Forecasting events amidst a global health crisis presents considerable challenges. The landscape shifts so rapidly on a day-to-day basis that any prior understanding is rendered obsolete with alarming speed. Even the fundamental certainties we believed were established – such as transmission rates, symptomatic manifestations, and effective quarantine protocols – are now subject to scrutiny and require reassessment.
However, amidst this pervasive uncertainty, one aspect has emerged that I could not have possibly anticipated: the discourse surrounding herd immunity. Its appearance is so unexpected that when a journalist initially inquired about its efficacy concerning the coronavirus, my immediate reaction was to laugh, assuming it was a jest.
Yet, here we are. A substantial volume of commentary and analytical pieces concerning the COVID-19 virus advocates for herd immunity as a potential, albeit precarious, response to our current predicament. Many proponents position herd immunity as a prophylactic measure intended to mitigate the impending deluge of illness that numerous forecasts predict.
This perspective is fundamentally flawed. Herd immunity, in the absence of a vaccine, cannot, by its very definition, constitute a preventative strategy.
Allow me to elaborate.
Herd immunity operates as an epidemiological principle describing a state where a population achieves a sufficient level of resistance to a pathogen, thereby impeding its transmission within that collective. In essence, a substantial proportion of individuals are rendered insusceptible to the disease – either through immunization or acquired resistance – thereby safeguarding those who remain vulnerable.
Consider the example of mumps. This highly contagious ailment, though generally considered mild, can inflict significant discomfort and, in some instances, lead to debilitating, life-altering sequelae. Fortunately, it is preventable through vaccination, with a highly effective vaccine that has dramatically reduced its incidence in contemporary times.
Mumps exhibits a basic reproductive number (R0) ranging from 10 to 12. This signifies that within a completely susceptible population – meaning no individuals possess immunity to the virus – each infected person will, on average, transmit the disease to between ten and twelve others.
Consequently, without widespread vaccination, approximately 95 percent of the population would contract the illness over time. Even with such a potent contagiousness, a small fraction, about 5 percent, would remain unaffected because the chain of transmission would have been broken by herd immunity, leaving no susceptible individuals to infect.
We can augment this protective barrier through vaccination initiatives. Immunization confers resistance to infection and also curtails the spread of the virus from infected individuals to their contacts. Achieving a critical mass of immune individuals effectively halts the pathogen’s proliferation throughout the populace.
This, in essence, is the concept of herd immunity.
For mumps, approximately 92 percent of the population must be immune to effectively cease its transmission. This is referred to as the herd immunity threshold. Fortunately, COVID-19 exhibits considerably lower infectivity than mumps, with an estimated R0 of around 3.
This reduced contagiousness implies a lower, though still substantial, proportion of infected individuals within the population is required to achieve herd immunity, estimated at approximately 70 percent of the total population.
This brings us to the fundamental reason why herd immunity cannot be classified as a preventative strategy.
If 70 percent of a population becomes infected with a disease, it is inherently not a scenario of prevention. How could it be? The majority of the nation’s inhabitants would be afflicted with illness! The notion that one can attain this 70 percent threshold solely through infecting younger demographics is preposterous. A strategy targeting only the youth would leave older individuals entirely unprotected, creating a significant risk for anyone above a certain age, effectively confining them to their homes indefinitely due to the omnipresent threat of infection.
Furthermore, it is imperative to consider the ramifications of such a dire outcome. Current estimations place the COVID-19 infection fatality rate at approximately 0.5-1 percent. If 70 percent of an entire population contracts the virus, this translates to a potential mortality rate of 0.35-0.7 percent of the national populace, a truly catastrophic consequence.
With roughly 10 percent of all infections requiring hospitalization, the healthcare system would be overwhelmed by a massive influx of severely ill individuals, posing profound challenges to national infrastructure and resources.
The unfortunate reality is that herd immunity is not a panacea for our current pandemic challenges. While it may eventually emerge as a natural consequence, relying on it as a savior is unequivocally unrealistic. The appropriate time to engage in discussions about herd immunity is concurrent with the development of a vaccine. Prior to that juncture, we are ill-equipped to genuinely halt the epidemic’s progression.
Until a vaccine is available, any assertion promoting herd immunity as a proactive measure against COVID-19 is demonstrably incorrect. Fortunately, alternative methods for curtailing the spread of infections exist, all of which fundamentally revolve around minimizing contact with infected individuals.
Therefore, prioritize staying home, ensuring your safety, and diligently practicing physical distancing to the greatest extent feasible.
Gideon Meyerowitz-Katz is an epidemiologist specializing in chronic disease research based in Sydney, Australia. He maintains a regular health blog that delves into science communication, public health, and the practical interpretation of newly published studies.
