The ongoing pandemic means countries have begun strategic discussions on how to achieve the “new normal.” This has led to renewed interest in a herd immunity approach to manage the pandemic. The concept is that after being widely infected, a population will develop herd immunity to the virus, eliminating or significantly reducing community transmission and protecting the most vulnerable, who must be shielded in the meantime. 

We would strongly advise that governments and public health advisers hold firm for the winter, promoting effective physical distancing and other non-pharmaceutical intervention strategies to compress virus transmission and limit infection rates. Vaccinations are already being rolled out quickly, and assuming high-uptake of the vaccine and an effective vaccination campaign, herd immunity should become a reality. 

But covid-19 herd immunity should only be relied on as part of a vaccination strategy and not based on “natural” infections, as advocated in some current thinking. The arguments against “natural” herd immunity include lack of evidence, inequalities that expose vulnerable groups, unattainable herd immunity thresholds just by infection, and moral and ethical considerations. 

Part of the infection-derived herd immunity argument is that vulnerable and older groups can be shielded while less vulnerable people acquire the infection and recover without consequence. This is incorrect and impractical. Underestimation of the severity of the infection is also of grave concern. 

There is a lack of evidence supporting herd immunity. It is a “hypothetical possibility” not borne out in the evidence. The empirical data emerging, such as from the city of Manaus, Brazil, have shown the fragility of natural herd immunity. Following a study showing that 66% of citizens had been infected with SARS-CoV-2 by the end of the first wave, some concluded that the number of people still vulnerable to SARS-CoV-2 was too small for transmission to survive, meaning herd immunity had been achieved. Those hopes were dampened by a subsequent surge in cases of infection, with exposure rising to 76% in what is now described as “unmitigated transmission.” 

There is also a lack of information about how the human immune system behaves with SARS-CoV-2. It is still unclear if antibodies against SARS-CoV-2 will protect people who have been infected fully, or generate wider herd immunity. A study in the UK suggests that the accuracy of the test for SARS-CoV-2 antibodies may be lower than previously suggested, increasing the risk of false positive results. If antibody responses are used as an indicator of immunity, there is a danger that individuals and the government will make decisions based on inaccurate information.

The herd immunity threshold is the proportion of people who need to become immune before the population is protected against further infection. Some studies are showing that the herd immunity threshold is impossible to calculate accurately because it does not take human behaviour into consideration. Using a statistic that might not be applicable to the current situation makes the idea of achieving herd immunity even more dangerous. According to Johns Hopkins University epidemiologists David Dowdy and Gypsyamber D’Souza, it is likely that 70% or more of the population would need to be immune to reach herd immunity for covid-19. To put this into perspective, “without a vaccine, over 200 million Americans would have to get infected before we reach this threshold.” 

The ethical and moral implications of the herd immunity argument are also crucial. Relying on a herd immunity strategy creates a dichotomy between lives that matter and lives that do not, which is deeply problematic. It would also lead to much pressure on national health services, with indirect damage to population health and an occupational mortality risk for health service personnel. Democratic societies have an obligation to uphold the equal value of all citizens. Allowing a viral infection to spread, either freely or at a “reasonable rate,” is unethical because it exposes large groups of vulnerable citizens to life-threatening risks.

It is an illusion to believe that the economy will benefit if the pandemic is allowed to go unchecked. There is no trade-off between the economy and health. With the exception of the East Asian and Australasian economies, who dealt successfully with the pandemic early on, there is no evidence that economic performance can be restored if the pandemic is not first dealt with. 

Reports are emerging which suggest that proponents of the herd immunity philosophy may have influenced UK government ministers in September 2020, allegedly delaying the “circuit breaker” advocated by the government’s own Scientific Advisory Group for Emergencies (SAGE). An additional 1.3 million cases of infection have occurred in the UK since the government’s inaction on a circuit breaker in September. Resistance to lockdown measures stemming from a desire to maintain economic activity has undoubtedly been an influence on the policies of many European nations and has resulted in massive increases in cases and deaths in the second wave. 

We need to focus on using successful virus suppression strategies until we reach herd immunity with the new vaccines. We need to track the epidemiology of the virus using population serology, but it is dangerous, and unfounded in science, to advocate natural herd immunity as a means to pandemic control.  

Nadav Davidovitch [1,2]

Carlo Signorelli [1,3]

Laurent Chambaud [1,4]

Arianne Tenenbaum [2]

John Reid [1,5]

John Middleton [1,6]

1 ASPHER COVID-19 Task Force, Brussels, Belgium

2 School of Public Health, Ben Gurion University of the Negev, Be’er Sheva, Israel

3 Università Vita-Salute, San Raffaele (UniSR), Milan, Italy

4 École des Hautes Études en Sante Publique (EHESP), Rennes, France

5 Department of Public Health and Wellbeing, University of Chester, Chester, United Kingdom 

6 Association of Schools of Public Health in the European Region – ASPHER, Brussels, Belgium 

Competing interests: none declared

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