What difference does mass vaccination make? The study referred to the difference in exposure to vaccinated or unvaccinated persons by vaccinated vs unvaccinated. In the study either all were vaccinated or all were not vaccinated. That was the point.
Mass vaccination is a major factor as the actual large scale data is very different to what they have found. As I stated earlier in NYC where we've had over a year and thousands of deaths, and lots of data on vaccination efficiency they have found the vaccinated subject is 5 times better off than unvaccinated. The CDC has released similar findings.
Now with regard to this study one massive issue with its credibility is this statement under limitations;
Fourth, owing to the age-stratified rollout of the UK vaccination programme, the age of the unvaccinated, delta variant-infected participants was lower than that of vaccinated participants. Thus, age might be a confounding factor in our results and, as discussed, peak viral load was associated with age.
There were fewer unvaccinated females than males (p=0·04) and, as expected from the age-prioritisation of the UK vaccine roll-out, unvaccinated participants infected with the delta variant were significantly younger (p<0·001; appendix p 3).
We know very well that infection rate and severity are significantly linked to age and vulnerability. So in such low cohort numbers such as this study to not even have matching age groups really means the numbers can not be compared accurately. Also the study only focuses on household spread, so not general spread through the community overall. One could expect higher transmission rates within close confines to be much higher especially between two sharing a bed, cleaning and eating facilities. The same as any other virus.
What it does display though that even with the age disparity and other issues the vaccine is still effective at preventing spread, just not foolproof.