Of Covid 19, Social distancing and herd immunity.

I’m practising social distancing, as prescribed in Scotland: out of the house only for food, exercise, and emergencies, which here translates as one of us walking the dog once each day, plus occasional food shopping, always maintaining the 2 metres distancing.

But what’s the aim? Clearly, slowing down the infection rate to a point where the health service (NHS) can have some hope of coping, and can have time to build up considerable extra capacity for those who require hospitalisation. But can we think just a little longer term?

While the measures taken here (and elsewhere in Europe, for example), are practical, there are many countries tries where these types of measures are simply impossible, or where the government of these countries do not wish to take such measures. At the same time, there are countries which are very organised, and manage to trace infected people effectively, reducing infection. But still, let’s take a longer view.

The UK 1957 flu epidemic had a much lower mortality rate, and while attempts were made to reduce the infection rate, it is reckoned about 9 million people in the UK had the disease (see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2714797/). Taking a 1% mortality rate (which is realistic only if the NHS can cope effectively), that would give about 90,000 deaths. More realistically, if more than 9 million people caught Covid19, nd the NHS couldn’t cope the figure would (I suspect) be more like 3 or 4%, taking the deaths to about 300,000 to 400,000, possibly quite a lot more depending on how many people actually caught the disease. That’s a lot: but in a country of 60 million people with mean death age of 80 (say), one would expect about (60,000,000/80 = 750,000) deaths per year, which puts the figure in some context. But the aim at the moment appears to be to stop the infection before it gets to this level.

How practical is this, in the long term? Let’s assume that there’s continuing outbreaks internationally. Let’s also assume that perhaps 2-3 million people in the UK get affected (with 20,000 deaths which seems to be one current estimate). Then there will be a large majority of the population with no immunity, at least until an effective vaccine is created. That would mean that travelling abroad would have new dangers, and people coming in from abroad would be likely to start off new outbreaks. If an effective vaccine existed, the former could be checked (rather like yellow fever is checked), but we might need quarantine for unvaccinated people arriving from abroad.

Of course, we may also find better treatments which might reduce mortality: I know there is research on this, but I have no idea of the timescale here. Viral diseases do mutate faster than bacterial ones, which makes (long-term) vaccination difficult – think of the annual influenza vaccination programme. We don’t really have medication agains influenza, as far as I know, except some anti-virals, some of which have side-effects – and anyway, influenza recovery rates are such that anti-virals seem inappropriate mostly. But that might not be the case for Covid19.

Conclusions? I don’t have any, but we do need to look forward. Are we aiming to cut the infection rate to where we can trace and isolate (like South Korea, and China), or are we aiming for slowly building uop herd immunity at a rate that the health service can cope? Or has this country not decided yet quite what it is aiming for?

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