Tag Archives: Natural 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!

Vagaries of Vaccine Efficacy

23 Sunday Jan 2022

Posted by in Coronavirus, Vaccinations

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There should never have been any doubt that vaccines would not stop you from “catching” the coronavirus. Vaccines cannot stop virus particles from lodging in your nose or your eyeballs. The vaccines act to prime the immune system against the virus, but no immune response is instantaneous. In other words, if you aren’t first “infected”, antibodies don’t do anything! A virus may replicate for at least a brief time, and it is therefore possible for a vaccinated individual to carry the virus and even pass it along to others. The Omicron variant has proven that beyond a shadow of a doubt, though the wave appears to be peaking in most of the U.S. and has peaked already in a few states, mostly in the northeast.

I grant that the confusion over “catching” the virus stems from an imprecision in our way of speaking about contracting “bugs”. Usually we don’t say we “caught” one unless it actually makes us feel a bit off. We come into intimate contact with many more bugs than that. The effects are often so mild that we either don’t notice or brush it off without mention. But when it comes to pathogens like Covid and discussions of vaccine efficacy (VE), it’s obviously useful to remember the distinction between infections, on the one hand, and symptomatic infections on the other.

Cases Are the Wrong Focus

Unless calibrated by seroprevalence data, these studies are not based on proper estimates of infections in the population. Asymptomatic people are much less likely to get tested, and vaccinated individuals who are infected are either much more likely to be asymptomatic or the test might not detect the weak presence of a virus at all. VE based on detected infections is essentially meaningless unless testing is universal.

We are bombarded by studies (and analyses like the one here) alleging that VE should be judged on the reduction in infections among the vaccinated. The likelihood of a detected infection by vaccination status is simply the wrong way to measure of VE. It’s not so much the direction of bias in measured VE, however. The mere presence of cases among the vaccinated has been sufficient to inflame anti-vax sentiment, especially cases detected in mandatory tests at hospitals, where the infections are often incidental to the primary cause of admission.

The typical evolution of a novel virus is further reason to dismiss case numbers as a basis for measuring VE. Mutations create new variants in ways that usually promote the continuing survival of the lineage. Subsequent variants tend to be more transmissible and less deadly to their hosts. Thus, given a certain “true” degree of VE, so-called breakthrough infections among the vaccinated are even more likely to be asymptomatic and less likely to be tested and/or detected.

There is the matter of immune escape or evasion, however, which means that sometimes a virus mutates in ways that get around natural or vaccine-induced immune responses. While such a variant is likely to be less dangerous to unvaccinated hosts, more cases among the vaccinated will turn up. That should not be interpreted as a deterioration in VE, however, because detected infections are still the wrong measure. Instead, the fundamental meaning of VE is a lower virulence or severity of a variant in vaccinated individuals than in unvaccinated individuals.

Interestingly, to digress briefly, while immune escape has been discussed in connection with Omicron, that variant’s viral ancestors may have predated even the original Covid strain released from the Wuhan lab! It is a fascinating mystery.

Virulence

In fact, vaccines have reduced the virulence of Covid infections, and the evidence is overwhelming. See here for a CDC report. The chart below is Swiss data, followed by a “handy” report from Wisconsin:

From the standpoint of virulence, there are other kinds of misguided comparisons to watch out for: these involve vaxed and unvaxed patients with specific outcomes, like the left side of the graphic at the top of this post (credit to Twitter poster aparachick). This thread has an excellent discussion of the misconception inherent in the claim that vaccines haven’t reduced severity: the focus is on the wrong conditional probability (again, like the left side of the graphic). Getting that wrong can lead to highly inaccurate conclusions when the sizes of the two key groups, hospitalizations and vaccinated individuals in this case, are greatly different.

Bumbled Messaging

The misunderstandings about VE are just one of many terrible failures of public health authorities over the course of the pandemic. There seems to have been fundamental miscommunication by the vaccine manufacturers and many others in the epidemiological community about what vaccines can and cannot do.

Another example is the apparent effort to downplay the importance of natural immunity, which is far more protective than vaccines. This looks suspiciously like a willful effort to push the narrative that universal vaccination as the only valid course for ending the pandemic. Even worse, the omission was helpful to those attempting to justify the tyranny of vaccine mandates.

Waning Efficacy

It should be noted that the efficacy of vaccines will wane over time. This phenomenon has been measured by the presence of antibodies, which is a valid measure of one aspect of VE over time. However, immune responses are more deeply embedded in the human body: so-called T-cells carry messages alerting so-called B-cells to the presence of viral “invaders”. The B-cells then produce new antibodies specific to characteristics of the interloping pathogen. Thus, these cells can function as a kind of “memory” allowing the immune system to mount a fresh antibody defense to a repeat or similar infection. The reports on waning antibodies primarily in vaccinated but uninfected individuals do not and cannot account for this deeper process.

Conclusion

Vaccines don’t necessarily reduce the likelihood of infection or even the spread of the virus, but they absolutely limit virulence. That’s why Jay Bhattacharya, one of the authors of The Great Barrington Declaration, says the vaccines provide a private benefit, but only a limited public benefit. Yet too often we see VE measured by the number of infections detected, and vaccine mandates are still motivated in part by the idea that vaccines offer protection to others. They might do that only to the extent that infections are less severe and clear-up more quickly.

In Praise of Voluntary Vaccination

31 Tuesday Aug 2021

Posted by in Coronavirus, Vaccinations

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I was vaccinated in March and early April and I’m damn glad to have done it. I have certain co-morbidities, and I’m of an age at which contracting COVID seems like a very bad idea, I felt a little run-down on the day after my second jab, but that was my only side effect, notwithstanding the unending litany of antivax hysterics to which we’ve all been subjected (even on certain sites to which I contribute).

Freedom Without Misinformation

In the context of the pandemic, it’s important to take a stand for liberty. In that spirit, I oppose the imposition of mandates requiring face masks and vaccinations against COVID. Furthermore, vaccination is at best unnecessary for those having acquired immunity from infection and for those at low risk, especially children. In fact, the younger, healthier, and fitter you are, the less important it is to be vaccinated.

It’s disappointing, however, to see completely innumerate people cite statistics purporting to show that COVID-19 vaccines are deadly or even particularly dangerous to those lacking contra-indications. Far worse, and far more idiotic, is to suggest that a conspiracy is afoot to kill large numbers of people via vaccination! I’m truly embarrassed to hear individuals who otherwise share my libertarian ideals say such irresponsible bullshit.

While the COVID vaccines seem to have more frequent side effects than earlier vaccines, they are not particularly risky. I’ll discuss the safety of the COVID vaccines in what follows. Even minuscule risks are unacceptable to some individuals, which of course is their right. However, others find these risks acceptable considering the far greater dangers posed by the early strains of COVID and even the more recent but less deadly Delta variant.

Unverified Adverse Events

The vaccine scaremongers often quote statistics from the CDC’s Vaccine Adverse Reporting Events System (VAERS). Here’s a disclaimer about the system from the CDC’s web site:

“Healthcare providers, vaccine manufacturers, and the public can submit reports to VAERS. While very important in monitoring vaccine safety, VAERS reports alone cannot be used to determine if a vaccine caused or contributed to an adverse event or illness. The reports may contain information that is incomplete, inaccurate, coincidental, or unverifiable.”

All kinds of reports are submitted alleging adverse events. For example, one observer cites the following reports from the system:

The link above refers to the following report as “Hulk Syndrome”, which was alleged to have been a side effect of the MMR vaccine:

Finally, we have this report related to the Pfizer COVID vaccine:

In case that’s hard to read, it claims that a COVID vaccine caused a third arm to grow from the reporting individual’s forehead, which constantly slaps him or her while trying to sleep. This report is a case of wonderful sarcasm, but it was submitted to VAERS! The real lesson is that the VAERS system collects many unverified accounts of side effects, so the aggregate counts of adverse events are not reliable, even by the CDC’s admission.

A More Sober Risk Assessment

Therefore, the VAERS system has obvious limitations. But even stipulating the use of VAERS reports, the risks of the COVID vaccines are vanishingly low. For example, roughly 198 million people in the U.S. have received at least one dose of a vaccine. As of last week, there had been about 13,600 reports of post-vaccination death in VAERS. The raw number is very high, and I don’t wish to minimize the seriousness of those losses. Nevertheless, if those deaths were all attributable to vaccination, and that is a BIG “if”, the risk of death from vaccination to-date is just 0.007%. That is seven thousandth of 1%. 

To put those VAERS deaths into perspective, I should first add the caveat that I am highly skeptical of the COVID case and death statistics. Nevertheless, let’s take the official U.S. COVID death toll of 646,000 at face value. I’m also treating reported vaccine deaths from VAERS at face value, which is a huge stretch. So, we have COVID mortality of 0.2% of the U.S. population, which is more than 28 times the risk of death from vaccination. I grant you the risk posed by COVID is lower going forward than in the past, which is due both to vaccinations and the declining virulence of the virus itself.

There are a range of vaccine side effects reported in VAERS, from pain near the injection site to such alarming conditions as anaphylaxis, Bell’s Palsy, and myocarditis/pericarditis. VAERS would attribute over 54,000 hospitalizations to the vaccines, a rate of 3 hundredths of one percent of those receiving at least one dose. Like COVID deaths, the number of COVID hospitalizations is likely inflated. Still, at 1.9% of the U.S. population, the risk of hospitalization from COVID is 68 times that of hospitalization from vaccine side effects reported in VAERS.

A large share of VAERS reports, covering all adverse events, are from middle aged individuals. It’s unclear how concentrated that reporting is among those with co-morbidities, including obesity, but I suspect they are heavily represented.

Coincidental Events

Perhaps less obvious is that many sincere reports to VAERS from both the public and health care providers represent coincidental events. A number of states have given heavy vaccine priority to the elderly and those with co-morbidities, and demand from those groups has been disproportionate in any case. Most of the VAERS-reported deaths also happen to be among the elderly and co-morbids.

For example, more than 38% of VAERS death reports come from the 80+ age cohort, accounting for roughly 5,200 deaths. That’s four hundredths of one percent of the 12.9 million people of ages 80+ in the U.S., most of whom have been vaccinated. Well over 1.2 million 80+ year-olds can be expected to die each year under normal circumstances. That a few would occur within days, weeks, or months of a vaccination should be expected. Furthermore, it would not be surprising, given the controversy surrounding vaccines and the suggestive power of antivax propaganda, for families or some caregivers to imagine a connection between vaccination and subsequent death. File a report! Who knows? Perhaps a class action award might be in it someday.

This is not to deny that a small number of individuals could be in such weakened states, or perhaps have unknown vulnerabilities, that the vaccines have catastrophic consequences. No doubt there are a few deaths precipitated by COVID vaccines in combination with other conditions. However, a large share of the deaths reported to VAERS are likely to have been coincidental. Likewise, people develop conditions all the time… sore joints, rashes, coughs, and headaches. It runs the gamut. Some of the VAERS reports of a less serious nature are undoubtedly coincidental, and perhaps some are due to the vivid imaginations of a subset of those having consented to the vaccine with great reluctance.

What Kills and What Doesn’t

Everything comes with a risk, and tradeoffs between risks must be balanced. The COVID pandemic was deadly, and I’ll be the first to admit that I underestimated its potential to kill. However, its deadliness was magnified by the non-pharmaceutical interventions imposed in many jurisdictions. Lockdowns and closures took a massive toll on the health of the population, cutting short many lives due to economic and personal despair as well as deferred and cancelled health care. While those interventions were deadly, I do not believe that kind of harm was intended. I do suspect the concomitant assault on liberty was welcomed in certain circles.

There are certainly downsides to the COVID vaccines. There have been more side effects and deaths than have ever been reported relative to earlier vaccines. It’s also a shame that public health authorities refuse to recognize the superior effects of natural immunity and the heightened risk of vaccinations to those with prior infections. And it’s a travesty that “vaccine passports” are now being demanded for various forms of travel, entertainment, and entry to some places of business. Despite these issues, it’s blatantly false to assert that the vaccines are generally harmful. Many more of the remaining vaccine-hesitant can benefit from vaccination. Let’s advocate for better assessments of risk by age and co-morbidity, and simply avoid the vaccines if that is your preference.

Addendum: I just came across this nice piece entitled “A Statistical Analysis of COVID-19 Breakthrough Infections and Deaths“. I thought I should share the link in case anyone supposes that so-called breakthrough infections somehow invalidate some of the comparisons I made above. This chart is particularly revealing:

On the Meaning of Herd Immunity

09 Saturday May 2020

Posted by in Pandemic, Public Health, Risk

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Immunity doesn’t mean you won’t catch the virus. It means you aren’t terribly susceptible to its effects if you do catch it. There is great variation in the population with respect to susceptibility. This simple point may help to sweep away confusion over the meaning of “herd immunity” and what share of the population must be infected to achieve it.

Philippe Lemoine discusses this point in his call for an “honest debate about herd immunity“. He reproduces the following chart, which appeared in this NY Times piece by Carl T. Bergstrom and Natalie Dean:

Herd immunity, as defined by Bergstrom and Dean, occurs when there are sufficiently few susceptible individuals remaining in the population to whom the actively-infected can pass the virus. The number of susceptible individuals shrinks over time as more individuals are infected. The chart indicates that new infections will continue after herd immunity is achieved, but the contagion recedes because fewer additional infections are possible.

We tend to think of the immune population as those having already been exposed to the virus, and who have recovered. Those individuals have antibodies specifically targeted at the antigens produced by the virus. But many others have a natural immunity. That is, their immune systems have a natural ability to adapt to the virus.

Heterogeneity

At any point in a pandemic, the uninfected population covers a spectrum of individuals ranging from the highly susceptible to the hardly and non-susceptible. Immunity, in that sense, is a matter of degree. The point is that the number of susceptible individuals doesn’t start at 100%, as most discussions of herd immunity imply, but something much smaller. If a relatively high share of the population has low susceptibility, the virus won’t have to infect such a large share of the population to achieve effective herd immunity.

The apparent differences in susceptibility across segments of the population may be the key to early herd immunity. We’ve known for a while that the elderly and those with pre-existing conditions are highly vulnerable. Otherwise, youth and good health are associated with low vulnerability.

Lemoine references a paper written by several epidemiologists showing that “variation in susceptibility” to Covid-19 “lowers the herd immunity threshold”:

“Although estimates vary, it is currently believed that herd immunity to SARS-CoV-2 requires 60-70% of the population to be immune. Here we show that variation in susceptibility or exposure to infection can reduce these estimates. Achieving accurate estimates of heterogeneity for SARS-CoV-2 is therefore of paramount importance in controlling the COVID-19 pandemic.”

The chart below is from that paper. It shows a measure of this variation on the horizontal axis. The colored, vertical lines show estimates of historical variation in susceptibility to historical viral episodes. The dashed line shows the required exposure for herd immunity as a function of this measure of heterogeneity.

Their models show that under reasonable assumptions about heterogeneity, the reduction in the herd immunity threshold (in terms of the percent infected) may be dramatic, to perhaps less than 20%.

Then there are these tweets from Marc Lipsitch, who links to this study:

“As an illustration we show that if R0=2.5 in an age-structured community with mixing rates fitted to social activity studies, and also categorizing individuals into three categories: low active, average active and high active, and where preventive measures affect all mixing rates proportionally, then the disease-induced herd immunity level is hD=43% rather than hC=1−1/2.5=60%.”

Even the celebrated Dr. Bergstrom now admits, somewhat grudgingly, that hereogeniety reduces the herd immunity threshold, though he doesn’t think the difference is large enough to change the policy conversation. Lipsitch also is cautious about the implications.

Augmented Heterogeneity

Theoretically, social distancing reduces the herd immunity threshold. That’s because infected but “distanced” people are less likely to come into close contact with the susceptible. However, that holds only so long as distancing lasts. John Cochrane discusses this at length here. Social distancing compounds the mitigating effect of heterogeneity, reducing the infected share of the population required for herd immunity.

Another compounding effect on heterogeneity arises from the variability of initial viral load on infection (IVL), basically the amount of the virus transmitted to a new host. Zvi Mowshowitz discusses its potential importance and what it might imply about distancing, lockdowns, and the course of the pandemic. In any particular case, a weak IVL can turn into a severe infection and vice versa. In large numbers, however, IVL is likely to bear a positive relationship to severity. Mowshowitz explains that a low IVL can give one’s immune system a head start on the virus. Nursing home infections, taking place in enclosed, relatively cold and dry environments, are likely to involve heavy IVLs. In fact, so-called household infections tend to involve heavier IVLs than infections contracted outside of households. And, of course, you are very unlikely to catch Covid outdoors at all.

Further Discussion

How close are we to herd immunity? Perhaps much closer than we thought, but maybe not close enough to let down our guard. Almost 80% of the population is less than 60 years of age. However, according to this analysis, about 45% of the adult population (excluding nursing home residents) have any of six conditions indicating elevated risk of susceptibility to Covid-19 relative to young individuals with no co-morbidities. The absolute level of risk might not be “high” in many of those cases, but it is elevated. Again, children have extremely low susceptibility based on what we’ve seen so far.

This is supported by the transmission dynamics discussed in this Twitter thread by Dr. Muge Cevik. She concludes:

“In summary: While the infectious inoculum required for infection is unknown, these studies indicate that close & prolonged contact is required for #COVID19 transmission. The risk is highest in enclosed environments; household, long-term care facilities and public transport. …

Although limited, these studies so far indicate that susceptibility to infection increases with age (highest >60y) and growing evidence suggests children are less susceptible, are infrequently responsible for household transmission, are not the main drivers of this epidemic.”

Targeted isolation of the highly susceptible in nursing homes, as well as various forms of public “distancing aid” to the independent elderly or those with co-morbidities, is likely to achieve large reductions in the effective herd immunity ratio at low cost relative to general lockdowns.

The existence of so-called super-spreaders is another source of heterogeneity, and one that lends itself to targeting with limitations or cancellations of public events and large gatherings. What’s amazing about this is how the super-spreader phenomenon can lead to the combustion of large “hot spots” in infections even when the average reproduction rate of the virus is low (R0 < 1). This is nicely illustrated by Christopher Moore of the Santa Fe Institute. Super-spreading also implies, however, that while herd immunity signals a reduction in new infections and declines in the actively infected population, “hot spots” may continue to flare up in a seemingly random fashion. The consequences will depend on how susceptible individuals are protected, or on how they choose to mitigate risks themselves.

Conclusion

I’ve heard too many casual references to herd immunity requiring something like 70% of the population to be infected. It’s not that high. Many individuals already have a sort of natural immunity. Recognition of this heterogeneity has driven a shift in the emphasis of policy discussions to the idea of targeted lockdowns, rather than the kind of indiscriminate “dumb” lockdowns we’ve seen. The economic consequences of shifting from broad to targeted lockdowns would be massive. And why not? The health care system has loads of excess capacity, and Covid infection fatality risk (IFR) is turning out to be much lower than the early, naive estimates we were told to expect, which were based on confirmed case fatality rates (CFRs).