COVID-19 and herd immunity

Herd immunity occurs when a large enough proportion of a population of humans or animals becomes immune to a disease in such a way as to make the spread of disease between members of the species involved unlikely. During the early weeks of the COVID-19 pandemic in the United Kingdom this apparently simple concept became highly political and controversial as a result of the intervention of the Government’s Chief Scientific Adviser, Sir Patrick Vallance.

By March 2020, confused by news of the rapid spread of COVID-19 around the world, together with vacillating and incoherent government statements, panic was beginning to spread around Britain. There was an increasing sense that the government was not in control despite efforts by the Chief Medical Officer to describe a framework for dealing with the virus consisting of four strands – ‘contain, delay, mitigate and research’.

Overnight in mid-march the Chief Medical Officer’s plan flipped into a potential one-point plan when the Chief Scientific Advisor appeared to float the idea of letting the virus run its course. The implied aim was to achieve herd immunity rather than containing the epidemic through traditional public health measures of tracking and tracing those who were infected and isolating them to prevent further spread. ¹ This suggestion was met by instant controversy, as the idea was digested and estimates were made that about 60% of the UK population, or 40 million people would need to catch the coronavirus to have any chance of building ‘herd immunity’. To many, in the context of a new virus about which little was known at that stage, jumping to simplistic conclusions as to the way forward seemed rash.

When challenged on SKY News as to where the evidence for this proposed course of action came from, Sir Patrick Vallance responded that ‘… we’ve got a panel of very experienced scientists’. Two days previously in a fireside chat with Prime Minister Boris Johnson, the Deputy Chief Medical Officer, Dr Jennie Harries, had implicitly supported such an approach by defending keeping schools open and was to argue on the Radio 4 Today Programme that

If you have a new disease the normal thing is it will take off gradually. It will rise very rapidly at one point and come back down again when it effectively runs out of people in the population to infect.

The fallacy behind such a view lies in its assumption of the unchanging nature of the virus rather than the recognition that viruses are constantly evolving in a dance with their animal hosts. In this dynamic situation, adaptation of both infecting agent and host to each other may occur over time but alternatively nature might throw up new threats through mutation and interaction with viruses from other animal species as happens with seasonal flu with the resulting necessity of refining the components of flu vaccines on a regular basis.

In addition, for example in the case of HIV/AIDS, the world would still be waiting for the virus to run out of people to infect, a reminder that the population of concern in the modern age is a global one and that in the words of the Director General of the World Health Organization (WHO) ‘Nobody is safe until everybody is safe’.

A parochial and domestic view on a pandemic may be comforting in the short term but without effective international collaboration, solidarity and health governance the problem ‘worthy of attack will prove its worth by hitting back’. ²

In the two years since the pandemic first emerged, there has been remarkable progress in understanding the new virus and not least in developing a menu of vaccines that are both effective and safe and have been produced at scale. However, the emergence of a succession of new variants from different parts of the world has reinforced the necessity of maintaining the capacity and capability for traditional public health interventions that are environmental and social in the form of population hygiene rather than assuming that laboratory science and pharmaceuticals are the only things that matter.

Throughout the pandemic, the tussle of concerns between the imperatives of public health, politics and the economy have been fought out as much behind the scenes as in public view with science being used as a weapon of enlightenment but also of obfuscation and as a shield when it suited political leaders. Statistical modellers of the pandemic have had a mixed war sitting somewhere between the tentative claims of imperfect methods based on variable data and the incredible punts into the crystal ball of Mystic Meg.

In this battle ground, the spectre of claims for herd immunity and more recently the idea that the future of living with COVID-19 will be no different from our perennial encounter with seasonal flu are never far away. During successive waves of the pandemic, tardiness in implementing measures such as extending vaccine coverage to younger groups, despite the clearance from international scientific advisory bodies and the WHO, have allowed the virus to run untrammelled among the school age and pre-school populations. This has contributed to the still-high infection rates and death toll among the most vulnerable especially as their vaccine-induced immunity has waned; such decisions give weight to the continuing currency of the ill-founded notion of herd immunity among those responsible for weighing up the options.

More recently, the dominance of politics over science as manifested by the rush to abandon all non-pharmacological methods of virus control have led to the emergence of a dominant domestic narrative that the pandemic is all but over bar the shouting. The assumption has been spreading that in future COVID-19 will only be of nuisance value on a par with the flu.

Such a claim may prove to be highly misleading assuming as it seems to that while the virus continues to circulate extensively around the globe it will be possible to keep ahead of the virus with ever-changing vaccines. This takes for granted that vaccines can be delivered to a majority of the world’s population despite the continuing weakness of health services in large parts of the globe. In addition, it plays down the associated morbidity and mortality associated with COVID-19 in comparison with influenza and in recklessly abandoning the bounty experienced from enhanced personal hygiene measures including mask wearing in reducing the annual toll from influenza itself risks throwing the baby out with the bath water.

In due course, the lessons and legacy of the COVID-19 pandemic of 2019–? will be crawled over by historians, social and biological scientists and others; let us hope that in the reckoning is included a commitment to population public health literacy so that in future the politicians claims to be ‘following the science’ might be better held to account by the citizens and that spurious notions such as that of ‘herd immunity’ are treated with the scepticism that they deserve.