That’s called a challenge trial, and it’s unethical to do when you have a disease that can kill people.
What they do instead is divide the volunteers into a group that gets the vaccine you’re testing and a group that gets a placebo (typically just a sugar pill or saline injection). Neither the patient nor the doctor giving the shot knows which the person is getting to avoid any tainting of the study. You then closely monitor everyone. After a period of time, a statistically significant number of people will get infected. You reduce the amount of time this takes by selecting volunteers who are most at risk for exposure and thus likely to get infected sooner rather than later.
Once you have enough people infected, you pull the data on who was in the control group and who was vaccinated to see how well you did. If you had 1,000 people in the control group get infected and 2 people in the vaccine group get infected, you did really well. If you had 1,000 people in each group get infected, your vaccine is worthless. You can further look at the people who got the vaccine and still got infected to see whether it made a difference in the severity of their illness. Again, you look at your control group and see what percentage of them got seriously ill, then you look at your vaccine group and see whether that percentage of seriously ill people was lower.
In the case of Moderna, not only was the vaccine 95% effective, but no one who got the vaccine got seriously ill. Given the large number of study participants, some of them SHOULD have gotten seriously ill, so the vaccine made a difference even when it didn’t 100% protect them.
So I’m certain they tested the subjects with the ultra reliable COVID testing we are so familiar with and then didn’t use any political bias or subjectivity when they checked the box to say the symptoms “weren’t really that bad I promise.”