The idea of voluntarily infecting the entirety of the population with a virus to prevent economic downturn seems a very dangerous idea to me. The purpose of society is to ensure the well-being of the people. It is not economic growth.

Furthermore, I see a few problems with that idea :

- it seems very hard to create a truly harmless virus. Viruses still destroy cells to replicate, they cause immune system response, inflammation, what would happen to weak or immunocompromised people ?

- if immunity through vaccines lasts 6 to 24 months, why wouldn't this be the case here too ? You'd still need to develop a vaccine or keep infecting people with new strains of this virus

- why would the deadlier strain disappear ? Unless you also engineer the mild strain to be much more infectious, both strains would still spread equally in the population. Presumably the milder strain would be less symptomatic so it would in fact spread less

- it's not demonstrated in what way economic downturn would be more harmful in the end. Economy is a means not an end.