The trails have tried to determine who gets actual symptoms from the virus, not whether it keeps you from getting infected at all. So for instance, they may not be measuring those who are asymptomatic.
I believe from what I read that the trials are indeed regularly taking blood samples from its participants, in order to check infection levels in both groups in the trial (vaccinated and placebo). This is in addition to documenting the development of symptoms. So theoretically, it looks like they will grab the asymptomatic infected.
For example (
from the NIH site) :
"Volunteers must provide informed consent to participate in the trial. After providing a baseline nasopharyngeal and blood sample, participants will be assigned at random to receive either a single dose of the investigational vaccine or a saline placebo. The trial is blinded, meaning neither investigators nor participants will know who is receiving the investigational vaccine. Participants will be followed closely for safety and will be asked to provide additional blood samples at specified time points after the injection and over two years. Scientists will analyze the blood samples to detect and quantify immune responses to COVID-19. Of note, specialized assays will be used that can distinguish between immunity as a result of natural infection and vaccine-induced immunity."
What is also still a bit unclear is how good is the statistical breakdown of the volunteers being tested, meaning how much are they truly representing a typical population.
In a rare move, four manufacturers have made their trial protocols publicly available while testing is being done. This has led to questioning certain points in their methodology (such as in
the British Medical Journal) :
"The first question is whether the right endpoints are being studied. Contrary to prevailing assumptions (including those of a former Food and Drug Administration commissioner
8), none of the vaccine trials are designed to detect a significant reduction in hospital admissions, admission to intensive care, or death.
9 Rather than studying severe disease, these mega-trials all set a primary endpoint of symptomatic covid-19 of essentially any severity: a laboratory positive result plus mild symptoms such as cough and fever count as outcome events (
table 1). These studies seem designed to answer the easiest question in the least amount of time, not the most clinically relevant questions."
And apologies to
@NoSpin for keeping things muddy....
. It's just that this whole health issue is quite fascinating and unprecedented in many ways. I'd much prefer to be talking about a beautiful country road in northern Italy....