The way covid-19 has progressed it looks that ultimately whole of the global population will be ultimately exposed to virus with morbidly ranging from very mild to severe in intensity which would be depending on the age and health of the individual.
Yes' if an effective vaccine for COVID-19 does not become available, most of the world population (at least 70%) will become affected when the pandemic will die down, not before killing millions of people.
Take Australia for example. It was hardly affected because of its sparse population. The problem was big cities: they all fared similarly. Hard to get away from other (infected) people within a big city.
Optimism is never a good strategy Teaba Wala Aldeen Khairi . Intelligent and capable people expect the best and spoil the worst. The truth, supported by evidence and background, is that this pandemic still affects many more people, Muhammad Ghafoor Ali . I fully agree with Fouad Yousuf. The actual impact of COVD-19 is not yet quantifiable or measurable. Please, see this information: https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situation-reports
I believe herd immunity will be achieved well before reaching the 70% mentioned by Fouad Yousuf because children are affected in a much lesser degree. Obviously it will depend on the age structure (Africa has 41% of population under 15 years, Europe only 16%). It will also be necessary to see if the victories over the virus in some countries such as New Zealand and Iceland are sustainable over time.
No body exactly knows when a herd immunity to COVID-19 for a country would be achieved. Only way to know would be when new cases continue to decrease despite ambivalence towards social distancing measures.