Three Hurdles to Herd Immunity

Photo by Daiga Ellaby on Unsplash

The concept of herd immunity is pretty simple: for nearly all human pathogens to which immunity is possible, if enough people are immune, the pathogen can’t keep itself afloat in the population, and subsides to very low levels. This phenomenon will protect even those without immunity, because they are very unlikely to be exposed to an infectious person. For each pathogen, epidemiologists try to calculate what percentage of the population needs to be immune in order to trigger the herd immunity effect. It’s a slippery number, influenced by both the characteristics of the virus and the behavior of the at-risk population.

For the SARS CoV-2 virus in its current form (mutation could change the number), the best guess right now is that herd immunity will blossom when 70-85% of the population is immune. At the low end of the range, herd immunity means the virus is under control, but present.  At the high end, herd immunity means the virus is effectively eradicated. With dependable herd immunity, relatively normal life becomes possible again. How might we get there?

Over the past year, herd immunity has often been proposed by advocates of achieving it naturally. That proposal went like this:

  1. Assume that contracting and recovering from the disease confers durable immunity, and that we won’t have a good vaccine anytime soon.
  2. Note that the people at greatest risk of dying from the disease are old.
  3. Stash the old people someplace safe, and then let everybody else get sick and then get well.
  4. Voila! Herd immunity.

For countries that followed this strategy, things have not gone well. It turns out that not just the old die, and death isn’t the only problem: serious long-haul consequences can occur at any age. It also turns out to be just about impossible for a virus-laden country to safely stash its elders; the virus finds them. Sweden, a country well-positioned to suppress the virus by conventional means, opted instead for natural herd immunity and now suffers from one of the world’s worst COVID-19 death rates. The UK started down the natural herd immunity road and then reversed course when things turned ugly. Things stayed ugly. The red half of the US marched proudly down this same road, led by our president, leaving us with 325,000 dead before Christmas, heading for north of 500,000 by April 2021.

Historically, herd immunity has usually been achieved by vaccination. Smallpox. Polio. Measles. Mumps. Rubella. Tetanus. The list is long. Once a good-enough vaccine becomes available, maintaining immunity in a population can be made routine through childhood vaccination programs and later booster shots if necessary.  Given the severe consequences of the COVID-19 pandemic, herd immunity is an overwhelmingly desirable goal. So desirable, in fact, that several countries have achieved it by non-pharmaceutical means: instead of infecting everybody, they protected everybody so completely that the virus died out, and normal life could resume. This has been achieved by New Zealand, Taiwan, Australia, and, it appears, much of China. If the new vaccines do indeed provide strong protection, this bet on interim non-pharmaceutical protection will pay off very handsomely, and the “natural herd immunity” countries will have brought about thousands of needless deaths.

What does the path to vaccine-enabled herd immunity look like for the United States? It will look like a track set up for hurdlers. If we clear them all, we can win full normalcy. If we clear most, we might be able to manage a more rickety normalcy. The hurdles fall into three groups: vaccine hurdles, vaccination hurdles, and globalization hurdles.

The Vaccine Hurdles

The first set of hurdles has to do with the vaccines themselves. We know that the first two vaccines, from Pfizer and Moderna, are effective at suppressing symptoms, but we don’t yet know how effective they are at preventing transmission. Experts have guessed that they might be 50% effective, but the data is scanty. If they don’t prevent transmission very well, we may spend a while in limbo, with reduced symptoms but continuing requirements for separation, masking, and testing, and periodic outbreaks to suppress. This would be a better life than today’s, but far from perfectly normal.

We also don’t know how durable these vaccines are, meaning how long the protection lasts once one is vaccinated. If the durability is less than a year, herd immunity will be fragile at best, depending on a major permanent vaccine infrastructure and the requirement for high community compliance. Life in this regime would be plagued by widespread chronic low levels of infection. It would be a better life than today’s, but not as good as we’re hoping for. Durability depends on the virus’ ability to mutate the proteins on its surface to become unrecognized again by our alerted immune systems. This process is called antigenic drift, and Fred Hutch’s Trevor Bedford has recently tweeted a summary of the evidence that SARV CoV-2 could be capable of such drift. The evidence doesn’t suggest, so far, that it is likely to be as drifty as the Flu, but it may resemble other Coronaviruses, which seem to evolve around our immune defenses every three years or so. This is preliminary data, but potentially significant, if it means we’d need to develop a new vaccine every few years. Fortunately, the new vaccine creation method using mRNA allows for very quick vaccine development.

We also know that these first two vaccines present obstacles to mass vaccination: they are difficult to transport, they require double injections several weeks apart, and they are expensive. Other vaccines in the pipeline are variously single-shot, transportable at normal refrigerated temperatures, and/or much less expensive, but the Emergency Use Authorizations granted to Pfizer and Moderna may prevent some of these other vaccines from coming to market. Why is that? Because once a vaccine is available, you can’t ethically recruit someone into a study for another vaccine where there’s a 50/50 chance they’ll receive a placebo without first telling them that there’s an actual working vaccine available. First-to-market vaccines have a tendency to scorch the earth behind them this way. The EUAs make the problem worse by speeding up availability of the first-past-the-post vaccines, thereby closing down the time window for competitors to run trials. The US has placed some big orders for vaccines from the vaccine-makers still in or entering Phase III trials. If these trials are rendered unworkable by the first two vaccines, we may face vaccine shortages this spring. Hurdles everywhere you look.

The Vaccination Hurdles

The second set of hurdles has to do with the challenges of unprecedented mass vaccination. As mentioned above, the first two vaccines require special handling. In Pfizer’s case, the cold chain has to keep them at -70C or below. In Moderna’s case, a more manageable -20C. Each requires two doses. In Pfizer’s case, the doses should be 3 weeks apart; in Moderna’s case, 4 weeks apart. Historically, it has proven difficult to get people to return for the second shot of a two-shot vaccine, yet the US protocol requires setting aside a second dose for each person who has received a first dose, even if this means delaying first doses for others. The fragility of the vaccines also means that they’ll only be available at well-established medical facilities, and typically won’t be readily available at small doctor’s offices in low-income neighborhoods, remote rural communities, etc. Mobile vaccination units can address some of this problem but, again, the road to herd immunity gets longer.

A big vaccination hurdle is the pandemic itself: ordinarily you would expect a mass vaccination program to be executed by our existing healthcare infrastructure—hospitals and clinics, doctors and nurses, and so forth. However, these resources are stretched to the breaking point already by the surge of new COVID cases: the very places you’d go to get a shot are places at high risk to give you the disease, and the very people who’d give you the shot are trying to save the lives of people already sick. This will slow down the rate of vaccination.

Perhaps the biggest vaccination hurdle is lack of willingness to be vaccinated. About 40% of Americans have expressed reluctance, ranging from “I’m not sure yet,” to “Over my dead body,” the latter being a grantable wish. Some of these people are probably unpersuadable because they are deeply committed to the perverse logic of the antivax movement. Others are probably unpersuadable because they bought so deeply into the “COVID is a hoax” argument made by Trump in earlier days, and they somehow conflate “being forced to take a vaccine” with “being forced to wear a mask,” a parallel government infringement on their personal sovereignty.

Another segment has bought into the odd conspiracies that seem normal in America now: the vaccine is a Trojan horse that will implant a chip that lets Bill Gates control your mind, for example. Personally, I think people who believe that Bill Gates wants to control their minds have un-supportably high regard for their minds, but I digress.

Beyond the crazies, a significant number of African Americans remember their history of being abused by American medicine, and are cautious even now, no matter what promises are made. Across all communities, some people are cautious about any new drug, some don’t trust big Pharma, some don’t trust anything touched by Donald Trump, some don’t like needles, some are just too overwhelmed leading difficult lives to organize two trips to the vaccination center. It adds up.

After the early waves of vaccination take care of the most willing, the most vulnerable, and the most available, the remaining 60% of the population will contain nearly 100% of the least available and most resistant. By the time 70% are vaccinated, the remaining 30% will consist almost entirely of people who are unwilling to be vaccinated. The road to herd immunity will be climbing a grade that gets steeper with every mile.

The Globalization Hurdles

This may be less obvious at first glance, but our ability to reach a new normalcy in the United States depends on having most of the rest of the world knock down the virus as well. The US is a global crossroads of travel and trade, and our citizens, businesses, and academics are connected everywhere. In order to reopen the US-Canadian border, we need both countries to achieve sustained herd immunity. In order to re-open the US-Mexican border, we need the same. Mexico in turn will need the same across its southern border.

For trade and travel to resume at normal levels with the rest of the world, we need the rest of the world to be a safe source of travelers and a safe place for us to travel to. This means that securing vaccinations for ourselves alone will not allow us to achieve normal life again. We need succor for the rest of the world, too.

Maintaining normalcy in an unprotected world will be even harder if the protection the vaccines provide against transmission is relatively low, or the durability of the vaccines is relatively brief. In these cases, our herd immunity will be held in a leaky bucket requiring constant topping-up. Opening up to travel and trade with countries without herd immunity will vastly compound that problem.

One hurdle facing a drive toward global immunity is the impact of the rich world on the supply of vaccines to the rest of the world: rich countries have signed contracts for many more vaccine doses than they may need, in order to be sure that they get enough. This has pushed the countries that can’t pay the highest price farther out in time.

Some countries won’t have access to enough doses to pursue their own herd immunity until 2022, 2023, or even 2024.  If the vaccines are not very durable, the rich countries will keep buying up more doses, further delaying global immunity. Eventually, the pharmaceutical industry could create enough doses to vaccinate the entire world annually, but the price would have to be high enough to incentivize that level of production.

Hybrid Herd Immunity, Novel Normal Life

As we slog through the later stages of vaccination, trying to reach the people with the greatest access challenges or the greatest aversion to being vaccinated, we should remember that non-pharmaceutical methods can accelerate herd immunity: continued tactical mask-wearing and social distancing, combined with ubiquitous personal daily testing can effectively close the gap between the actual level of vaccination and the level needed for purely vaccine-based herd immunity. We can collapse the virus population faster this way, making ourselves less vulnerable to each other, and making foreign trade and travel safer sooner.

Herd immunity is not a goal in itself, it’s a means to an end, and that end is, in Warren G. Harding’s memorable phrase, “a return to normalcy.” We can think of normalcy in degrees. We can probably get close to economic normalcy even before achieving vaccine-based herd immunity: we can make it safe for most people to do most jobs most of the time. We can also achieve an acceptable-enough degree of healthcare normalcy, where the level of new cases and deaths is relatively low and predictable, and the healthcare system is not overwhelmed. We can get the schools open. People can plan ahead again. We may even enjoy a burst of new economic activity as the pandemic’s grip is partially broken.

Given that 60 million Americans say they won’t accept vaccination and another 60 million say they probably won’t, it’s going to be difficult to quickly achieve pure vaccine-driven herd immunity in this angry country of ours.  A better path would be to remember that we can get to a better normalcy sooner if we add non-pharmaceutical measures to our vaccination program. Ubiquitous personal testing, common-sense distancing, better ventilation and tactical masking could bridge the gap between almost-good-enough vaccine-driven immunity and the new normal life we need. The project of persuading at least half of the obdurately vaccine-averse to cross over is going to take a while.


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