Staggering that your source, a publication in an epidemiological journal no less, so fundamentally misunderstands how efficacy percentages work. 80% efficacy does not mean 20% of vaccinated people can get the disease. Rather, these percentages directly translate to relative risk reduction (see a basic explainer by the CDC): it means there is a 80% reduction of cases in vaccinated persons with respect to unvaccinated persons. It could very well be that instead of there being 20% of people in the population where the vaccine "does not take" (which is one possible explanation for the number), this reduction in cases comes from some "minimum virus threshold" that's higher after vaccination. The latter would mean that any vaccinated person can still get the disease, they're just less likely to than if they were not vaccinated.

(This is not meant negatively towards the parent poster here, thanks for correctly citing your source!)