Gerry Killeen: Slowing the growth of epidemic is simply not enough
The approach taken to containment in Ireland thus far remains too tentative as many people will be infected without clinical symptoms, writes .
Since I returned to Ireland last Wednesday, I’ve been frustrated to hear the Covid-19 virus repeatedly described as “unstoppable” and authorities prematurely emphasise strategies to merely slow or mitigate the epidemic.
Without in any way detracting from essential efforts to urgently bolster our health care services, we must accept there are limits to what can be achieved by merely delaying and mitigating rather than suppressing the epidemic.
As summarised by the World Health Organisation (WHO) Collaborating Centre for Infectious Disease Modelling in the UK, “epidemic suppression is the only viable strategy at the current time.
Many countries have adopted such measures already, but even those countries at an earlier stage of their epidemic (such as the UK) will need to do so imminently.”
I am therefore hugely relieved to read that we’ve finally moved beyond stepwise ratcheting up of response efforts, and suggestions that the Irish people lack sufficient patience for the decisive interventions required.
Rather than de-emphasise containment so soon, we should pursue it with greater aggression and urgency, not to mention an inclusive global view of how epidemics are successfully contained on a regular basis elsewhere.
However, the approach taken to containment in Ireland remains too tentative and pays insufficient attention to emerging evidence that many people will be infected without obvious clinical symptoms.
Several reports over recent days confirm how cryptically infectious carriers can quietly spread Covid-19 through a community.
The lead author of the most authoritative recent study in China concludes: “By our most conservative estimate, at least 59% of the infected individuals were out and about, without being tested and potentially infecting others.”
This convincing evidence comes too late to help Italy, reflecting a missed opportunity to act faster based on earlier, if less conclusive, warning signs.
While asymptomatic carriage remained to be proven as a widespread phenomenon at the time, it was already flagged up over a month ago as the real joker in the deck of cards with which Covid-19 plays.
I was therefore disappointed this weekend to hear our chief medical officer describe transmission by asymptomatic carriers as merely theory.
I was even more disappointed when, immediately afterwards, I heard Michael Ryan at WHO dismiss an astute suggestion that the true number of cases globally may be 1m, but currently underestimated because of asymptomatic carriage and limiting testing capacity.
In practical terms, frequent asymptomatic carriage means that all contacts of known cases need to be followed up, regardless of how healthy they may appear.
While all potential contacts should ideally be tested, that may not always be possible.
However, self-isolation requires no new technology or laboratory capacity and can be implemented near-instantaneously.
High frequency of asymptomatic carriage also means that self-isolation, rather than merely self-quarantine, should apply to entire households, regardless of symptoms: Most onward transmission of Covid-19 occurs within families.
Cases may remain infectious for as long as three weeks, so longer isolation periods merit consideration.
Silent transmission by carriers who appear perfectly healthy also necessitates suspension of all non-essential travel and businesses, including construction and manufacturing.
The latest announcements, closing a wide range of non-essential retail outlets, services, events, and venues, as well as banning public gatherings larger than four people, are welcome and fully justified.
However, they don’t go far enough, in my view, and the tone of the advice provided remains too timid.
Apart from expanding isolation criteria and extending isolation periods, our Government should also close all popular public spaces and restrict takeaway and deliver services of food to those who really need them.
Covid-19 can be contained with robust political will and comprehensive public participation.
China has proven that, and we would be unwise to disregard their experiences, societal values, or successes.
And if some of the poorest countries in Africa can contain ebola epidemics, even in conflict-prone areas like the eastern Democratic Republic of Congo, surely we can learn from those societies and the traumatic experiences they’ve survived?
A universal truth that holds across all known human pathogens is that a hefty majority of transmission (80% or more) always comes from a small minority of people (20% or less) with unusually high levels of contact with others, often at popular venues or near breeding sites for mobile vectors like mosquitoes.
Consequently, the golden rule of pathogen elimination is that these exceptional individuals, and the circumstances in which they exchange pathogens with so many others, need to be targeted first and foremost.
While keeping shops open is a justifiable way to keep daily life functioning, allowing households to calmly stock up just in case, the way we have allowed restaurants, cafes, and pubs to remain open for so long flouts the so-called 80-20 rule of disease control.
Fortunately, Covid-19 viral infection is self-limiting and lasts less than a month before the immune system eliminates it from the body.
This kind of rapid and complete immunity is not only good news for the patient, it’s even better news for the public-health practitioner because it limits the period over which the virus can reproduce itself.
Consequently, what epidemiologists call the reproductive number of the virus, equivalent to the number of children a human family has per parent, looks to be manageably low.
Estimates for Covid-19 range from about one to seven new cases per initial case, comparing favourably with measles, for example (12 to 18).
Inclusive isolation measures and comprehensive suspension of gatherings, especially at shared venues, should therefore be sufficient to eliminate transmission, as exemplified by ongoing progress in China.
Every transmission event requires exposure behaviours by two people, so the good news is it responds in proportion to the square of the relative exposure reduction achieved by preventative interventions.
If everyone reduces their exposure behaviours by half, the reproductive number should drop from four to one and the epidemic should level out.
Push a little harder to get the reproductive number below one and the viral population will fade away sooner or later.
Sooner is better, so aggressively squeezing the reproductive number as close to zero as possible will accelerate dissipation of the epidemic.
However, merely slowing the growth of the epidemic is not enough. The bottom line is this outbreak still appears to be growing and has yet to peak.
It is far too early to celebrate the contraction of contact clusters or relax messages about staying away from friends and family.
Anyone who accepts that aiming to “flatten the curve” is adequate should take a close look at the horrifying curves predicted by the WHO Collaborating Centre for Infectious Disease Modelling in the UK, which contrast starkly from those projected behind our chief medical officer every evening.
The sooner we tackle this epidemic decisively, the more lives will be saved and sooner we’ll all be able to get back to normal.
While cases will keep rising for the next few weeks because of the lag times involved, that also means we’ll inevitably see the benefits of prevention measures if we persist.
Professor Gerry Killeen is incoming Axa research chair of applied pathogen ecology at the School of Biological, Earth, and Environmental Sciences, University College Cork.
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