Note: This thread is related to #Coronavirus #COVID19

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Francois Balloux @BallouxFrancois Computational biologist, director of @UGI_at_UCL. Interested in most things genetics, in particular when they involve pathogens. Mar. 19, 2020 5 min read

I felt it may useful to summarise the various intervention strategies available to us to manage the Covid-19 pandemic. So far, there are three basic strategies that have been considered. (1/29)

the chin’; (2) SUPPRESSION-SOC, which aims to stem the epidemic through behavioural changes; and (3) SUPPRESSION-EPI, which aims to stem the epidemic through enhanced testing and contact tracing. (2/29)

MITIGATION, SUPPRESSION-SOC, and SUPPRESSION-EPI represent extremes of a wider continuum, and they could in principle be used in conjunction. Though there can be trade-offs. For example, it could be challenging to implement SUPPRESSION-EPI in a society already in lockdown. (3/29)

I do not consider the concept of ‘flattening the curve’ (FTC) that has been widely circulated on social media, as a strategy. Indeed, FTC is too loosely defined to be implementable in terms of epidemiological mitigation measures. (4/29)

‘Flattening the curve’ represents an objective (i.e. avoiding hospitals to be overrun) rather a means, and could in principle be based on any epidemiological measure. Though, I assume many of its proponent may favour behavioural (‘social distancing’) measures. (5/29)

The ‘flattening the curve’ idea has been influential in shaping policy and raising awareness to the severity of the Covid-19 pandemic, but it remains a ‘hopeful slogan’, and all the representations I’ve seen felt unrealistically optimistic. (6/29)

Before I discuss strategies in more detail, I’ll summarise the key features of the Covid-19, as I understand them. I wish to stress that there are many unknowns left at this stage, and that I could have gotten some things deeply wrong (which I would be delighted about). (7/29)

The average mortality rate of Covid-19 lies around ~1-2%, ≥10x higher than seasonal flu. Though, there isn’t an ‘intrinsic mortality rate'. The probability of death depends on the pathogen’s virulence, the host’s condition and the environment (i.e. healthcare) (8/29)

Healthcare is essential in keeping mortality down. In China, mortality was ~4x higher for patient treated in rural China than in the city of Wuhan. Mortality also increases drastically when hospitals get overrun, as happened in Northern Italy. (9/29)

Each infected patient transmits to ~2.5 people (R0~2.5). ~50% people infected develop no symptoms. ~50% transmissions from symptomatic patients, happen before the onset of symptoms. This means that at most 25% of transmissions are from symptomatic patients. (10/29)

The most plausible scenario for the long-term dynamic is for Covid-19 to become seasonal with future epidemics re-occurring each winter in the foreseeable future. This will strain healthcare in particular in years coinciding with a difficult seasonal flu season. (11/29)

Healthcare capacity is already stretched in most countries. For example, the UK currently has ~4000 beds in Intensive Care Units (ICU), which are occupied at ~80% (≥100% in winter). There are an additional 100,000 beds, a proportion of which could be repurposed as ICU. (12/29)

MITIGATION is a light-touch management of the first (ongoing) wave of Covid-19. The outbreak may still be allowed to grow in the immediate (R0~1-2.5) but more slowly. This is the strategy the UK government seemed to favour initially before changing course. (13/29)

The upsides of MITIGATION are lesser short-time disruption to the economy and the ability to keep society functional. Another objective is to build up ‘herd immunity’ in the population to lessen the impact of future Covid-19 epidemic waves. (14/29)

The major downside of MITIGATION is the heavy short-term death toll anticipated during the first wave, largely due to under-preparedness of healthcare to deal with an outbreak of this scale. (15/29)

SUPPRESSION-SOC aims to curtail the epidemic through behavioural changes (‘social distancing’) leading to each infected person transmitting to less than one other person (R0<1). It is the backbone of the strategy China and an increasing number of countries implemented (16/29)

The upside of SUPPRESSION-SOC is that it can be effective at reducing transmission at least locally and in the short term. Thereby, it greatly reduces the death toll in the short term. (17/29)

The downsides of SUPPRESSION-SOC include a massive strain to the global economy and extreme societal disruption. It is questionable whether the required lockdowns can be maintained for very long without irremediably damaging the very fabric of our societies. (18/29)

A major global economy crash may lead to far more deaths than Covid-19 pandemic could ever have claimed. The societal disruption may also challenge our ability to prepare for the next waves of the pandemic, which will likely hit us every winter in the future. (19/29)

Also, while there is no doubt a lockdown on society works at stemming the covid-19 epidemic, any success will only be short-term and local. It cannot be imposed to the whole world (e.g. Syria) and will have to be lifted eventually. (20/29)

As soon as countries will lift the lockdowns, the Covid-19 epidemic will restart, and gain steam in the coming winter. As such, SUPPRESSION-SOC is only beneficial long-term as long as it helps (rather than hurts) our readiness to face the next waves of the pandemic. (21/29)

The SUPPRESSION-EPI strategy is based on widespread testing and aggressive follow-up of people having been in recent contact with diagnosed cases (‘contact tracing’). This was the mainstay of the South Korean strategy to deal with their Covid-19 epidemic. (22/29)

The SUPPRESSION-EPI strategy is by far the best possible strategy. South Korea showed to the world that it can allow curtailing a pandemic with limited infringements to individual freedom and disruption to the economy and the fabric of society. (23/29)

Though, widespread testing combined with aggressive ‘contract tracing’ requires a level of preparedness and epidemiological expertise available in very few countries in the world at this stage. (24/29)

Widespread testing combined with aggressive ‘contract tracing’ also works best when implemented decisively in the early stages of an outbreak. This window of opportunity has arguably been missed by most countries. (25/29)

The countries that are still in a position to implement widespread testing combined with aggressive ‘contract tracing’ should decisively implement it now. This would highly beneficial to their populations, and could prove invaluable for humanity as a whole. (26/29)

Maintaining some parts of the world economically somewhat functional over the coming months feels to me the key short-term objective. This is our best hope for us to get ready for the winter, when the next wave of Covid-19 is likely to be upon us. (27/29)

This Covid-19 pandemic was in principle largely preventable. With a few exceptions, we failed, globally, at all levels. We will eventually prevail but the cost will be far higher than could have been if the world had approached the incoming threat less complacently (28/29)

Covid-19 won’t be the last pandemic that humanity will be facing. Once we have prevailed, we will have to reflect on the lessons learned, to ensure we won’t get caught napping again. Though, this is a discussion for another day. (29/29)


You can follow @BallouxFrancois.



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