Even if there was - testing and controlled-infecting 50% of the population would be a huge effort and probably not shorter than isolating old people and allowin

Even if there was - testing and controlled-infecting 50% of the population would be a huge effort and probably not shorter than isolating old people and allowing an uncontained infection to burn through. But the big problem is that it might not even help, the worst case scenario is that absent a vaccine the virus becomes just a more infectious and deadlier cold that people risk getting every season.