Dr Gerry Killeen: Just like measles, elimination of Covid is still feasible in Ireland

Elimination doesn't mean global reduction to zero, it means elimination from an area, where there is no locally sustained human-to-human transmission, writes Dr Gerry Killeen

If we want to stop the emergence of new variants, we need to look after our most vulnerable, and the only way to do that over the long term is to eliminate transmission one country, region, and continent at a time.

Britain's chief medical officer, Professor Chris Whitty, justified his country’s plan to comprehensively unravel restrictions intended to suppress Covid transmission on July 19 by saying: “At a certain point, you move to the situation where instead of actually averting hospitalisations and deaths, you move over to just delaying them. So you’re not actually changing the number of people who will go to hospital or die, you may change when they happen.”

Prof. Whitty has been consistent in his view that elimination of Covid is “not realistic”, insisting that we must accept and learn to live with Covid as an endemic pathogen. More strident advocates for herd immunity in the UK have gone further, making dangerously misleading comparisons with influenza, while also disregarding the impact of long Covid on at least a million people in the UK.

However, these perspectives are founded on misconceptions and even misrepresentations of what these two standard epidemiological terms actually mean, along with several others like virulence, epidemic, and eradication.

In a recent Independent Scientific Advocacy Group (ISAG) webinar, entitled 'Choosing our future relationship with Covid-19: Elimination or sustained endemic transmission?', my colleagues and I explained what these key concepts really mean in practical terms, using established knowledge of Covid and a range of other existing diseases to illustrate their implications. 

To begin with, most non-specialists are unaware of the critical distinction between elimination and eradication. In formal technical parlance, the word 'eradication' is a strictly global term, meaning that human-to-human transmission of a pathogen no longer exists anywhere on the planet and looks unlikely to return for the foreseeable future. 

Although this has been achieved for smallpox, SARS-1, and rinderpest, a virus that used to decimate cattle populations all over the world, this remains a rare achievement. Even the long-standing programme to end polio has struggled to reach that end-point in conflicted countries of central Africa and central Asia.

However, elimination means something very different and has been so routinely achieved for so many pathogens, across so much of the world, that many of us have never experienced these illnesses and take their absence for granted. Elimination doesn't mean global reduction to zero, it means elimination from an area, where there is no locally sustained human-to-human transmission.

UK chief medical officer Professor Chris Whitty thinks the elimination of Covid is “not realistic”

Polio is an excellent example with which to illustrate the difference between the two terms, and how a pathogen can remain endemic on a global level while also eliminated across most of the planet. Although sustained human-to-human transmission of polio persists in parts of Nigeria, Afghanistan, and Pakistan, and therefore remains endemic in those countries, it has been forced out of existence and largely forgotten across the rest of the world.

While a local epidemic involves a surge of pathogen through a population with inadequate immunity to stifle it, and a global pandemic just extends that process around the world, an endemic pathogen is one that has indefinitely settled into a human population, with transmission patterns that may range from slow and steady like malaria to intermittent and sporadic like dengue fever. 

The differing biology of pathogens, notably their virulence and genetic diversity, can make them easier or harder to control. Drawing on comparisons between rinderpest (eradicated) and foot-and-mouth (eliminated from much of the world), I’m firmly of the view that Covid remains in the part of the biological spectrum where elimination is still feasible.

Measles

Unbeknown to most of us, Ireland has eliminated several pathogens that we used to live and die with, perhaps the most relevant of which is measles because it is similarly transmissible and also more virulent than most people appreciate. Ireland has WHO-certified measles elimination status. It doesn't mean that there are no measles outbreaks but they're all traceable to introductions.

Prior experience of adopting a “vaccines plus” measles elimination policy over the last decade has proved remarkably successful: After living with sustained measles transmission for several decades, despite vaccine coverage levels of about 90% from the 1980s onwards, it was only after we decided we could no longer tolerate endemic measles transmission that it was halted.

In 2010 Europe decided to eliminate measles. It took another couple of years to get the machinery up through the gears and then it disappeared pretty fast. We interrupted endemic measles transmission in 2015 and were then certified as one of the 35 European countries that had eliminated it by 2018. 

A key point to bear in mind is that there are still dozens of measles cases in Ireland every year but that doesn’t make it an endemic disease: These outbreaks are all traced to inbound travellers from overseas by our public health teams, who routinely track down every last case to prevent measles from re-establishing itself.

Our future relationship with Covid could and should be the same: Once we are all vaccinated, even if we fall short of herd immunity, we should only require a modest nudge with complementary measures like ventilation, masks, and outbreak investigation to interrupt and then eliminate persisting residual transmission.

But why wait? And what are the risks of waiting, persisting with our current mitigation strategy to see how that works out?

The case for an elimination strategy

First of all, an aggressive “vaccines plus” elimination strategy that maintains non-pharmaceutical interventions just a few months longer, will achieve synergistic impacts to bring our national epidemic to a much faster conclusion. Israel is currently reaching the end stages of such an offensive strategy, with outbreaks among unvaccinated school children now the last bastion of locally sustained transmission. 

Even if successful, the Irish Government’s defensive mitigation strategy will take much longer, allowing considerable levels of sustained transmission to threaten our schools and universities when they reopen this autumn.

Second, for those who remain vulnerable despite being vaccinated, it will be similar to the experiences of children growing up today in parts of Africa with greatly reduced but nevertheless intransigent levels of malaria transmission: Sooner or later, they all get it and need urgent care. 

At incidence rates similar to those we are experiencing now, despite considerable ongoing suppression efforts, Covid infection would become a “when, not if” issue for such vulnerable individuals who know they are likely to be part of the 5% that vaccines cannot adequately protect. That’s exactly what Prof. Whitty spelled out to the indefinitely vulnerable citizens of the UK yesterday.

Beyond the unacceptable situation that endemic Covid would place our vulnerable friends, neighbours, and family members in, we should also be aware of the potentially catastrophic risks associated with the extended chronic infections they often experience. While most of us will clear Covid from our system within two to three weeks, chronic infections can persist for months in individuals with weakened immune systems.  

Multiple variants rapidly proliferate that then jostle for advantage over each other (and the misfortunate patient), resulting in what are known as the accelerated evolution events that give rise to variants of concern like alpha, beta, gamma, and delta. 

If we want to stop the emergence of new variants, we need to look after our most vulnerable, and the only way to do that over the long term is to eliminate transmission one country, region, and continent at a time.

• Dr Gerry Killeen is a founding member of the Independent Scientific Advocacy Group (ISAG) and the AXA Research chair in applied pathogen ecology at the School of Biological, Earth, and Environmental Sciences at University College Cork