Tag Archives: Acquired Immunity

Chill-Out Advisory: Pandemic to Endemic Means Live Again

13 Sunday Feb 2022

Posted by in Pandemic, Public Health, Uncategorized

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We might be just be done with the coronavirus pandemic. That is, it appears to be transitioning to a more permanent endemic phase. What follows are a few details about the Omicron wave and its current status, an attempt to put the risks of Covid in perspective, and a few public policy lessons that are now gaining broad currency but should have been obvious long ago.

What’s The Status?

The Omicron variant became the dominant U.S. strain of the coronavirus in December. Omicron outcompeted Delta, which was very good news because Omicron is far less severe. The chart below (from the CDC Data Tracker site) shows Omicron’s rapid ascendance and displacement of the Delta variant. The orange bar segments represent the proportion of cases of the Delta strain, while the purple and pink segments are Omicron sub-variants known as BA.1 and BA.2, respectively. BA.2 is even more transmissible than BA.1 and is likely to become dominant over the next month or so. However, the BA.2 sub-variant appears to be far less virulent than Delta, like BA.1.

Despite a record number of infections over a period of a month or so, the Omicron wave is tapering just as rapidly as it ramped up, as the next chart demonstrates. In fact, covidestim.org shows that cases are now receding in all states, DC, and Puerto Rico. Here are new cases per million people from Our World in Data:

Whether BA.2 causes cases to plateau for a while, or even a secondary Omicron “wavelet”, is yet to be seen. That would be consistent with the normal Hope-Simpson seasonal pattern of viral prevalence in the northern hemisphere (hat tip: HOLD2):

Data problems make the Omicron wave difficult to assess, however. We don’t know the share of incidental infections for the U.S. as a whole, but more than half of hospitalized Covid patients in Massachusetts and Rhode Island are classified with incidental infections. The proportion in the UK is estimated to be rising and approaching 30% of total cases, with much higher percentages in many regions of England, as shown below.

As I’ve emphasized in the past, case numbers should not be the primary gauge of the state of the pandemic, especially with a more highly contagious but relatively mild variant like Omicron. Hospitalizations are a better measure, but only if “incidental” infections are removed from the counts. That’s been acknowledged only recently by the public health establishment, and even the Biden Administration is emphasizing it as a matter of sheer political expediency. Another measure that might be more reliable for assessing the pandemic in the community as a whole is the number of emergency room patients presenting Covid-like symptoms. From the CDC Data Tracker:

There is no doubt that incidental infections create complications in caring for patients with other ailments. That has a bearing on the utilization of hospital capacity. Generally, however, strains on hospital capacity during the pandemic have been greatly exaggerated. This is not to diminish the hard work and risks faced by health care workers, and there have been spot shortages of capacity in certain localities. However, in general, staffed beds have been more than adequate to meet needs. This chart, like a few others below, is courtesy of Phil Kerpen:

With the more highly transmissible variants we have now, it’s not at all surprising to see a high proportion of incidental cases among inpatients. Incidental infections are likely to inflate counts of Covid deaths as well, given the exceptional and odd way in which Covid deaths are being recorded. It will be some time until we see full U.S. data on cases and deaths net of incidental infections. Moreover, many of the Covid deaths in December and January were from lingering Delta infections, which might still be a factor in the February counts.

How Are Your Odds?

The mild or asymptomatic nature of most Omicron cases, the large proportion of incidental hospitalizations, and the knowledge that Omicron is not a deep respiratory threat should offer strong reassurance to healthy individuals that the variant does not pose a great risk. According to a recent CDC report, in a sample of almost 700,000 vaccinated individuals aged 65 or less without co-morbidities, there were no Covid fatalities or ICU admissions during the 10 months from December 2020 through October 2021. There was only one fatality in the sample of healthy individuals older than 65. There were just 36 fatalities across the full sample of over 1.2 million vaccinated individuals, so COVID’s fatality risk was only about 0.3%. Of those deaths, 28 were among those with four or more risk factors (including co-morbidities and > 65 years). And this was before the advent of Omicron!

I have a few doubts about the CDC’s sample selection and vagaries around certain definitions used. Nevertheless, the results are striking. However, the study did not address risks to unvaccinated adults. Another more limited CDC study found that vaccinated patients were still less likely than the unvaccinated to require critical care during the Omicron wave.

A separate CDC study found a 91% reduction in the likelihood of death for Omicron relative to Delta. A study from the UK (see summary here) found that Omicron cases were 59% less likely than Delta cases to require hospitalization and 69% less likely to result in death within 28 days of a positive test. Omicron was far less deadly among both the vaccinated and the unvaccinated, and the latter had a larger reduction in the likelihood of death. The study was stratified by age as well, with less severe outcomes for Omicron among older cohorts except in the case of death, for which there was no apparent age gradient.

Another unnecessarily contentious issue has been the risk to children during the pandemic. Based on the data, there should never have been much doubt that these risks are quite low. Apparently, however, it was advantageous for teachers’ unions to insist otherwise. Phil Kerpen soundly debunks that claim with the following chart:

Covid has been less deadly to children from infancy through 17 years than the pre-pandemic flu going back to 2012! Oh yes, but teachers FEAR transmission from the children! That claim is just as silly, since children are known to be inefficient transmitters of the virus (and see here).

Now that Omicron has relegated the Delta variant to the history books, the risks going forward seem much more manageable. Omicron is less severe, especially for the vaccinated. Levels of acquired (natural) immunity from earlier infections are now much higher against older strains, and Omicron infections seem to be protective against Delta.

In commentary about the first CDC study discussed above, John Tierney lends perspective to the odds of death from pre-Omicron Covid:

“Those are roughly the same odds that in the course of a year you will die in a fire, or that you’ll perish by falling down stairs. Going anywhere near automobiles is a bigger risk: you’re three times more likely during a given year to be killed while riding in a car, and also three times more likely to be a pedestrian casualty. The 150,000-to-1 odds of a Covid death are even longer than the odds over your lifetime of dying in an earthquake or being killed by lightning.”

Yet with all this research confirming the low odds of death induced by Omicron, why have we seen recent deaths at levels approaching previous waves? First, many of those deaths are carried over from Delta infections. That means deaths should begin to taper rapidly as February reports roll in. And remember that daily reports do not show deaths by date of death. Deaths usually occur weeks or even months before they are reported. That also means some of the deaths reported might be “harvested” from much earlier fatalities. Second, given the high levels of incidental Omicron infections, some of those deaths are misattributed to Covid, an issue that is not new by any means. Finally, while Omicron is relatively mild for most people, the high rate of transmission means that a high number of especially vulnerable individuals may be infected with severe outcomes. We have seen much more severe consequences for the unvaccinated, of course, and for those with co-morbidities.

Things We Should Have Known

I’ll try to keep this last section brief, but as an introduction I’ll just say that it’s almost as if we’ve been allowing the lunatics to run the asylum. To paraphrase one comment I saw recently, if you wonder why there is so much dissent, you ought to consider the fact the much of what our governments have done (along with many private organizations) was to prohibit things that were demonstrably safe (e.g., going outside, using swing sets, or attending schools) and to encourage things that were demonstrably harmful (e.g., deferring medical care, or masking small children).

The following facts are only now coming into focus among those who’ve been “following the politics” rather than “the science”, despite pretensions to the latter.

Grow Up and Chill Out!

Life is full of risks, and nothing has changed to alter wisdom gained in earlier pandemics. For example, this pearl from a 2006 publication on disease mitigation measures should be heeded (hat tip: Phil Kerpen):

If there is one simple message everyone needs to hear, it is to stop allowing the virus bogeyman to rule your life. It will never go away completely, and it is likely to present risks that is are comparable to the flu going forward. In fact, it might well compete with the flu, which means we won’t be dealing with endemic Covid plus historical flu averages, but some smaller union of the two case loads.

So get out, go back to work, or go have some fun! Get back truckin’ on!

Herd Immunity To Public Health Bullshitters and To COVID

16 Monday Aug 2021

Posted by in Coronavirus, Herd Immunity, Uncategorized

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My last post had a simple message about the meaning of immunity: you won’t get very sick or die from an infection to which you are immune, including COVID-19. Like any other airborne virus, that does NOT mean you won’t get it lodged in your eyeballs, sinuses, throat, or lungs. If you do, you are likely to test positive, though your immunity means the “case” is likely to be inconsequential.

As noted in that last post, we’ve seen increasing COVID case counts with the so-called Delta variant, which is more highly transmissible than earlier variants. (This has been abetted by an uncontrolled southern border as well.) However, as we’d expect with a higher level of immunity in the population, the average severity of these cases is low relative to last year’s COVID waves. But then I saw this article in ScienceAlert quoting Sir Andrew Pollard, a scientist affiliated AstraZeneca and the University of Oxford. He says with Delta, herd immunity “is not a possibility” — everyone will get it.

Maybe everyone will, but that doesn’t mean everyone will get sick. His statement raises an obvious question about the meaning of herd immunity. If our working definition of the term is that the virus simply disappears, then Pollard is correct: we know that COVID is endemic. But the only virus that we’ve ever completely eradicated is polio. Would Pollard say we’ve failed to achieve herd immunity against all other viruses? I doubt it. Endemicity and herd immunity are not mutually exclusive. The key to herd immunity is whether a virus does or does not remain a threat to the health of the population generally.

Active COVID infections will be relatively short-lived in individuals with “immunity”. Moreover, viral loads tend to be lower in immune individuals who happen to get infected. Therefore, the “infected immune” have less time and less virus with which to infect others. That creates resistance to further contagion and contributes to what we know as herd immunity. While immune individuals can “catch” the virus, they won’t get sick. Likewise, a large proportion of the herd can be immune and still catch the virus without getting sick. That is herd immunity.

One open and controversial question is whether uninfected individuals will require frequent revaccination to maintain their immunity. A further qualification has to do with asymptomatic breakthrough infections. Those individuals won’t see any reason to quarantine, and they may unwittingly transmit the virus.

I also acknowledge that the concept of herd immunity is often discussed strictly in terms of transmission, or rather its failure. The more contagious a new virus, like the Delta variant, the more difficult it is to achieve herd immunity. Models predicting low herd immunity thresholds due to heterogeneity in the population are predicated on a given level of transmissibility. Those thresholds would be correspondingly higher given greater transmissibility.

A prominent scientist quoted in this article is Paul Hunter of the University of East Anglia. After backing-up Pollard’s dubious take on herd immunity, Hunter drops this bit of real wisdom:

“We need to move away from reporting infections to actually reporting the number of people who are ill. Otherwise we are going to be frightening ourselves with very high numbers that don’t translate into disease burden.”

Here, here! Ultimately, immunity has to do with the ability of our immune systems to fight infections. Vaccinations, acquired immunity from infections, and pre-existing immunity all reduce the severity of later infections. They are associated with reductions in transmission, but those immune responses are more basic to herd immunity than transmissability alone. Herd immunity does not mean that severe cases will never occur. In fact, more muted seasonal waves will come and go, inflicting illness on a limited number of vulnerables, but most people can live their lives normally while viral reproduction is contained. Herd immunity!

Sadly, we’re getting accustomed to hearing misstatements and bad information from public health officials on everything from mask mandates, lockdowns, and school closings to hospital capacity and vaccine hesitancy. Dr. Pollard’s latest musing is not unique in that respect. It’s almost as if these “experts” have become victims of their own flawed risk assessments insofar as their waning appeal to “the herd” is concerned. Professor Hunter’s follow-up is refreshing, however. Public health agencies should quit reporting case counts and instead report only patients who present serious symptoms, COVID ER visits, or hospitalizations.

Effective Immunity Means IF YOU CATCH IT, You Won’t Get Sick

12 Thursday Aug 2021

Posted by in Coronavirus, Uncategorized, Vaccinations

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Listen very carefully: immunity does NOT mean you won’t get COVID, though an infection is less likely. Immunity simply means your immune system will be capable of dealing with an infection successfully. This is true whether the immunity is a product of vaccination or a prior infection. Immunity means you are unlikely to have worse than mild symptoms, and you are very unlikely to be hospitalized. (My disclaimer: I am opposed to vaccine mandates, but vaccination is a good idea if you’ve never been infected.)

I emphasize this because the recent growth in case numbers has prompted all sorts of nonsensical reactions. People say, “See? The vaccines don’t work!” That is a brazenly stupid response to the facts. Even more dimwitted are claims that the vaccines are killing everyone! Yes, there are usually side effects, and the jabs carry a risk of serious complications, but it is minuscule.

Vaccine Efficacy

Right out of the gate, we must recognize that our PCR testing protocol is far too sensitive to viral remnants, so the current surge in cases is probably exaggerated by false positives, as was true last year. Second, if a large share of the population is vaccinated, then vaccinated individuals will almost certainly account for a large share of infected individuals even if they have a lower likelihood of being infected. It’s simple math, as this explanation of base rate bias shows. In fact, according to the article at the link:

“… vaccination confers an eightfold reduction in the risk of getting infected in the first place; a 25-fold reduction in risk of getting hospitalized; and a 25-fold reduction in the risk for death.”

The upshot is that if you are vaccinated, or if you have acquired immunity from previous exposure, or if you have pre-existing immunity from contact with an earlier COVID strain, you can still “catch” the virus AND you can still spread it. Both are less likely, and you don’t have as much to worry about for your own health as those having no immunity.

As for overall vaccine efficacy in preventing death, here are numbers from the UK, courtesy of Phil Kerpen:

The vertical axis is a log scale, so each successive gridline is a fatality rate 100x as large as the one below it. Obviously, as the chart title asserts, the “vaccines have made COVID-19 far less lethal.” Also, at the bottom, see the information on fatality among children under age 18: it is almost zero! This reveals the absurdity of claims that children must be masked for schools to reopen! In any case, masks offer little protection to anyone against a virus that spreads via fine aerosols. Nevertheless, many school officials are pushing unnecessary but politically expedient masking policies

Delta

Ah, but we have the so-called Delta variant, which is now dominant and said to be far more transmissible than earlier variants. Yet the Delta variant is not as dangerous as earlier strains, as this UK report demonstrates. Delta had a case fatality rate among unvaccinated individuals that was at least 40% less than the so-called Alpha variant. This is a typical pattern of virus mutation: the virus becomes less dangerous because it wants to survive, and it can only survive in the long run by NOT killing its hosts! The decline in lethality is roughly demonstrated by Kelly Brown with data on in-hospital fatality rates from Toronto, Canada:

The case numbers in the U.S. have been climbing over the past few weeks, but as epidemiologist Larry Brilliant of WHO said recently, Delta spreads so fast it essentially “runs out of candidates.” In other words, the current surge is likely to end quickly. This article in Issues & Insights shows the more benign nature of recent infections. I think a few of their charts contain biases, but the one below on all-cause mortality by age group is convincing:

The next chart from Our World In Data shows the infection fatality rate continuing its decline in the U.S. The great majority of recent infections have been of the Delta variant, which also was much less virulent in the UK than earlier variants.

Furthermore, it turns out that the vaccines are roughly as effective against Delta and other new variants as against earlier strains. And the newest “scary” variants, Kappa and Lambda, do not appear to be making strong inroads in the U.S. 

Fading Efficacy?

There have been questions about whether the effectiveness of the vaccines is waning, which is behind much of the hand-wringing about booster shots. For example, Israeli health officials are insisting that the effectiveness of vaccines is “fading”, though I’ll be surprised if there isn’t some sort of confounding influence on the data they’ve cited, such as age and co-morbidities. 

Here is a new Mayo Clinic study of so-called “breakthrough” cases in the vaccinated population in Minnesota. It essentially shows that the rate of case diagnosis among the vaccinated rose between February and July of this year (first table below, courtesy of Phil Kerpen). However, the vaccines appear only marginally less effective against hospitalization than in March (second table below).

The bulk of the vaccinated population in the U.S. received their jabs three to six months ago, and according to this report, evidence of antibodies remains strong after seven months. In addition, T-cell immunity may continue for years, as it does for those having acquired immunity from an earlier infection. 

Breakthroughs

It’s common to hear misleading reports of high numbers of “breakthrough” cases. Not only will these cases be less menacing, but the reports often exaggerate their prevalence by taking the numbers out of context. Relative to the size of the vaccinated population, breakthrough cases are about where we’d expect based on the original estimates of vaccine efficacy. This report on Massachusetts breakthrough hospitalizations and deaths confirms that the most vulnerable among the vaxed population are the same as those most vulnerable in the unvaxed population: elderly individuals with comorbidities. But even that subset is at lower risk post-vaccination. It just so happens that the elderly are more likely to have been vaccinated in the first place, which implies that the vaccinated should be over-represented in the case population.

Conclusion

The COVID-19 vaccines do what they are supposed to do: reduce the dangers associated with infection. The vaccines remain very effective in reducing the severity of infection. However, they cannot and were not engineered to prevent infection. They also pose risks, but individuals should be able to rationally assess the tradeoffs without coercion. Poor messaging from public health authorities and the crazy distortions promoted in some circles does nothing to promote public health. Furthermore, there is every reason to believe that the current case surge in Delta infections will be short-lived and have less deadly consequences than earlier variants.

CDC Flubs COVID Impact on Life Expectancy

03 Wednesday Mar 2021

Posted by in Coronavirus, Public Health

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The CDC choked on a new analysis estimating COVID-19’s impact on U.S. life expectancy as of year-end 2020: they reported a decline of a full year, which is ridiculous on its face! As explained by Peter B. Bach in STAT News, the agency assumed that excess deaths attributed to COVID in 2020 would continue as a permanent addition to deaths going forward. Please forgive my skepticism, but isn’t this too basic to qualify as an analytical error by an agency that subjects its reports to thorough vetting? Or might this have been a deliberate manipulation intended to convince the public that COVID will be an ongoing public health crisis. Of course the media has picked it up; even Zero Hedge reported it uncritically!

Bach does a quick calculation based on 400,000 excess deaths attributed to COVID in 2020 and 12 life-years lost by the average victim. I believe the first assumption is on the high side, and I say “attributed to COVID” as a reminder that the CDC’s guidance for completing death certificates was altered in the spring of 2020 specifically for COVID and not other causes of death. Furthermore, if our objective is to assess the impact of the virus itself, under no circumstances should excess deaths induced by misguided lockdown policies enter the calculation (though Bach entertains the possibility). Bach arrives at a reduction in average life of 5.3 days! Of course, that’s not intended to be a projection, but it is a reasonable estimate of COVID’s impact on average lives in 2020.

The CDC’s projection essentially freezes death rates at each age at their 2020 values. We will certainly see more COVID deaths in 2021, and the virus is likely to become endemic. Even with higher levels of acquired immunity and widespread vaccinations, there will almost certainly be some ongoing deaths attributable to COVID, but they are likely to be at levels that will blend into a resumption of the long decline in mortality rates, especially if COVID continues to displace the flu in its “ecological niche”. I include the chart at the top to emphasize the long-term improvement in mortality (though the chart shows only a partial year for 2020, and there has been some flattening or slight backsliding over the past five years or so). As Bach says:

“Researchers have regularly demonstrated that life expectancy projections are overly sensitive to evanescent events like pandemics and wars, resulting in considerably overestimated declines. … And yet the CDC published a result that, if anything, would convey to the public an exaggerated toll that Covid-19 took on longevity in 2020. That’s a problem.”

There were excess deaths from other causes in 2020, which Bach acknowledges. Perhaps 100,000 or more could be attributed to lockdowns and their consequences like economically-induced stress, depression, suicide, overdoses, and medical care deferred or never sought. The Zero Hedge article mentioned above discusses findings that lockdowns and their consequences, such as unemployment spells and lost education, will have ongoing negative effects on health and mortality for many years. The net effect on life expectancy might be as large as 11 to 12 days. Again, however, I draw a distinction between deaths caused by the disease and deaths caused by policy mistakes.

The CDC’s estimate should not be taken seriously when, as Kyle Smith says, there is every indication that the battle against COVID is coming to a successful conclusion. Public health experts have not acquitted themselves well during the pandemic, and the CDC’s life expectancy number only reinforces that impression. Here is Smith:

“We have learned a lot about how the virus works, and how it doesn’t: Outdoor transmission, for the most part, hardly ever happens. Kids are at very low risk, especially younger children. Baseball games, barbecues, and summer camps should be fine. Some pre-COVID activities now carry a different risk profile — notably anything that packs crowds together indoors, so Broadway theater, rock concerts, and the like will be just about the last category of activity to return to normal.”

But return to normal we should, and yet the CDC seems determined to poop on the victory party!

Fauci Flubs Herd Immunity

03 Sunday Jan 2021

Posted by in Coronavirus, Herd Immunity, Public Health, Vaccinations

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Anthony Fauci has repeatedly increased his estimate of how much of the population must be vaccinated to achieve what he calls herd immunity, and he did it again in late December. This series of changes, and other mixed messages he’s delivered in the past, reveal Fauci to be a “public servant” who feels no obligation to level with the public. Instead, he crafts messages based on what he believes the public will accept, or on his sense of how the public must be manipulated. For example, by his own admission, his estimates of herd immunity have been sensitive to polling data! He reasoned that if more people reported a willingness to take a vaccine, he’d have flexibility to increase his “public” estimate of the percentage that must be vaccinated for herd immunity. Even worse, Fauci appears to lack a solid understanding of the very concept of herd immunity.

Manipulation

There is so much wrong with his reasoning on this point that it’s hard to know where to start. In the first place, why in the world would anyone think that if more people willingly vaccinate it would imply that even more must vaccinate? And if he felt that way all along it demonstrates an earlier willingness to be dishonest with the public. Of course, there was nothing scientific about it: the series of estimates was purely manipulative. It’s almost painful to consider the sort of public servant who’d engage in such mental machinations.

Immunity Is Multi-Faceted

Second, Fauci seemingly wants to convince us that herd immunity is solely dependent on vaccination. Far from it, and I’m sure he knows that, so perhaps this too was manipulative. Fauci intimates that COVID herd immunity must look something like herd immunity to the measles, which is laughable. Measles is a viral infection primarily in children, among whom there is little if any pre-immunity. The measles vaccine (MMR) is administered to young children along with occasional boosters for some individuals. Believe it or not, Fauci claims that he rationalized a requirement of 85% vaccination for COVID by discounting a 90% requirement for the measles! Really???

In fact, there is substantial acquired pre-immunity to COVID. A meaningful share of the population has long-memory, cross-reactive T-cells from earlier exposure to coronaviruses such as the common cold. Estimates range from 10% to as much as 50%. So if we stick with Fauci’s 85% herd immunity “guesstimate”, 25% pre-immunity implies that vaccinating only 60% of the population would get us to Fauci’s herd immunity goal. (Two qualifications: 1) the vaccines aren’t 100% effective, so it would take more than 60% vaccinated to offset the failure rate; 2) the pre-immune might not be identifiable at low cost, so there might be significant overlap between the pre-immune and those vaccinated.)

Conceptual Confusion

Vaccinations approaching 85% would be an extremely ambitious goal, especially if it is recommended annually or semi-annually. It would be virtually impossible without coercion. While more than 91% of children are vaccinated for measles in the U.S., it is not annual. Thus, measles does not offer an appropriate model for thinking about herd immunity to COVID. Less than half of adults get a flu shot each year, and somewhat more children.

Fauci’s reference to 85% – 90% total immunity is different from the concept of the herd immunity threshold (HIT) in standard epidemiological models. The HIT, often placed in the range of 60% – 70%, is the point at which new infections begin to decline. More infections occur above the HIT but at a diminishing rate. In the end, the total share of individuals who become immune due to exposure, pre-immunity or vaccination will be greater than the HIT. The point is, however, that reaching the HIT is a sufficient condition for cases to taper and an end to a contagion. If we use 65% as the HIT and pre-immunity of 25%, only 40% must be vaccinated to reach the HIT.

Heterogeneity

A recent innovation in epidemiological models is the recognition that there are tremendous differences between individuals in terms of transmissibility, pre-immunity, and other factors that influence the spread of a particular virus, including social and business arrangements. This kind of heterogeneity tends to reduce the effective HIT. We’ve already discussed the effect of pre-immunity. Suppose that certain individuals are much more likely to transmit the virus than others, like so-called super-spreaders. They spur the initial exponential growth of a contagion, but there are only so many of them. Once infected, no one else among the still-susceptible can spread the virus with the same force.

Researchers at the Max Planck Institute (and a number of others) have gauged the effect of introducing heterogeneity to standard epidemiological models. It is dramatic, as the following chart shows. The curves simulate a pandemic under different assumptions about the degree of heterogeneity. The peak of these curves correspond to the HIT under each assumption (R0 refers to the initial reproduction number from infected individuals to others).

Moderate heterogeneity implies a HIT of only 37%. Given pre-immunity of 25%, only an additional 12% of the population would have to be infected or vaccinated to prevent a contagion from gaining a foothold for the initial exponential stage of growth. Fauci’s herd immunity figure obviously fails to consider the effect of heterogeneity.

How Close To the HIT?

We’re not as far from HITs as Fauci might think, and a vaccination goal of 85% is absurd and unnecessary. The seasonal COVID waves we’ve experienced thus far have faded over a period of 10-12 weeks. Estimates of seroprevalence in many localities reached a range of 15% – 25% after those episodes, which probably includes some share of those with pre-immunity. To reach the likely range of a HIT, either some additional pre-immunity must have existed or the degree of heterogeneity must have been large in these populations.

But if that’s true, why did secondary waves occur in the fall? There are a few possibilities. Of course, some areas like the upper Midwest did not experience the springtime wave. But in areas that suffered a recurrance, perhaps the antibodies acquired from infections did not remain active for as long as six months. However, other immune cells have longer memories, and re-infections have been fairly rare. Another possibility is that those having some level of pre-immunity were still able to pass live virus along to new hosts. But this vector of transmission would probably have been quite limited. Pre-immunity almost surely varies from region to region, so some areas were not as firmly above their HITs as others. It’s also possible that infections from super-spreaders were concentrated within subsets of the population even within a given region, in certain neighborhoods or among some, but not all, social or business circles. Therefore, some subsets or “sub-herds” achieved a HIT in the first wave, but it was unnecessary for other groups. In other words, sub-herds spared in the first wave might have suffered a contagion in a subsequent wave. And again, reinfections seem to have been rare. Finally, there is the possibility of a reset in the HIT in the presence of a new, more transmissible variant of the virus, as has become prevalent in the UK, but that was not the case in the fall.

Fragility

Tyler Cowen has mentioned another possible explanation: so-called “fragile” herd immunity. The idea is that any particular HIT is dependent on the structure of social relations. When social distancing is widely practiced, for example, the HIT will be lower. But if, after a contagion recedes, social distancing is relaxed, it’s possible that the HIT will take a higher value at the onset of the next seasonal wave. Perhaps this played a role in the resurgence in infections in the fall, but the HIT can be reduced via voluntary distancing. Eventually, acquired immunity and vaccinations will achieve a HIT under which distancing should be unnecessary, and heterogeneity suggests that shouldn’t be far out of reach.

Conclusion

Anthony Fauci has too often changed his public pronouncements on critical issues related to management of the COVID pandemic. Last February he said cruises were fine for the healthy and that most people should live their lives normally. Oops! Then came his opinion on the limited effectiveness of masks, then a shift to their necessity. His first position on masks has been called a “noble lie” intended to preserve supplies for health care workers. However, Fauci was probably repeating the standing consensus at that point (and still the truth) that masks are of limited value in containing airborne pathogens.

This time, Fauci admitted to changing his estimate of “herd immunity” in response to public opinion, a pathetic approach to matters of public health. What he called herd immunity was really an opinion about adequate levels of vaccination. Furthermore, he neglected to consider other forms of immunity: pre-existing and already acquired. He did not distinguish between total immunity and the herd immunity threshold that should guide any discussion of pandemic management. He also neglected the significant advances in epidemiological modeling that recognize the reality of heterogeneity in reducing the herd immunity threshold. The upshot is that far fewer vaccinations are needed to contain future waves of the pandemic than Fauci suggests.