Global Health Experts Warn Against Herd Immunity Approach to Covid-19

In an open letter, 80 health experts call the approach ‘a dangerous fallacy unsupported by the scientific evidence’

Photo by Mika Baumeister on Unsplash

Herd immunity has received significant attention since the beginning of the Covid-19 pandemic. Several countries hoped that herd immunity would offer protection against the virus, and some even went so far as to loosen restrictions to allow for the natural spread of the virus. In an open letter published today in The Lancet, dozens of health officials publicly criticized herd immunity approaches to slowing Covid-19 as flawed and dangerous.

Herd immunity relies on an adequate number of immune individuals within a population such that a pathogen is unable to spread. Beyond that, herd immunity differs widely from one disease to another. Vaccines offer a powerful example of how herd immunity can control the spread of a disease. For example, a measles vaccination rate above 95% is sufficient to halt the spread of the measles virus.

However, reliance on herd immunity for a disease such as measles is underscored by years of scientific data that supports the efficacy of the vaccine, and the durability of immunity conferred. Just seven months into the Covid-19 pandemic, this level of scientific certainty is lacking.

The 80 authors of the letter published today in The Lancet argue that herd immunity approaches to stopping Covid-19 are deeply flawed and potentially dangerous.

The herd immunity approach to stopping Covid-19 can best be explained as allowing the virus to spread within low-risk populations — namely younger populations — to the point where enough people have caught the virus and developed immunity that’s sufficient to protect high-risk populations such as older adults.

However, as the authors of the letter underscore, one flaw to this approach is that the durability of Covid-19 immunity after an infection is unknown. At this point, researchers have not been able to demonstrate conclusively that antibodies from Covid-19 offer lasting protection.

Beyond this, recent outbreaks of Covid-19 have been linked to younger populations, including those who are asymptomatic for the disease. Research has shown that spikes in cases within older populations rise shortly after, indicating that the disease quickly passes between the two groups. Authors of the letter also say that keeping low-risk and high-risk populations isolated from one another is virtually impossible.

The authors agree that without a complete understanding of how Covid-19 affects different populations, relying on herd immunity could lead to large and lasting spikes in new cases, an excessive burden on health care systems, economic slowing, and potentially thousands of deaths.

Sweden made headlines early in the pandemic by adopting a more lax approach to Covid-19. By avoiding a shutdown and not encouraging the use of masks, Swedish officials appeared to be placing their bets on achieving herd immunity quickly. Sweden suffered one of the highest death rates of any European nation and their approach has been decried by health officials around the world.

Experts who signed on to this letter unanimously agree that herd immunity is an unreliable and dangerous approach to controlling Covid-19. They argue that protective measures such as mask-wearing, crowd size restriction as well as testing, contact tracing, and isolation of positive cases offer the best hope of controlling the virus until a vaccine is available.

“The evidence is very clear: controlling community spread of Covid-19 is the best way to protect our societies and economies until safe and effective vaccines and therapeutics arrive within the coming months,” they conclude. “We cannot afford distractions that undermine an effective response; it is essential that we act urgently based on the evidence.”

Medical student, molecular biologist and educator. I write about science and medicine.

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