Do participants of the COVID-19 vaccine trials get exposed to the virus?

Medical Sciences Asked by mgrstnr on November 20, 2021

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

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

Answered by Thilo on November 20, 2021

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:

Answered by Thilo on November 20, 2021

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