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Covid-19

Here is a collage built of various recent emails about the Coronavirus pandemic. I have not fully tidied these up yet.

Covid-19

The true prevalence of this disease is, quite likely, at least 10 or more times higher than the reported sufferers. There is a silent cohort of infectors. Basically, “the genie is already out of the bottle”. We are shutting the stable door when most of the horses have already bolted.

The history of virus eradication is not a glorious success story. We seem to have done it with smallpox and polio – but that by immunisation and attention to infection vectors. I guess that, with draconian measures and a disregard for human rights, an infection such as Covid-19 might be eradicated if we caught it in an easily isolated population in the very early stages of its spread. But that needs clear identification of the infected and then contact tracing and their isolation for a suitable period. We don’t yet know if the occasional person continues to shed infective virus particles after the 14 days regarded as suitable at the moment.

Should the prevalence of Covid-19 be much higher that we have assumed (due to a large cohort of asymptomatic individuals), then this would help explain why the epidemic wave of infected people follows the same pattern regardless of our total lockdowns or our concentration on the at risk group alone. At this rate, herd immunity might kick in earlier than we have so far anticipated (basically, immunity in surrounding people reduces the number that a single person can infect – hopefully to below one). This herd immunity kicks in when about 60% of local interacting populations become immune (eg, children in school, socially gregarious networks of 20-35 year olds)

There is already much suggestive evidence – when we can study a defined population – that more people are infected than those who develop significant symptoms (cruise ships for example)..

Should the scale of the spread in the population already be high, then we should concentrate on blocking spread into the susceptible group (which is, by and large, well defined). This group need active cocooning to carry then through the peak infective period without needed to access critical care at this time. It is unlikely that we will alter the 3-4 year “saturation” of spread within the general population – we will at best just delay its spread (unless we can introduce immunisation).

The economic consequences of an attempted eradication may be catastrophic – the “cure” might prove much worse than the disease. Having said that, if we could stop it spreading beyond small isolated groups of 2-5 people, it should fade away in 3 weeks. But everything – just everything - would have to stop. That’s just too unlikely.

In all probability we will all meet this virus over the new 2-3 years and many of us will get infected. But, we don’t want to be accessing intensive care in the bulge of the epidemic.

There are a lot of unknowns about Covid-19. But, it's a virus and probably follows typical patterns. That probably means that reinfection is unlikely unless the virus "mutates" (that usually takes many months even with the notoriously re-emergent influenza). The point that plasma infusions from the recovered helps in infections suggests that there is reasonable immunity after infection. The obvious crisis with this virus is that it results in differentially serious consequences in the elderly and in those with compromised health. It would be interesting to know whether cocooning the susceptible during an unrestricted rapid spike then fall of cases would have been more "cost" effective than trying to slow down the entire infection. This virus seems to be spreading "silently" though large numbers of people whilst becoming most manifest in the more susceptible few. Pandemics peak whilst the virus is capable of infecting a majority of the population it meets (one person infecting more than one other victim – sometimes three or four per spreader and thus, potentially, exponential growth). As the population becomes more immune (more and more are post infective) the population growth is dampened (drops below 1 person infected per spreader). I suspect that this "experiment", to let it run riot in the less susceptible, might have worked where the alternative - the attempt to halt its spread - has had disastrous economic consequences. An interesting point is that the biggest spreaders are probably the mildest sufferers - do the children and young adults stop their socialising significantly unless "physically" restrained? But, this of course is a potentially dangerous experiment with an uncertain outcome and it’s an ethical minefield that the government is now moving away from. I don't think that it is necessarily wrong in its anticipated outcome. Cocooning might have proven very expensive but probably a lot cheaper than the scenario we now have. Now Sweden seem to be following this experiment and – lo and behold – the epidemic curve behaves very similarly there and they seem to be “getting away with it”.

Let me go back to the point that “at some stage, we are all likely to be exposed to, and a large number of us will get infected by, Covid-19.” So, it is all about managing which groups of people to optimise the ability of our healthcare systems to work most efficiently – let the virus rush through the young. That is why allowing it to rush through the low risk cohort has its attractions for me. Once the big bulge has gone, the virus will find it harder to trigger a pandemic and the cases will start to trickle rather than rush. The currently strategies are concentrating on “we can stop people getting this virus” rather than “can we manage when various groups get this virus?”

Here’s some more thoughts:

First a note about infectivity:

Base reproduction number – each infected person will pass a disease on to R0 other individuals.

Now TB has a potentially high R0 but that is spread over the duration of its persistence in a single sufferer, so we don’t see explosive growth – just progressive or even static growth in the infected population (indeed, the diseased population is fairly stable in numbers.)

Flu has an R0 of around 0.8 – 2.1.

Covid-19 has an R0 of 2 – 2.5 and it also appears to have a very large cohort of asymptomatic and unsuspecting spreaders – so its growth rate is potentially explosive.

That explosive growth is the real problem – it overwhelms the healthcare systems in the short term.

So, what this means – particularly with the suspected large cohort of asymptomatic spreaders – is that the growth of Covid-19 in the population is explosive. However, that also implies that it will reach saturation proportionately quickly. Should the illness provoke a protective immune response (the IS remembers the pattern of inflammation that occurs in Covid-19 and ramps it up quickly on a re-encounter) then we can expect to see not only an explosive outbreak but this will also be followed by a precipitous decline as herd immunity reduces the local base reproduction number (R0 ) of Covid-19 to well below 1.

Provided that we accept the premise that Covid-19 has a large cohort of asymptomatic-infected-spreaders, the mortality drops to around 1% or less. That is lower than the flu but the flu epidemic is spread over a somewhat longer time frame.

We tend to think of flu viruses and now Covid-17 as “causing” deaths, However, they probably precipitate imminent deaths rather than cause them. So Covid-19 is precipitating its deaths to occur slightly earlier in the aged and in those compromised by pre-existing conditions. Over the course of 2 years the statistics will show that those who died with coincident Covid-19 infection were likely to have been brought down anyway by other incipient factors (flu, injury, operations, accumulating co-morbidities and more). So Covid-19 is like “the final straw that breaks the camel’s back”.

So what is it about Covid-19 that has got us into a panic? It is its potent infectivity – that is, how it can race exponentially through a population. If this compression into a short time frame could be spread over a 2 year period, the excess deaths would come down to no more than 5% of the total number of deaths in the aging/compromised population and possible less. And, of course, we meet this same sort of pressure every year with the flu virus during the winters BUT, the Covid-19 pandemic is so concentrated over a short period of time that the healthcare systems are temporarily overwhelmed and bought to their knees. That explosive growth of an epidemic is the problem. But like any tsunami, there is both a surge and a draw back. Once the susceptible elderly and the susceptible people with compromised health have succumbed in a pandemic, the remainers are somewhat more resilient as a population and the next wave is delayed in compensation.

So, Covid-19 is probably less “lethal” than influenza. Jo’ average has little to fear from this virus if he/she is in good health. But the elderly and those with compromising health conditions need to be shielded from this virus during the crazy heights of the pandemic.

Is the current policy of economic crippling a cure that will prove much worse than the Covid-19 disease itself?