Collective immunity or abandoned herd? The Unique Problem with COVID-19 | Print edition

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By Dr Changa Kurukularatne

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Collective immunity represented by red (Alpha), green (Beta) and blue (Gamma) individuals depending on their immunity. The blue barrier (Gamma) prevents red (Alpha) from infecting green (Beta).

Collective immunity ”is one of the most misunderstood and misused terms in the context of COVID-19. The fundamental principle of collective immunity is the protection of sensitive individuals within a population without immunity against a particular contagion, by individuals immunized against this contagion. To better understand this concept, consider three categories of individuals within a population with respect to immune status: Alpha, infected (propagators); Beta, uninfected (susceptible); and Gamma, immune to that particular agent. When the proportion of Gamma individuals is above the collective immunity threshold, they form a protective interstitial barrier to prevent an Alpha from directly infecting a Beta.

The term herd immunity dates back to the pre-World War I epidemic of “contagious abortion” among American cattle. Kansas veterinarian George Potter opposed the practice of killing infected animals, saying that “the disease of abortion can be compared to a fire which, if new fuel is not constantly added, goes out quickly “. Potter advocated that infected cows be allowed to naturally clear the infection and continue to breastfeed their calves. Until new, non-immune cattle were introduced (Potter’s “new fuel”), the contagion would naturally subside.

The term was reused in the 1970s to estimate the number of people in a population who needed to be vaccinated in order to eradicate smallpox.

The concept

The basic reproductive number, or R0 (R-naught), is the average number of secondary infections produced by an infected individual in a fully susceptible population. As intuitively apparent, where R0 is> 1, epidemics will worsen; when R0 0 is used to estimate group immunity thresholds using the (simplified) equation of 1-1 / R0.

Older variants of SARS-CoV-2 had an R0 about 3, which means that each infected person would infect 3 more. Extremely simplified 1-1 / R application0, 67% of the population must have developed immunity to protect the non-immune. Newer variants such as Delta (B.1.617.2) have a much higher R0, ranging from 3.2 to a whopping 8 (average R0, about 5), beating SARS, MERS, Ebola and influenza. For context, seasonal influenza has an average R0 of 1.3, and measles, an exceptionally cruel 15-18. Extremely simplified 1-1 / R application0, we see that the collective immunity threshold for the Delta variant is 80%, a jump of 67% for the older variants.

The problem with
oversimplification

The life assumptions are problematic, and 1 – 1 / R0 is no exception. The formula assumes that the immunity conferred (by vaccination or natural infection) is complete and lasting. It further assumes a uniformly stable R0 and the random movement of people within the population.

In reality, human movements are not random, and R0 varies considerably within a population.

COVID-19 vaccines do not provide 100% protection against infection or zero risk of transmission. Vitiello et al. (2021) were very careful not to be mistaken in their conclusion laden with adverbs “People vaccinated are likely to be less likely to transmit the virus”.

Since vaccines are “likely to be less likely to transmit the virus,” we are a long way from achieving herd immunity. Scientists and public health officials who understand that COVID-19 vaccines do not completely prevent infection or eliminate transmission will talk about achieving herd immunity through vaccination campaigns – a brain decoupling indicative of today’s buzzword culture. Simply put, if one can still contract and transmit the virus despite vaccination, and indeed be re-infected after natural infection, then herd immunity as a concept is struggling to find a foothold in COVID-19.

Abandoned herd?

Recall what COVID-19 has taught us so far. One – you can get COVID-19 more than once (uncommon, but it does happen). Two – the probability of reinfection increases with new variants. Three – previous infections in a population, whether asymptomatic or symptomatic, are poor predictors of future protection. While these weren’t daunting enough for anyone trying to calculate herd immunity thresholds, we also know that the new viral strains are able to evade vaccine-induced immune responses, selling the phantom head to antibodies. hastily assembled and playing around the lazy legs. , clueless immune cells.

A R0 under another name? Implement physical distancing, good face mask compliance, responsible population behavior, effective test-trace-isolation programs, effective early treatment at home, and voila! Your R0 will drop, guaranteed. When distancing practices are conveniently forgotten, face masks hang thoughtlessly from people’s chins and ears, and large gatherings resume with reckless abandon, the R0 will surely rise like the eastern morning sun; ergo, not collective immunity.

If it’s not collective immunity, then what?

Manaus in northern Brazil was devastated by a particularly vicious outbreak of COVID-19 last year. It is estimated that 76% of the population of Manaus have been exposed to SARS-CoV-2 – above the herd immunity threshold of 70% based on their R0. As deaths and hospitalizations decline, herd immunity has been hailed as the savior – until new infections, hospitalizations and deaths rise again dishearteningly, possibly due to waning immunity, a new strain P1 and other unforeseen population factors.

Here’s what we have to accept, no matter how bitterness the pill: The main principles of collective immunity just don’t apply to the elusive SARS-CoV-2, its slippery and supercharged variants, our answers. immune system and currently available vaccines. This idea is not without company; see Asch-wanden’s “Five Reasons Why Herd Immunity COVID Is Likely Impossible” published in Nature just a few months ago, the proper adverbs and all.

COVID-19 vaccines are very effective in reducing deaths and preventing hospitalizations due to serious illness. Hospitals need to operate at a manageable capacity. If this is not an important referral, simply visualize 10% of the 20 million people flooding our hospitals over the next 2-4 weeks and try to access care for an injury or illness that is afflicting you. , you or your family. When hospitals exceed their threshold of treatment capacity, people die of heart attacks, strokes, blood poisoning, internal bleeding and trauma that otherwise could and would have been saved.

Regardless of the ability of COVID-19 vaccines to reduce mortality and prevent serious illness (translation: get vaccinated), they lack the firepower of preventing infection and eliminating the transmission of, by example, Max Theiler’s yellow fever vaccine of 1937. Boasting lifelong immunity with a single dose, 98% seroconverting, and inducing high levels of neutralizing antibodies in just 10 days, the yellow fever vaccine has helped suppress every epidemic since its inception. Viruses vary widely, as do vaccines. COVID-19 vaccines are simply not in the same league when it comes to preventing infection and transmission. Significant reductions in both of these measures are observed, but not to the level required for collective immunity.

So where do we go from here?

We just have to – and just have to – use interventions that have been proven in the fight against COVID-19. We hope that the proven recipe for test-trace-isolation will be used to its full potential in every country in a meaningful and effective way, without interference or unscientific idiocy. We hope that our border security and testing capabilities will be improved enough to identify new variants as soon as possible. Where early treatment at home is essential, we hope that personal gain or political advantage will not preclude the rigorous scientific analysis of ivermectin as a preventive and therapeutic strategy. We hope that natural immunity, especially the cell-mediated immune response, will manifest over the long term, conferring lasting protection long after neutralizing antibody levels have declined. We hope that future vaccination strategies elicit robust immune responses without those clearly marked neon escape hatches for the new variants.

We also hope against all hope that SARS-CoV-2 will somehow lose its infectivity, virulence and lethality over time, and agree to live among us in relative harmony, much like his cousins ​​causing the cold.

None of these hopes, that we know, involve “collective immunity” as we know it. Let us not use collective immunity as vaccine propaganda; COVID-19 vaccines are already credible enough in benchmarks that really matter. Instead, let’s move on to making a real difference by saving lives and livelihoods through strong leadership and good governance, as responsible citizens. Compassion, unlike collective immunity, is achievable in COVID-19. Compassion will allow us to remove the stigma so unpleasantly associated with this disease and to face its challenges with dignity and humanity.

(The author is an infectious disease specialist)

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