@yaksha single digit ARR doesn't mean that vaccination is useless/ gives no significant protection. To imagine things better you can invert the number which gives you NNV (number needed to vaccinate) - how many people need to be vaccinated to protect one individual from getting sick.
Fe. NNV for AZ is 77 (according to data in the picture). So when you vaccinate just 77 people, you'll save 1 from the illness, which can result in hospitalization or even death.

And ofc ARR is highly dependant on specific epidemiologic situation = higher ARR in place with higher infection rate.