Herd Immunity in the SARS-CoV-2 Pandemic

This week, I wanted to focus on the idea of herd immunity: what it is, how efforts have panned out in the current pandemic, and what the scientific community proposes as we move forward. Like other aspects of the pandemic, the idea of herd immunity is one that has become politically charged, with proponents and opponents on both sides of the aisle. It is important, however, in deciding whether or not to pursue a policy of advocacy for herd immunity that we consider the theory behind it, previous instances of this method of preventing transmission, and how it may relate to the current circumstances.

As Paul Fine explains in his article “Herd Immunity: History, Theory, Practice”, herd immunity “has to do with the protection of populations from infection which is brought about by the presence of immune individuals” (Fine, 1993). When a theoretical threshold percentage of a population has become exposed to a pathogen – whether by infection or vaccination – and developed resistance such that the incidence of new infections begins to decrease as a result of a limited host availability, the population has achieved herd immunity (Fine, Eames, Heymann, 2011). In theory, the development of herd immunity in a population means that the transmission of the pathogen will be held at a constant low level, preventing further widespread outbreaks or epidemics. Additionally, when most people in a community are immune to a given pathogen, they in effect protect the few susceptible individuals, because it is unlikely the pathogen will find a susceptible host. The term “herd immunity” was coined by George Potter, Adolph Eichhorn, and W. J. MacNeal in 1911, who proposed a mechanism by which farmers could save far more heads of cattle through the development of herd immunity than by slaughtering those individual cows which were sick (Potter, Eichhorn, MacNeal, 1911). Similarly, A. W. Hedrich, who studied the spread of measles among children in the early 1900s, noted a decline in cases following mass immunization (Hedrich, 1933). Throughout the 20th century, large-scale vaccination efforts for smallpox and whooping cough have resulted in mass immunity. In these instances, herd immunity functions to protect susceptible populations who may still contract these diseases: infants who cannot receive their first vaccination until 1 year of age, immunocompromised people, and pregnant women (APIC, 2020). While the threshold differs for each disease based on its contagiousness (of which the R0 value is a proxy measurement), herd immunity has proven successful in helping contain the spread of many diseases in the US and elsewhere.

While several countries – among them, the UK and now the US – have at some point allegedly considered pursuing some level of herd immunity as a potential strategy, the most significant example of an attempt at establishing herd immunity against SARS-CoV-2 infection comes from Sweden. Their policy of “folkvett”, relying on the supposed common sense and responsibility of the Swedish population to prevent the spread, ultimately mutated into a ramrod policy in which public spaces – gyms, stores, restaurants – and schools for students up to the age of 16 remained open when countries elsewhere were experiencing their first surges of cases. According to several reports, health authorities in Sweden predicted that at least 40% of the population would have contracted and developed antibodies against the disease by May of this year. The actual figure for seroprevalence is closer to 7% (Medical Xpress, 2020; CGTN.com, 2020). By mid-May, Sweden’s per-capita death toll was the highest of any nation in the world, and it remains the highest of any Nordic country (Financial Times coronavirus tracker). Despite this fact, some experts have contended that Sweden’s method of containment has achieved results that are similar to – if not better than – other countries in terms of hospitalizations and even mortality rates through mid-summer, though by far less coercive and restrictive means (Pierre, 2020; Kamerlin, Kasson, 2020). These authors argue that Sweden’s ordeal may actually set a precedent for other countries in future epidemic or pandemic outbreaks.

Here in the US, speculation has arisen that the Executive Branch is advocating for a containment policy that is “in line” with a herd immunity strategy, according to CNN and others. This idea, though largely decried by the Trump administration, has attracted – unsurprisingly – the attention of media outlets comprising the spectrum of political ideology. And though the idea of a herd immunity strategy has its proponents, the consensus among news sources seems to be that any policy advocating for the lifting of restrictions at this stage would spell disaster for the US. Experts agree that a successful attempt at achieving herd immunity would require between 60% and 80% of individuals to become infected: equivalent to roughly 200 million Americans contracting the disease. And with the current fatality rate in the US just below 3% (Johns Hopkins University coronavirus tracker), this means there may be as many 6 million more deaths before truly protective herd immunity is reached, a price most people are not willing to accept. Dr. Maria Van Kerkhove of the World Health Organization pointed out in an August 27 media briefing that herd immunity is usually only discussed in the context of vaccination levels, not in the context of pandemics (WHO.com). So it seems that herd immunity may not be the silver bullet for SARS-CoV-2 after all – the mortality rate is far too high to support this type of strategy, and as of yet, it is unclear whether such a policy would have enough benefits to outweigh the enormous risks. In an op-ed in The Wall Street Journal, former FDA commissioner Scott Gottlieb wrote that Sweden should not be held as the idyllic model of a pandemic response. In his article, Gottlieb recapitulates the larger sentiment that the Swedish population is still far from their desired rates of exposure. Howard Koh, former professor at the Harvard T.H. Chan School of Public Health, echoed this with a warning that Sweden’s “attempt to create ‘herd immunity’ has been woefully unsuccessful and is definitely not a strategy we should seek to replicate in the United States”.