Why would you expect a large gap, that's not how statistics work.

Any study of a sufficiently large population, like the phase 3 trials shouldn't vary much at all when scaled up to millions of participants.

That's assuming the error bounds are small with the clinical trials, but I think that is universally the case here.

Efficacy describes what happens in a carefully controlled study. Vaccine efficacy is the percent reduction in disease incidence in a test group of vaccinated subjects compared to an unvaccinated group under optimal conditions. Such subjects usually do not have any underlying medical illnesses.

Effectiveness describes what happens in the real world. Some tend to use ‘efficacy’ as if it is synonymous with the concept of ‘effectiveness’. This misuse is even rampant amongst doctors, who should know better.

In fact, vaccine effectiveness is the ability of a vaccine to prevent outcomes of interest in the real world.

For example, the influenza vaccine has been in use since 1938. It is spectacular at causing test subjects to make antibodies to the influenza virus. Now take a look at flu vaccine actual effectiveness (or lack thereof):

There are universal recommendations to give flu vaccine to the elderly starting at age 65. A 2020 study [1] on the influenza vaccine in the UK compared outcomes when persons aged 65 to 75 got their yearly flu vaccines, compared to those of the same age who didn’t receive it. They were all followed for 4 years. The ones who got flu vaccine did not have any lower hospitalization rates or lower death rates (from all causes).

This study showed that flu vaccine may be efficacious in controlled studies, but in real life it is often not effective in reducing death or hospitalizations – the outcomes of interest.

[1] https://pubmed.ncbi.nlm.nih.gov/32120383/

I wasn't aware of the difference in terminology, thanks.

In this case, despite the different terminology, the study efficacy and real world effectiveness should be very similar, because they're measuring the same thing - number of infections and serious disease.

I hope so. I haven’t looked at how healthy the study cohorts were to begin with. I doubt they had the same distribution of conditions that you’d find in the general population.

If you take into account herd immunity the actual effectiveness should be >99%.

Yeah, that's a good point. Obviously vaccinations benefit from network effects as the number of vaccinated individuals in the population increases - so over time this will skew the results towards higher efficacy.

What do you mean by lab efficacy? These vaccines are using large scale real life clinical trials.

I meant “trial efficacy”. See my comment on elof above.