"Herd immunity" is the level of individual immune response such that, as a sufficient proportion of and sufficiently distributed across a population, a specific pathogen can't spread in that population.
It's usually used about vaccines.
So there are three parts here:
- individual immunity; there's enough immune response this person won't get sick
- there are enough individuals as a proportion of the population to allow for herd immunity
- those individuals are evenly distributed throughout the population
With COVID-19, we run smack into the limits of our ignorance.
For item 1, the question is "Can you become lastingly immune to the SARS-CoV-2 virus that causes COVID-19?"
Nobody knows; the virus has existed for about six months. There really isn't any reliable way to tell except wait and see who gets it again, when. The painstaking work to determine what an immune condition looks like hasn't been done in large part because there hasn't been time, but the component of time in the human immune system's natural responses can't be reduced. There are a great many unanswered questions about this; research is ongoing, and consensus hasn't emerged yet.
For item 2, you figure out how many people need to be immune as a proportion of the population with the formula 1 - 1/R₀ and we don't confidently know what R₀ is for this disease. We're reasonably confident it's more infectious than the flu, which has an R₀ of about 2 and requires a population where 50% of people are individually immune to have herd immunity.
Since there's no vaccine, that's talking about recovered survivors. There isn't anywhere that has had as many as 10% of the population contract COVID-19. That isn't enough, and can't possibly be enough. Anybody talking about herd immunity as a means to re-open the economy is either totally unclear on the concept or necessarily talking about making sure absolutely everyone gets infected with COVID-19, with the concomitant health-care system collapse and excess deaths in excess of 2% of the population.
(Most people actively trying to kill 2% of the population would expect to be arrested, but apparently it's OK if you're a capitalist.)
For item 3, it does no good if all the cities contain people who are immune and all the hinterlands contain people who are not immune; you might meet your population percentage for herd immunity that way, but as soon as the disease gets into a region where people aren't sufficiently immune, off it goes. So again, anybody talking about herd immunity is either unclear on the concept or asserting that they want absolutely everyone to be infected.
Well... why not pay the one-time cost and get it over with? That's awful, but it might be the least awful option out of a selection of dire consequences?
Go look at item 1 again. We don't know herd immunity to SARS-CoV-2 is possible in principle. (While we certainly do know COVID-19 kills people.) There are lots of diseases where you just don't get herd immunity because you don't get lasting individual immunity. (All those common-cold coronaviruses, for example.) (Yes, this means we don't know for sure a useful vaccine is possible.)
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Looking ahead, even in a best case scenario the disease will continue to circulate, finding pockets of non-immune people around the world. High infectiousness plus long asymptomatic times plus air travel mean that the only safeguard for countries that do get it under control (say, New Zealand and Australia) is stringent testing and long quarantine. Goodbye tourism.
+Peter T best case is someone figures out how to produce a cheap, sufficiently effective vaccine and we repeat the smallpox eradication effort.
Not betting on it, but it's there.
(Tourism is not a big worry; forced autarky would be a huge problem.)
The smallpox eradication program was made possible only by the fact that smallpox immunity is really long-lasting. Here we're talking about something vaguely similar to measles, where every year more non-immunes enter the population and after four or five years there are enough of them to ignite another mass outbreak.
The solution there is early childhood vaccination as a precondition for school or daycare, along with ring vaccination similar to smallpox.
One key advantage, such as it is, is that it will be a relatively long time before we start getting the nonsense about "harmless childhood disease" being widely accepted.
+D.C. in my comment, "sufficiently effective" is doing a LOT of work!
I don't expect we're going to get a vaccine, and I certainly don't expect a sufficiently effective one, any time this decade. I would not be surprised if we never get a vaccine. (This doesn't mean I don't think it would be great if we DID get a vaccine; that would be splendid. It's just not looking real likely.)
The "harmless childhood disease" unfortunates are doubling down and holding COVID-19 parties. It's really really easy to underestimate the cost an authoritarian attaches to admitting they're wrong; it's functionally infinite.
(I am in favour of treating the anti-vaxxers as bioterrorists, no asterisks, no exceptions, no mitigation of sentencing. Because functionally and materially, bioterrorists is just precisely what they are.)
I'm a bit more sanguine regarding the prospect of a vaccine. SARS-CoV-1 patients are still showing high antibody titres after 15 years. Given that we can now trick microbes into producing antigens specific to inducing immune responses to the ACE2 receptor agonists that SARS-CoV-2 uses to invade human cells, a vaccine sometime in the 18 to 30 month range looks reasonable.
OTOH, I'm biased. I would really like to think I'll be seeing my grandchildren again. Yes, serum tests might be effective (although the social implications are problematic) but that's a seriously "if nothing else works" Plan B. Plan C is massive test, retest, trace, retest, quarantine, etc.
RE: bioterrorists. Since I'm in a presumptively high-risk group, no argument here.
+D.C. Measles frequently gets eliminated from countries and it could in fact be eradicated from the world if we got our act together just a *little* more.
(My anti-antivax conspiracy theory conspiracy theory is: if measles was properly wiped out then Big Pharma couldn't sell measles vaccinations any more. Therefore who profits the most from antivax conspiracy theories? --I don't actually hold to this... though I wouldn't be exactly surprised either.
(I do think there are two kinds of anti-vaxxers: those who are in it for the ticket sales; and those who've been fooled by the former. I'm willing to extend sympathy in the abstract to the latter even though when faced with them in the concrete I get very frustrated; especially because at the moment, in New Zealand, they don't have as big a negative impact on immunisation rates as general health inequities do.)
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In New Zealand at least, though tourism is not going to be on the cards for A Good While Yet, the government's been working really hard to keep trade flowing freely. It's also starting to talk in a plan-for-the-future kind of way of a) creating a shared bubble with eg Australia, Taiwan, any Pacific Islands who feel safe, etc; and/or b) opening to international students who could realistically be quarantined for two weeks before starting their studies. Similarly I'd expect room for people to continue immigration [or emigration if you *want* to move to a country where a pandemic is rife I guess].
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