The number of people positive for COVID-19 out of the entire population is called prevalence – a number expert can only roughly estimate, based on other factors including COVID-19 deaths and recovered patients.
The positivity rate is the percentage of people who test positive for COVID-19 out of all testing conducted in a country or specific area.
It’s testing that determines whether an individual decides to self-isolate or not, and the number of total positive cases is what governments use to determine whether to impose stricter restrictions or ease them.
The World Health Organization recommended in May that the positivity rate be kept below five percent for at least two weeks before governments consider easing restrictions.
A high positivity rate might mean the risk of infection is really high in a certain area, or it might mean not enough testing has been done. Generally, the higher the positivity rate, the higher is the risk of infection.