On May 13, 2020 Oxford and NIAID scientists published the pre-clinical animal study looking at vaccinated versus un-vaccinated rhesus macaques (monkeys). They “observed a significantly reduced viral load in bronchoalveolar lavage fluid and respiratory tract tissue of vaccinated animals challenged with SARS-CoV-2 compared with control animals, and no pneumonia was observed in vaccinated rhesus macaques.” [1]
But the Oxford/AstraZeneca vaccine in this animal challenge study, “did not provide sterilizing immunity” which is considered the “gold standard for any vaccine.” Vaccinated monkeys could still become infected and had viable virus in their nose which could be transmitted and infect others!
Even though “no evidence of immune-enhanced disease following viral challenge in vaccinated animals was observed”, experience with other vaccines tells us that is not a firm guarantee that such will be the case for humans, states William
Based on the observation that their vaccine could “moderate the disease” they proceeded into human clinical trials.