The first indicator we will see is likely to be a reduction in the number of deaths in hospital from COVID-19. Current data suggest that around 75% of deaths from COVID-19 have been in the over 75 age group. You can read more here.
Assuming COVID vaccines are 70% effective on average then death rates should reduce by approximately 50%. This will hopefully be followed by a decrease in numbers of people requiring admission to hospital with this virus. Sadly there are very few vaccines for the treatment of bacterial infections and much more research is needed.