Herd immunity: a zombie idea and a lethal strategy

If you’re feeling that herd immunity is the zombie idea of the pandemic that refuses to die, you’re about to see it resurrected in earnest. This time, however, the talk ought to be about the context in which herd immunity was only ever meant to be applied: a vaccination programme.

Herd immunity refers to the situation in which an infectious disease can no longer propagate in a population because enough people have immunity to it that it has very little chance of being passed on. Generally this is achieved by vaccination. It happens when the proportion of people with immunity exceeds some threshold, the value of which depends among other things on how infectious the pathogen is. For measles it is around 95 per cent; for flu it can be as low as 25 per cent or so. For the SARS-CoV-2 coronavirus that causes Covid-19, it was thought originally that the herd immunity threshold would be around 60 to 70 per cent, although the new, more infectious variant that has appeared in the UK in the second wave this winter might raise that to around 80 per cent. This rather high threshold makes it all the more important to explain to people that the vaccines have been thoroughly tested and shown to be as safe as any other officially approved medicines. 

Yet it’s unlikely that herd immunity is going to remain an issue about vaccines. The Great Barrington Declaration – a statement issued in October by a collection of scientists and others, supported by a right-wing US think tank, the AIER – argues, contrary to the view of most experts, that the pandemic response should eschew lockdowns in favour of “focused protection”. It offered succour to the “lockdown sceptics” who insist that the only way out of this crisis is for people to continue getting infected until “natural herd immunity” is achieved (perhaps now with some help from vaccines). “The most compassionate approach that balances the risks and benefits of reaching herd immunity”, says the Declaration, “is to allow those who are at minimal risk of death to live their lives normally to build up immunity to the virus through natural infection, while better protecting those who are at highest risk.”

We have just seen a tragic indication of why this would be a lethal strategy. In Manaus, the capital of the Brazilian state of Amazonas, the coronavirus has spread more or less unmitigated. It has infected an estimated 76 per cent of the population, and even though this was an unusually young demographic with a relatively low risk of death from Covid-19, the mortality rate by the start of October was around 1,200 per million people – about twice that of the hard-hit UK over the same period. Yet even with this enormous extent of infection, it appears that herd immunity has not been achieved in the Manaus population. “The deaths that would accrue in pursuit of naturally acquired herd immunity would be catastrophic”, say public-health experts Devi Sridhar and Deepti Gurdasani of Edinburgh Medical School. Adjusting for the age demographic, infection of that extent in the UK would cause around 350,000 deaths. The Manaus outcome shows that “pursuing herd immunity through naturally acquired infection is not a strategy that can be considered”, Sridhar and Gurdasani conclude.

Yet a handful of scientists continue to advocate the idea. Prominent among them is Sunetra Gupta, of the department of zoology at Oxford University, one of the principal authors of the Great Barrington Declaration. In March her group released a study (before peer review) suggesting that around half the UK population had already been infected with the virus, and that naturally acquired herd immunity might already be close, if not indeed already achieved. She also suggested that the infection threshold for achieving it might be as low as 10 to 25 per cent. As it became clear from the continued spread of Covid-19 that this was far from the case, Gupta said that it might be necessary to tell people that “not only is it a good thing for young people to go out there and become immune [by being infected], but that is almost their duty. It’s a way of living with this virus.”

In the summer, Gupta asserted that, if she was right that there was already widespread immunity in the population, Covid-19 was already on the way out and we would not see a second wave. Others who share such views include Carl Heneghan of Oxford’s Centre for Evidence-Based Medicine and Karol Sikora, an oncologist at the University of Buckingham and a former cancer adviser to the World Health Organization. Sikora (a signatory of the Great Barrington Declaration) asserted in June that the virus might simply just “fade out”, despite “a lot of fuss and a lot of noise”, and that it was likely we’d be done with Covid-19 by August. In late September, faced with evidence that he was wrong, he argued that “this second wave is simply not as deadly.” In later September, as the more infectious new variant of the virus was just taking off, Heneghan argued it was too early to implement a lockdown; by 9 October he was still casting doubt on the reality of a second wave. 

You might imagine that being proved so catastrophically wrong would instill some humility. But Gupta and others who share her views have doubled down. 

Lockdown sceptics cling to the fact that Gupta is a professor at Oxford specialising in the evolution of infectious-disease agents like viruses – so who are the rest of us to argue? This “appeal to authority” is the resort of the scientific ingénu(e) who lacks any other discriminatory tools. Of course someone qualified in the field they are speaking about deserves to be listened to more than, say, a journalist who has made a career from contrarianism. But as a former editor of Nature, I can assure you that such qualifications offer no guarantee of being right. Experts in a given topic won’t always agree with one another, and when they do not, it becomes important to judge them by the quality of their record – especially if they hold a minority view. The lockdown sceptics lack the skills – or the inclination – to make such judgements, so they simply recite Gupta’s job title (and those of other scientists, such as Sikora’s previous WHO position) like a mantra.

Gupta has been consistently wrong. That her study in March about the level of infection in the population, and subsequent statements about the imminent appearance of natural herd immunity in the population, were utterly mistaken was in itself no disgrace; in the early days of Covid, with good data so sparse, errors were inevitable. But rather than simply admit them, Gupta has compounded her mistakes without the slightest admission of error or contrition. This is not the way a scientist is meant to behave. It echoes the tactics of the tiny minority of climate scientists sceptical about human-induced warming, who – rather than conceding to the overwhelming evidence – simply shift their arguments to new turf. 

For all this, it was not maverick scientists who introduced the notion of herd immunity into the debate about pandemic (as opposed to vaccination) strategies. That genie was let out of the bottle in early March by the scientists advising the UK government, including the CMO, GCSA and others on the Scientific Advisory Group for Emergencies (SAGE).

This has been one of the most murky aspects of the UK’s pandemic response, and needs to be given careful attention in the public enquiry to which everyone in the country is now entitled. Health Secretary Matt Hancock has denied that the initial strategy was ever about herd immunity, saying: “Herd immunity is not a part of it. That is a scientific concept, not a goal or a strategy.”

Setting aside the fact that this statement is incoherent (a scientific concept can furnish a goal), it seems very hard to reconcile with what the scientists said at the time. Sir Patrick Vallance, the Chief Scientific Officer, said on 13 March that the idea was to “allow enough of us who are going to get mild illness to become immune to this to help with the whole population response.” At the Government press briefing the previous day, he stated that “it’s not possible to stop everybody getting [the virus], and it’s also not desirable, because you need to have some immunity in the population.” 

(Left to right) Boris Johnson and Sir Patrick Vallance.

SAGE member Graham Medley, an infectious-disease modeler at the London School of Hygiene and Tropical Medicine, could not have been more clear about the role of herd immunity in the Government’s strategy when he spoke on BBC’s Newsnight in early March: “We are going to have to generate what we call herd immunity. So that’s a situation where the majority of the population are immune to the infection. And the only way of developing that, in the absence of a vaccine, is for the majority of the population to become infected…So what we are going to have to try and do ideally is…manage this acquisition of herd immunity and minimise the exposure of people who are vulnerable.” 

This makes it all the more puzzling that Jeremy Farrar, Director of the Wellcome Trust and a leading member of SAGE, told me in June that “I was never aware at SAGE of the discussion of herd immunity”.

The picture was muddied all the more by Boris Johnson’s notorious statement on This Morning, a breakfast TV show, on 5 March: “One of the theories is, that perhaps you could take it on the chin, take it all in one go and allow the disease, as it were, to move through the population, without taking as many draconian measures. I think we need to strike a balance.”

These discrepancies have never properly been explained. Yet I think they can be.

Johnson’s position is perhaps the easiest to understand: it seems to have been a classic example of his basic inability to state matters clearly and carefully. He begins with language that seems to imply there is a “theory” that you just let the virus rip, deploying his habitual Etonian stiff-upper-lip lexicon. But he does not seem to mean quite that, for he then speaks of fewer mitigating measures, not a total absence of them. Some feel he might have been alluding to the kind of laissez-faire approach that a few extremists in his party and in the right-wing media might have advocated – which can sound perilously close to a eugenic “let the weak die” approach. If the Times source of 22 March is to be believed, such ideas were by no means absent from policy circles: Dominic Cummings is reported to have said that the strategy at the end of February was “herd immunity, protect the economy, and if that means some pensioners die, too bad.” (No 10 strongly denies this was ever said.) In this case, maybe Johnson was wanting to portray the Government’s strategy as a measured middle ground between the libertarian and the draconian.

Whatever scenario was playing out in the Prime Minister’s imagination, the fact is that herd immunity was absolutely and undeniably a part of the course the Government had, on the advice of its chief scientists, chosen to take in early March. In short, this was what was being called at the time a “mitigation” approach: not trying to stop the virus spreading entirely (“it’s not possible and it’s also not desirable”) but slowing it down to “flatten the peak”, ensuring that the NHS was not overwhelmed. It was a fatalistic view: there is no way we can prevent many, perhaps most people from getting Covid, so we must ensure that this doesn’t all happen at once, and also try to shield the most vulnerable until there is enough immunity in the population that they will no longer be at risk. In other words, this is very close to the “focused-protection” herd-immunity strategy still advocated by the Great Barrington crowd.

It is arguably true that herd immunity is not exactly a goal of this approach, but an outcome. That, however, is mere semantics; the approach is predicated on achieving it (for the vulnerable could never be protected indefinitely otherwise). As Richard Horton, editor of the Lancet, told me, “I think herd immunity was central to our initial response” – because, he thinks, “we got locked in this cognitive bias of thinking this was an influenza-like epidemic, even though we knew in January it wasn’t.”

This UK strategy differed considerably from those adopted in many other countries at the time, especially in its avoidance of a strong lockdown that tries to suppress rather than mitigate transmission. So much so that some experts outside the UK could hardly believe what they were hearing: Bill Hanage, an epidemiologist at the T H Chan School of Public Health of Harvard Medical School, said that when he first heard about the strategy, he figured it must be satire. A study by Hanage and colleagues released on 14 March showed that the strategy would lead to a level of hospitalisation among 20- to 40-year-olds alone that would totally overwhelm the available NHS intensive-care-unit capacity.

Vallance explained in March that the problem with lockdowns is that they just postpone the problem. Without developing herd immunity, he said, “all the evidence suggests that when you [lock down] and then release it, it all comes back again” – possibly with greater force as the weather starts to get colder after the summer and transmission could thereby be boosted.

Chris Whitty (L) and Patrick Vallance leave 10 Downing Street (Photo by Alberto Pezzali/NurPhoto)

In retrospect, it is hard to understand the logic of this statement. No one ever imagined that a lockdown amounted to staying at home and hoping the virus would magically vanish. Faced with a new disease, about which little was known and no effective treatments were available, a lockdown buys vital time. It offers space to look for antivirals, and to begin at least to work on vaccines (Vallance did not expect to see those before spring of 2021, but in fairness neither did anyone else.) A lockdown buys time to prepare schools, workplaces and public transport for social distancing measures, better ventilation, and other protective interventions. Perhaps most crucially, it would create time to build up a comprehensive testing infrastructure: the most vital tool, as the WHO stressed, for combating the pandemic. For these reasons alone, the case for a lockdown was overwhelming.

What’s more, the notion of “protecting the vulnerable” was a fantasy. We knew already by then that older people were at greater risk of severe Covid and death – but it was clear too that the disease could be very nasty for many others too. Did such vulnerability have a genetic component, perhaps even correlated with ethnicity? How much did other pre-existing conditions contribute to risk? What about those who would not die but would carry the debilitating effects of their infection for many months, perhaps even for a lifetime? The simple fact is that, aside from the elderly (who were, in the event, given entirely the opposite of shielding in care homes), we had very little idea who would need to be shielded. And trying to enact strategy that walked the tightrope between unmitigated spread and suppression, in the absence of a large-scale testing system that could supply detailed information on the extent of infection, was ludicrous. Even if a strategy like this were to be adopted eventually, once the technical infrastructure was in place, a lockdown made sense to properly prepare for it.

Invoking the hope that herd immunity would develop among those at low risk of serious illness was in any event nothing more than that: hope, based on no evidence. As Farrar told me, “we didn’t know anything about immunity to this virus in March… we couldn’t be sure that, after infection, you were immune” – or for how long.

So this “mitigation” strategy, based on herd immunity, never made sense last March. That it was entertained for so many days at such a crucial stage is one of the most problematic aspects of the UK’s response. The Government only changed course when it became clear that “mitigation” would lead to massive mortality. Famously, this point was made in a paper by the infectious-disease modeler Neil Ferguson of Imperial College London and his colleagues, released on 16 March. They showed that the mitigation strategy being pursued at that stage was likely to result in about 250,000 deaths, and concluded that only a suppression strategy – a lockdown – could avoid it. They admitted that it wasn’t clear how to get beyond that in the longer term without vaccines, but that was never the point.

But in truth, Ferguson’s paper, however important for the policy change, was not rocket science. Given what was already known then about the likely mortality rate of Covid – around 0.3-1.0 per cent  on average – simple maths suggests that infecting 60 per cent of the population will kill around 200,000.  

It seems likely that there was a fatal match between the scientists’ hesitant and flawed search in March for a way to avoid draconian lockdown measures – not in itself an unreasonable impulse – and the Government’s (certainly the Prime Minister’s) fervent libertarian inclinations, that would have seized on any excuse for light-touch intervention. The UK’s pandemic response has been dogged all along by the inability of the Prime Minister to take swift and decisive action, meaning that decisions have been made too late – with disastrous consequences – not once, not twice, but no fewer than four times (in March, November, before Christmas, and at the start of January). This eagerness to persuade themselves of an alternative to lockdowns even led to an extraordinary audience that the Prime Minister and the Chancellor Rishi Sunak granted to the “herd immunity” advocates Gupta and Heneghan in late September, despite these scientists being by that stage clearly outside the mainstream of opinion. They were joined by Anders Tegnell, architect of the lockdown-avoidant Swedish pandemic strategy which has now proved so catastrophically mistaken that even Sweden’s king has condemned it. There could be no gathering of “experts” better suited to offer an excuse for Johnson’s inclination to inaction and Sunak’s opposition to lockdowns on economic grounds.

Given the harm that the notion of herd immunity has done and continues to do to as a putative way to deal with the coronavirus, the Government and its chief scientists should now come clean: herd immunity was an aspect of their initial pandemic strategy, whether that was explicitly stated in SAGE meetings or not. Such an admission would make it easier to explain why the idea should no longer be a part of the debate. It would be deluded to hope for the same integrity from lockdown sceptics. But, as is the case with climate-change sceptics, we should declare that this argument is now over and we must move on.

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