>>12610687Vaccine trials only looked at whether those vaccinated later became ill (i.e. had symptoms of illness for 48 hours, got a test, and test came back positive, see
https://www.modernatx.com/sites/default/files/mRNA-1273-P301-Protocol.pdf). Thus, it’s possible the vaccine only reduces symptom severity to non-noticable levels, but doesn’t stop infection and by extension transmissibility of disease to others. A methodology which tested continuously for infection would require very frequent COVID tests of participants, and would therefore have high dropout rates, making it hard to get sufficiently large sample sizes
