"Volunteer" shouldn't be the word here.
It should be "paid".
If the pay was $10k per participant, the world would happily pay 100 people for each of 10 vaccine trials.
The people would understand the risk they were taking and be happy that the $10k was worth it for them. Countries would be happy that the payment was good value for money for finding a vaccine for their population.
"Volunteering" is in my mind ethically wrong - you are paying people in 'goodwill', which doesn't and shouldn't cover the risk satisfactorily.