What is “Herd Immunity” and How to End a Pandemic?
by Nick Tiller, PhD
A question on the minds of many as we approach the 1-year anniversary of the COVID-19 pandemic is, how can we end this pandemic more quickly?
The best strategy to end the pandemic is to attain “herd immunity”, also called “community immunity”. Herd immunity is achieved when 50–90% of the population develop and retain effective antibodies to the virus, ultimately reducing its transmission. However, because COVID-19 is still relatively new, we do not know where in that spectrum (50–90%) population-based immunity will be achieved, and it’s different for each virus. A commentary at Science-Based Medicine suggests that the most effective means of achieving herd immunity in any pandemic is the development of an effective vaccine that would mitigate the spread of the virus to levels that would not be disruptive.
Another strategy that’s been proposed is to accelerate herd immunity through natural infection; i.e., permitting the virus to circulate freely within the community, while simultaneously isolating its elderly and most vulnerable members. The premise is that those who survive will retain antibodies to the virus and eventually reduce its spread. While this strategy may quickly achieve high rates of community infection, it has been heavily criticized by most scientists and medical experts, including the World Health Organization who referred to herd immunity through natural infection as “unscientific” and “unethical”. So why isn’t natural herd immunity a valid means of addressing the pandemic, and what are the potential risks and benefits of this strategy?
There are major risks and potentially severe consequences of pursuing natural herd immunity. Recent estimates are that approximately 9% of the US population currently has immunity to COVID-19. Accordingly, it would require a five-fold increase in the number of total US cases just to achieve the lowest (most optimistic) threshold for herd immunity (i.e., 50%). It is estimated that this would result in approximately 1 million American fatalities, which clearly represents an unacceptable risk to Americans.
Fortunately, the death rate of COVID-19 (the percentage of those who die after infection) is declining, in part because a greater percentage of those infections are recorded in groups at lower risk for severe illness. So, why not allow the virus to spread exclusively among only lower-risk individuals? This approach is unethical because younger people (or others at lower risk) are not immune to serious illness or even death. Also, the long-term effects of COVID-19 are still only beginning to be understood. A post-viral syndrome commonly termed “long COVID” causes serious and long-lasting side effects that include breathlessness, fatigue, cardiovascular and renal dysfunction. The other risk of this strategy is the effects of prolonged isolation of vulnerable populations – as would be obligatory in the natural herd immunity model. This is not only impractical, but itself would result in enormous mental health and socioeconomic burden.
So much for the risks. What are the benefits of natural herd immunity? One problem is that current studies are not yet clear that infection leads to lasting immunity against SARS-CoV-2. An optimistic scenario would see post-infection antibodies remaining for one year. But other coronaviruses do not generally evoke lasting immunity, and so there’s no reason to think SARS-CoV-2 will be different. It could take several years for natural herd immunity to be achieved (if at all), by which time any effective antibodies resulting from natural infection may have subsided. As a recent letter in the Lancet highlighted, such a strategy wouldn’t put an end to the pandemic, but would instead result in recurrent worldwide outbreaks, as was the case with numerous infectious diseases before the advent of vaccination. Conversely then, far from hastening the end of the pandemic, a strategy to pursue natural herd immunity would result in more deaths and damage to the economy, and overwhelm the ability of health-care systems to provide acute and routine care to both COVID and non-COVID patients.
Everyone wants to see an end to the pandemic and for COVID-19 to be relegated to a manageable background level. But experts agree that “natural herd immunity” is the worst possible strategy. Of course, the emphasis should be on developing effective treatments and preventative measures, but these take time. A safe and effective vaccine is being pursued, but it will need to be administered to a large proportion of the population, and this will take many months to achieve. It may also require regular booster shots before vaccine-based herd immunity is obtained.
In the interim, we must have the patience to strive to flatten the curve with proven methods; mask-wearing, social distancing, good hygiene, and the avoidance of mass gatherings. There must also be an effective system of contact-tracing to efficiently isolate infections when they occur and reduce community spread. This must all be complemented with financial and social programs that support the economy and encourage community engagement with a collective response. While such an integrated and multifaced strategy is no mean feat, continued talk of a natural herd immunity only serves as a distraction that undermines the genuine efforts to end the pandemic.