The vaccines which are administered intramuscularly aren't expected to prevent infection at all.
You won't have neutralizing antibodies in the mucosal surfaces in the upper respiratory tract which would be required to completely prevent infection.
You will have NAbs in the bloodstream which should very effectively isolate the infection to the upper respiratory tract. Immune responses will also be primed so that the infection in the upper respiratory tract will be dramatically reduced. In many cases though the infection will be detectable via rtPCR but that won't be significant since the person won't be symptomatic, won't "feel sick" (or very sick) and will be very unlikely to transmit the virus. It should also entirely cut out the multi-organ involvement of covid since the neutralizing antibodies in the blood will prevent spread to the lungs/kidney/heart/brain/etc.
Testing via rtPCR for "infection" would be an expensive waste of time in a vaccine which isn't expected to confer that level of protection. And when it failed to confer that level of protection you're left trying to explain why that low number doesn't matter to a population expecting some kind of medical miracle.
Instead they watch for symptoms and then they confirm that is a SARS-CoV-2 infection via rtPCR, and this is the typical "efficacy number" which is reported (I think the initial Oxford trial results did report the results of randomized rtPCR testing so their numbers were dramatically lower, which has caused all kinds of confusion since its not apples-to-apples with the mRNA vaccine results).
Reduction in transmissibility is more difficult to determine which is why those numbers aren't reported, it is assumed that reduction in symptoms correlates with reduction in transmissibility. So none of the vaccine trials can conclusively rule out asymptomatic spread in vaccinated individuals. However since the people are being followed over time what is caught is any infection which subsequently produces symptoms. So the vaccinated asymptomatic individuals would be truly asymptomatic and not just presymptomatic. Dramatically cutting down symptoms is expected to be a pretty good proxy to dramatically cutting transmission.
Relevant quote from a recent BMJ article on truly asymptomatic transmission:
> The transmission rates to contacts within a specific group (secondary attack rate) may be 3-25 times lower for people who are asymptomatic than for those with symptoms.1121415 A city-wide prevalence study of almost 10 million people in Wuhan found no evidence of asymptomatic transmission.16 Coughing, which is a prominent symptom of covid-19, may result in far more viral particles being shed than talking and breathing, so people with symptomatic infections are more contagious, irrespective of close contact.17 On the other hand, asymptomatic and presymptomatic people may have more contacts than symptomatic people (who are isolating), underlining the importance of hand washing and social distancing measures for everyone.
https://www.bmj.com/content/371/bmj.m4851
The title study here on HN on the Oxford vaccine is pretty clear that they're looking for "primary symptomatic COVID-19" which means they're not screening for asymptomatic infection, they're waiting for symptoms, then confirming.
They found that one shot conferred good protection (76%) for at least 90 days post vaccination (after the first 2 weeks) with little waning and some evidence that it gets better. They also found that the BEST boosting interval was 12+ weeks or more, reaching 82% efficacy. This matches what I've heard some virologists yap about informally online that longer boosting intervals are generally what is recommended (more or less letting the learning immune response bake longer while letting the immediate response to the vaccine vector wane) but the early studies used short intervals due to pandemic driven concerns around giving health providers and the elderly the best immunity in the shortest period possible. Now that we're transitioning to needing to vaccinate literally everyone the message may unfortuntely shift in ways that make people suspicious.