Pepe Alcamí
IDIBAPS researcher and scientific director of the HIV Unit at Hospital Clínic de Barcelona
In my opinion in the answer the Pfizer delegate confuses a word because she is asked about stopping transmission and she answers: "Did we know about stopping immunization [sic] before it entered the market? No!". I understand that what she meant to say was: "Did we know about stopping transmission before it entered the market? No!".
The designs of all the phase III studies of the COVID vaccines were not designed to measure this parameter - protection against transmission/infection - because the endpoints of the trials, the efficacy parameters that were assessed, were the development of covid symptoms, mild, moderate or severe. What the studies show, and what they say in the publications, is that the vaccines protected against the development of symptoms, mild or severe, i.e. they protected against the development of symptomatic disease or death.
To know whether they blocked transmission, it would have been necessary to perform a weekly or bi-weekly PCR on all trial participants to see if those vaccinated suffered less asymptomatic infection or did not become infected, which is not feasible given the number of patients included in these trials. In fact, for many vaccines the endpoints are protection against disease, not germ transmission.
Subsequent work by various groups including the CDC and the UK Vaccine Study Group concluded, particularly in cohorts of healthcare workers who had routine PCRs even if they had no symptoms, that vaccines, including Pfizer's, had a high degree of protection against infection, indicating that they blocked transmission.
These data on protection against transmission refer to the pre-omicron era, with D614G, alpha and delta variants. They are not extrapolable to omicron against which protection is diminished, as it partially escapes the immune response and where two-dose vaccination has been shown to be much less protective against disease and also against transmission.