As has been posted before, herd immunity has never been acheived in the modern age without a vaccination program. We could definitely go that route, but the deaths would likely be in the 3M+ range for the US (there are models out there for this) and unfortunately we would break down both our economic and health systems long before we got to that point.
Humans have achieved herd immunity with infectious diseases before but that has largely caused the upheaval of societies to happen and the total re-ordering of humans on the planet.
NY was likely at less than 10% infection rate when it was overwhelmed by the virus. So getting to the 75% level would be horrific and no community would be able to stay standing. Seroprevalence studies around the world generally put the infection rate around 3% in most Western countries (there are areas with higher rates within countries, but even the highest of those are areas like NYC and the Lombardy region of Italy where the rate is at around 20%). Needing to increase the infection rate 25X worldwide is not something that is remotely a working possibility. For starters, the fatality rate would increase from the current estimated fatality rate as health systems worldwide are overwhelmed and more people die because we lack the manpower and resources to help them.
Data shows that 75% of the drop in economic activity from pre-COVID baseline in the US occurred before stay at home orders were implemented in each state. People started seeing things happening and adjusted their behavior even before the government reacted to it. So if you just allow it to spread you end up creating panic and crash your economy.
The 1919 Pandemic killed an estimated 5M in the US (when the population was 1/3 of what it was today) and 50M+ worldwide ( when the worldwide population was below 2B).
This particular virus in the most like that one since then in terms of its combination of transmission and fatality. Our science and medicine is so much more advanced then than, but we still have somewhat limited options in fighting virus'. I think of this like a football or basketball game where you are behind. What do you do? You try to lengthen the game as much as possible, give your team time to come back. That is what I think we are doing now. We are just trying to lengthen the game to allow the coaches (scientiests and doctors) to come up with plans to win in the end. Don't let the other team run up the score where you can't makle a comeback. (I know its not a perfect analogy, but its the one I have).
Also, Sweden wasn't exactly trying to get to herd immunity - their own health leaders will tell you that. it is more that they were relying on their citizens to behave in a certain manner without the government mandating them to. They closed secondary schools and universities. They closed some large factories. They asked citizens to work from home if they are able to. They asked them not to visit the elderly. They allowed restuarants and shops to stay open but asked that people be respectful and use social distancing measures in them (stay 1.5 meters from each other). It has allowed them to stay more open than other countries, but they are still in a recession and suffering job losses at a lower rate than the hard closing countries and their death rate is currently 6th per capita in the world and rising (like to be 2nd or 3rd per capita by the end of the summer since their cases are still occurring at a higher rate than other European countries and the US right now). Keep in mind that Swedish citizens have a very high level of trust in their government leaders and experts, unlike in the US. All Swedish residents have the right to read any official documents (letters, emails, documents, etc) they want as part of the Principal of Public Access. Only personal financial information, ongoing criminal investigations, and security of the realm are exempted.
Hopefully they will find some combination of medical therapeutics and a vaccine that work. If they don't we will likely need to go to more of an Asian model worldwide to make the economy work. That would first require getting the numbers down to where you can legitimately trace and isolate outbreaks. Once you do then you open up your economy (I mean really open up, not just partial openings), but anytime you start to have outbreaks people need to be conditioned to wearing masks and engaging in social distancing programs (not lockdowns), so that things can proceed. Asian countries have alot of muscle memory from previous outbreaks and so very naturally take preventive measures as soon as an alarm is raised about a new virus.
Right now the majority of outbreaks in the US are happening due to large groups of people being in close proximity to each other. It's going to be quite a balancing act allowing some more normality to return but not having too many large groups of people together. the better contact tracing you have the more you can allow larger groups.
