Any notion that Covid-19 was going to last for only a few months was very much misplaced in 2020. Especially after it was recognized that the SARS-CoV-2 virus was largely spread through the airborne route, all indications were that it would cause repeat bouts of waves.
This is what happened in the flu epidemic of 1918. In addition, very few scientists predicted that we would see the type of mutations that occurred over such a short period of time. This has resulted in the virus becoming both more transmissible and more able to evade immune responses.
The evolution of the virus has been so rapid that the Delta variant, which is now dominating the world, is at least twice as transmissible as the ancestral virus that was circulating.
Read more at “Why herd immunity isn’t going to happen”