https://www.biorxiv.org/content/10.1101/2021.04.20.440647v1.full.pdf
Moderna is apparently thinking about ways to understand what antibody levels are needed to get protection from infection. They did a series of experiments in monkeys in which the monkeys were given the vaccine at different doses to get different levels of antibodies to the spike protein, or no vaccine as a zero antibody control. Then they exposed them to a fixed amount of virus. They then tested them for virus growing in their lungs (BAL) and noses (nasal swab). As we've seen in the past, it's easier to protect the lungs than it is to protect the upper respiratory tract. In the breakthrough infections in humans, they are getting symptoms like sore throat, nasal congestion, and loss of smell. But it's the infections in the lungs that are killing people, so that's definitely better than the other way around.
For the lungs, the amount of virus growing was roughly linearly proportional to the amount of antibodies that the monkey had before the viral challenge. That is, if the antibody levels are ten times higher, the amount of virus in the lungs is about ten times lower.
They also did an experiment where they correlated the antibody levels before viral challenge with the antibodies to nucleoprotein, which is in the virus but not the vaccine, and thus serves as a proxy for infection. There was a general correlation, but not a super tight one. Many lab animals are inbred, which means they give a more consistent result than a population of humans. But here they used rhesus macaques in these experiments, a type of monkey which are genetically diverse, like humans. So apparently antibodies are important, but they are not the only factor. We have seen in many studies in humans, the innate immune system, especially interferon, appears to be important in fighting off this virus, so that is probably another part of it. T-cells might be a third.
