Because a lot of people will be dead by the time we get to ~70% immunity.

And the huge number of severe cases will overwhelm health care systems by an order of magnitude, risking the lives of other people who will need medical care to survive.

As I understand it, the healthy majority would get the immunity while continuing to isolate the at risk groups.

The "at risk" groups include a sizable percent of the US population, and isolating all of their first degree daily contacts alone is likely impossible.

Hospice patients, and the hospice workers.

People already in treatment, and the healthcare professionals providing them with care.

People who smoke, have diabetes, a heart condition, or are immune compromised, and so on, and everyone they know.

This has got to be millions and millions of people, and this isolation will be extraordinarily porous. Those folks have friends and family too, they will slip up, exposing their contacts and then the virus is on the other side of a firewall. And we've seen how quickly the virus spreads in isolated groups!

Which is not very realistic as a possibility given the number of people that fall into the at risk groups. It sounds simple in theory but seems very unlikely to be something workable in practice.

It's impossible to isolate only the at risk group. That's like 30% of the population. Also, there is growing evidence that even mild cases can leave more or less permanent damage.

Big IF we ever reach those numbers at all. (!)

The path to herd immunity includes a lot of death.

Uh... from what do you understand that? Sweden isn't even close. Their deaths/case ratio isn't too far off from New York. We have reasonably good evidence now that New York is maybe 30-50% of the way to herd immunity (plus or minus a factor of two or so).

New York has six times the per capita deaths that Sweden does. Just extrapolating and multiplying their current number by 3x6==18, that's about 35000 dead swedes until they get to herd immunity.

I genuinely don't understand the alternative world some people live in where Sweden has already beaten this. They're current curve is about 3 weeks behind most of Europe, largely due to the lack of lockdown that allowed the early outbreak to spread for longer.

That's a statement from a politician, not a scientific result. He's not predicting "almost there", he says they "could" reach herd immunity within a month. And that 30% with immunity number quoted is wildly off from the actual research already done on this stuff in Italy and New York. In fact the back of my envelope tells me that it requires an even higher asymptomatic undercount than even the Stanford study did.

This isn't evidence. You're reading spin. The article even tells you that.

Almost to herd immunity? No. They're at less than 1% infected (like most places), with more deaths per capita than the us despite a younger healthier population.

That’s a political answer to a question they’ve staked their political future on. Let’s wait for actual scientific evidence before accepting it.

I read The Model Thinker some time last year. It includes a chapter on the SIR model used in epidemiology. The book states the basic mathematical model for working out how much of the population needs to be immune before 'herd immunity' is achieved is (R0 - 1)/R0.

I have heard R0 estimated around 4. So, technically you would need 3/4 of the population to be immune. The result of this being that R_t drops below 1 thus no longer becomes an epidemic and eventually either disappears or becomes seasonal.

They'd rather not (and I agree) have the accompanying death toll that comes with getting to herd immunity by having lots of people get infected.

(we also get the benefits of herd immunity if enough people get an effective vaccine)