HCoV-OC43 is an endemic betacoronavirus very similar to SARS-CoV-2 (COVID-19). We don't have significant herd immunity to it. Instead most people get infected as youths and develop a level of immunity which protects against severe symptoms during subsequent reinfections. But it still kills frail elderly and immunocompromised patients.

We don't have proof of when HCoV-OC43 first emerged but circumstantial evidence indicates that it probably caused the 1889-1890 pandemic and killed about 0.06% of the world population. If we allowed SARS-CoV-2 to run it’s course without vaccination then the outcome would probably be similar or a little worse because the population today is on average older with a higher incidence of co-morbid conditions. (I am not suggesting we follow that approach.)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7252012/#!po=92...