later stages, the trials would be bigger. Have a look at clinical trial phases (0-4) eg. here: https://en.wikipedia.org/wiki/Phases_of_clinical_research

  • Challenge trials for viruses like influenza have been done, so I'd disagree with "no ethical committee would allow anyone to expose study subjects on purpose to a potentially life-threatening infectious agent". The answer is correct regarding plans for covid-19 study, though.
    – Bryan Krause
    Jul 23, 2020 at 13:43
  • Yeah, that is true that such studies have been done. I do remember some studies with low-risk viruses such as the common cold. There was a trial link in exposing participants to the common cold to answer whether being cold makes you more susceptible (It does not). However, my point was maybe not clear enough that it would be hard to believe that approval would be granted for a novel virus where the long-term effects are not yet known (as long as there is ethical oversight).
    – Thilo
    Jul 23, 2020 at 14:10
  • 2
    Agreed, I think the novelty and unknown factors are bigger barriers than danger itself - makes it harder to give informed consent. Though it's not just for the cold, like I said, influenza has been used in challenge trials yet is potentially deadly. There are likely some countries doing covid challenge trials anyways, but not necessarily to any ethical standards and with questionable resulting data...
    – Bryan Krause
    Jul 23, 2020 at 14:30

It appears that the UK is actually planning a challenge trial. This is rather interesting from an ethical point of view.


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