[id2000 (OP)]

The Oxford vaccine is reported as having lower efficacy than the Pfizer vaccine.

There have also been reports that the Oxford & Pfizer trials used different methods to monitor for infections.

I believe that the Pfizer trial only tested for infection when symptoms were present while the Oxford trial regularly tested for infection irrespective of symptoms.

Have I misunderstood the reports? If not, does that not suggest that the measures of efficacy are not comparable? I.e. the Oxford trial is measuring the impact of the vaccine on asymptomatic cases while the Pfizer trial is not?

[alancalverd]

Efficacy of a vaccine is a measure of its ability to prevent disease. So you have to start with known uninfected people who have no known immunity, vaccinate some of them, then see how many of the vaccinatees actually contract the disease compared with the control group who received a placebo. So no "asymptomatic cases" - vaccines do not treat, they prevent.

All the research groups were faced with similar problems: considerable urgency; very few but rapidly increasing actual cases; reluctance to intentionally infect more than a tiny cohort of fit volunteers; known, unknown and suspected  variance of susceptibility within the community; probable rapid mutation of the target virus; little knowledge of the persistence of immunity; huge variance in chance personal exposure......so however you define and measure efficacy, it is prone to huge uncertainty at the Phase 2 trial stage.

Best response: take whatever you are offered. Any parachute is better than none.

[id2000 (OP)]

- vaccines do not treat, they prevent.

Sorry Alan, I think my final question was ambiguous. I appreciate that vaccines do not treat, they prevent. But during the trial, some months after vaccination, a subject may become infected but asymptomatic. If one trial is testing the subject every week - then it will identify the asymptomatic infection; if another trial is only testing when symptoms develop then it will miss the asymptomatic infection and so give a different measure of efficacy?

I am just trying to understand what I am reading - I will certainly take whatever is offered.

[alancalverd]

I assume that everyone had enough knowledge of virology to test for the presence of antibodies and viruses. Plenty of antibodies and no virus = effective. But you still don't know how many people were exposed to a potentially infective quantity of virus, so you have to take the difference between two large cohorts (active and placebo) and hope that the average is meaningful. Comparing the relative frequencies of needles in two haystacks is quite an art.

[Colin2B]

If one trial is testing the subject every week - then it will identify the asymptomatic infection; if another trial is only testing when symptoms develop then it will miss the asymptomatic infection and so give a different measure of efficacy?

Both measured efficacy in the same way, the primary outcome - efficacy against symptomatic cases of covid-19 - which showed that the Oxford vaccine was 62.1% effective in participants who received two standard doses.
The Oxford trial also monitored for asymptomatic infections as a secondary outcome, which the Pfizer trial does not appear to have done.