If the immune reaction in the vaccinated was good enough to avoid symptoms but a few still tested positive then those five are surely not the only ones. What are the chances that a random person without symptoms would be getting tested exactly those days that a very mild infection lasts? Those studies don't observe the subjects with daily PCR or something like that (impossible for ten thousands) and most mild infections remain undiscovered. And mild, hard to discover infections aren't exceptionally rare even without a vaccine, as antibody testing studies have shown again and again.
The pressing follow-up question is this: how many of the control group were discovered with (and despite of) equally weak symptoms? The answer could be anything between many more and many less. If it's more (more discovered very mild cases amongst the placebo group) then the vaccine apparently prevents most infections from happening at all, but if it's less then the vaccine doesn't really reduce the number of infected (and infective), it only prevents bad outcomes (which usually remain undiscovered, even in a phase III trial group).
A vaccine that only prevents bad outcomes would still be very valuable, but only to the vaccinated themselves because it would not create a herd immunity effect. This virus is very good at spreading from a mild case, so if the vaccinated still get unnoticeable mild cases they would still serve the virus as stopovers.
Contract tracing is more likely to identify sources of infection when the people remember being in the same room with someone who had symptoms. This is also why public transport is virtually absent as a infection source in contact tracing data, but private parties are very prominent.
From what I've heard, virus concentration in the upper respiratory tract (i.e. where the aerosols come from) peaks two or three days before symptoms start.
Here in Germany there's the statistic of "75% of infection sources are not found by tracing" circulating hard through public discussions. I doubt that many of those untraceable infections could be coming from symptomatic carriers, given the level of awareness. Coughing people basically do not exist in the 2020 public. And tracing should still be good enough to recognize most cases when the source became symptomatic after the contact (which would very likely be before the receiver became aware of the infection, triggering tracing). I consider this a pretty strong indication of asymptomatic spread, or very weakly symptomatic (thresholds are very subjective).
I would be surprised if they didn't specifically test all participants in the trial for covid antibodies, symptoms or not. Does anybody know what validation was done?
Depends a lot on the question of whether the antibody test can distinguish the immune system's reaction to the virus from the immune system's reaction to the vaccine (or in this case: to the proteins built from mRNA blueprints in the vaccine). If they can't, and this is quite likely given that they all target the same spike protein, the outcome would hopefully be positive for all of the vaccinated.
In the Pfizer trial, all the subjects in the interim evaluation had symptomatic infection confirmed by PCR test. I'm not sure about Moderna, but it's likely to use the same or similar methods.
The pressing follow-up question is this: how many of the control group were discovered with (and despite of) equally weak symptoms? The answer could be anything between many more and many less. If it's more (more discovered very mild cases amongst the placebo group) then the vaccine apparently prevents most infections from happening at all, but if it's less then the vaccine doesn't really reduce the number of infected (and infective), it only prevents bad outcomes (which usually remain undiscovered, even in a phase III trial group).
A vaccine that only prevents bad outcomes would still be very valuable, but only to the vaccinated themselves because it would not create a herd immunity effect. This virus is very good at spreading from a mild case, so if the vaccinated still get unnoticeable mild cases they would still serve the virus as stopovers.