Tag: herd immunity

Post #978: The two distinct levels of herd immunity, Part I

Edit:  Read Post #982 first.

I think this is an important post.  So, no kidding around. Minimal entertainment value.  And you’re going to have to follow a little bit of arithmetic in the followup post.

And at the end, you’re going to say, well, that’s obvious.  And yet, nobody seems to have grasped this.  Yet.  Or, at least, I have yet to find a single discussion that makes this point.

The point being that there are two distinct herd immunity levels that matter.  One is the fraction of the population that needs to be immune, to stop a pandemic, if immunity is the only tool used to limit spread of the virus.  That’s our classic “70% required for herd immunity”.  The other is the level of immunity needed to stop a pandemic while all the other infection limiting tools (masking, distancing, limits on gatherings) are still in place.  That’s a much lower limit that I have not seen discussed anywhere.  But that’s the level of immunity that’s relevant to ending this third U.S. COVID wave.  And I crudely estimate that second version of herd immunity to require something like 40% of the population to be immune.

That’s a little controversial, I think.  Or, at least, under-discussed.

So here goes.  Words today, crude numerical estimates tomorrow. Continue reading Post #978: The two distinct levels of herd immunity, Part I

Post #976: Simple projection to March 1, and herd immunity.

How close will the U.S. be to herd immunity for COVID-19, on March 1 2021?  Even with the significant uncertainty involved, it’s worth making a few simple projections of that.  If nothing else this will put the current vaccination effort into perspective.

Even under a rosy scenario where vaccine injections double from the current 1M/day, and new infections continue to fall, only about half the population will be immune to COVID-19 as of the start of March 2021.  That’s well below the 70% that was thought to be required for herd immunity under the original strains of COVID, let alone the higher percentage that will be required for the more infectious U.K. strain.

Why does March 1, 2021 matter?  If the U.S. COVID third wave truly has crested, our next test will come as the more-contagious U.K. variant becomes the dominant U.S. COVID strain.  That’s predicted to happen sometime in March 2021.  Near as I can tell, that prediction is based solely on the amount of time it took for that strain to become dominant in the London area.  So that amounts to a crude guess.  But, at present, that’s the only guess we’ve got.

And if that’s correct, then vaccinations should proceed as quickly as possible.  But under any plausible scenario, vaccination alone won’t prevent a fourth U.S. wave of COVID-19.  We really need to be thinking about what else we’re going to do — such as making N95 masks available to citizens — in addition to vaccinations.

Details follow.

Continue reading Post #976: Simple projection to March 1, and herd immunity.

Post #958: About a third of U.S. residents have already had COVID

In the spirit of “it can’t get but so much worse”, here’s an estimate of the fraction of the U.S. population that has already been infected with COVID-19.

As of today (1/17/2021), about 24 million cases have been reported.  It is well- established that there are several actual infections for every one that is reported.  That’s due to asymptomatic and mildly symptomatic individuals who don’t bother to go to a doctor about their symptoms, and so don’t get tested.

Here, I am assuming a conservative five-to-one ratio between total infections and formally-diagnosed infections.  That five-to-one splits the difference between two different estimates from CDC staff, as documented in Post #940.  A CDC estimate based on likelihood of getting tested would put that ratio at closer to 8-to-1.  A CDC estimate based on presence of antibodies in a recent (non-random) sample of blood draws would put it closer to 4-to-1.  I chose 5-to-1 as a seemingly reasonable estimate, as documented in that post.

Post #947: COVID trends through 1/12/2021. Is North Dakota the bellwether?

Source: Calculated from NY Times Github COVID data repository.  Data reported through 1/12/2021.

North Dakota now has the second-lowest rate of new COVID-19 cases in the country.  (Well, maybe third — they are tied with Vermont, rounded to the nearest whole number.)  Only Hawaii has a materially lower rate. Continue reading Post #947: COVID trends through 1/12/2021. Is North Dakota the bellwether?

Post #940: Seroprevalence surveys

Source:  Calculated from supplemental data, round 4, Bajema KL, Wiegand RE, Cuffe K, et al. Estimated SARS-CoV-2 Seroprevalence in the US as of September 2020. JAMA Intern Med. Published online November 24, 2020. doi:10.1001/jamainternmed.2020.7976

I’m still trying to nail down the actual fraction of the US population that currently has antibodies to COVID-19.  Bottom line is that the best you can do is take a conservative, educated guess.  I’ll be assuming a 5-to-1 ratio of total cases to diagnosed cases.  The rationale for that follows. Continue reading Post #940: Seroprevalence surveys

Post #928: Which state has the 4th lowest rate of new COVID-19 cases in the U.S.?

And, a couple of weeks from now, when that state achieves the lowest rate in the nation, will people finally pay some attention to how they achieved that?  And will our public health officials then stop lying about it?

And I am going to tell you which state, and what lie?

Eventually. Continue reading Post #928: Which state has the 4th lowest rate of new COVID-19 cases in the U.S.?

Post #925: Has Tennessee turned the corner? The jackknife says yes.

Source:  Analysis of NY Times Github COVID-19 data reported through 12/23/2020.

Today’s question is, what’s likely to happen next, on the graph shown above?  And how on earth could you attempt to quantify that?

Today’s answer is, the odds overwhelmingly suggest that Tennessee has in fact peaked.  And that line will probably continue downward.

Why?  I’d like to go all science-y on you, but the simple fact is, we’ve seen something like this 12 times before.  And every time, it was a true peak in the rate.  So, chances are pretty good that this is number 13.

And I’d say that there’s a single underlying reason for this.  My best guess is that once states hit those high rate of infection, they simply start to run out of un-infected people.  I think that this is how herd immunity is playing out, in the U.S., right now, for the third wave of the pandemic.

To be clear, I’m not advocating herd immunity as a strategy.  Far from it.  I think that’s an appalling inefficient way to deal with a pandemic.  I’m just pointing out that it’s happening, regardless.

Continue reading Post #925: Has Tennessee turned the corner? The jackknife says yes.

Post #921: North Dakota: Is this what herd immunity looks like?

The news media don’t pay much attention to places that aren’t having trouble.  But in the case of North Dakota, I think it’s pretty important to keep your eye on them even as their situation brightens.

I started out by joking about North Dakota and herd immunity  (Post #889, Post #890).  Then, at some point, that wasn’t a joke any longer (Post #900, Post #901).  So in this post, I’m not even going to try to be amusing, because this is going to be a hard thing for many people to accept.

When looking at the situation in North Dakota, I think it’s important to try to separate out your value judgments from your analysis of the facts.  As a value judgement, I think that letting many people die because you’re too stubborn to wear masks and want to return to a more relaxed lifestyle is a piss-poor way to respond to a deadly pandemic.   As well as being an inefficient way to deal with it.

But purely as a matter of fact, I can’t deny that it should work. Even if they stumbled into it by denying and ignoring all the best public health advice.  Even if nobody planned to have it play out this way.  Purely as matter of fact, if enough people get infected, your population ought to get to herd immunity.  Minus the ones that died on the way. 

And ignoring the unnecessary risk imposed on hospital staffs and first responders, the permanent organ damage for some survivors, the substantial burden on the public purse for paying for those unnecessary hospitalizations, and so on.   It’s a strategy, but don’t expect me to say it’s a good strategy.

So I’ll just go ahead and say it.  I think this is the best best explanation of what’s happening now in North Dakota.  I’m guessing they’re closing in on herd immunity, and that the pandemic there is winding down of its own accord. Continue reading Post #921: North Dakota: Is this what herd immunity looks like?

Post #901: Maybe ND really has achieved herd immunity.

Source:  NY Times Github data repository, data reported through 11/27/2020.

EDIT:  This post is updated in Post #921.  And in Post #928.  And others. 

As of 12/27/2020, ND had the fourth-lowest rate of new COVID infections in the U.S.

As of 1/14/2021, ND had the second-lowest rate of new COVID infections in the U.S.  Only Hawaii has  lower rate.  See Post #951.

Original post follows:

This is one of those seemingly simple 2+2=4 analyses.   In this case, it’s literally 8*10 > 70.

The arithmetic isn’t rocket science.  Anybody can do that.  My only value-added here has been in keeping an eye on the situation, and realizing why that arithmetic might matter.

Right now, 10% of the population of North Dakota has been formally diagnosed with COVID-19.   As of data reported through 11/27/2020, they’ve had 77,242 known cases.  That’s out of a population of about 760,000 (per the US Census Bureau).  Or (77,242/760,000 = ~) 10%.

A 10/25/2020 publication by CDC staff says that, best estimate, on average, 8 people have had COVID-19 for every one that has been diagnosed. 

IF CDC staff are right, and IF that US average applies to the US Midwest, then North Dakota has probably achieved COVID-19 herd immunity.  Or is close to it.  And much of the US Midwest has or will be following suit in the near future.

Obviously, that’s two big ifs.  But anybody can follow the math.  That’s 8*10% = 80%, and that’s higher than the 70% conventionally thought to be required to achieve herd immunity to COVID-19.

Oh, and note the peaks on all the curves at the top of the graph above.

Discussion follows.  This brings together several points that I’ve brought up over the past two months or so. Continue reading Post #901: Maybe ND really has achieved herd immunity.