COVID-19 Science World

A crash course in epidemiology

Written by Callum Murison

I think the vast majority of the world’s population is both worried and concerned, but now also at a stage where it is almost sick and tired of hearing nothing else but that of COVID-19 and its extraordinary impact on human life. But, importantly to remember, there is still light at the end of the tunnel – even if it is not currently in view. So how far down the tunnel are we, and how far do we still have to go?

Rather frustratingly, I must begin by stating that we simply cannot be certain. Epidemiology (the study of diseases) is not an exact science, by any means. The main problems are as follows:

  • no two diseases are identical, meaning evidence from and responses to past epidemics cannot necessarily be directly transferred to this one
  • no two countries are directly comparable with regards to the spread and impact of a disease

We’ve heard a lot in the news about the UK government’s approach to tackling the spread of the virus; awash with opinions about how it may or may not be worse than some of the more radical approaches initially taken by other countries. Over the past week (more or less since the UK lockdown was instigated), the growth rate of the number of cases has fluctuated – initially around 13% and now slightly higher, around 20%. But elsewhere, at a comparable stage, the growth rate was actually slightly higher – in Italy it was, on average, somewhere between 20 and 25%.

This is where the genius of logarithmic scales comes into play. Whilst a linear scale is helpful in showing how the total number of cases varies over time, a logarithmic scale is far more useful in showing how the rate of change – the growth rate – of the epidemic varies as time progresses (i.e. as the number of confirmed cases rises). That is, whether the number of new daily cases is staying constant, increasing or, hopefully, decreasing. However, an even more powerful comparison can be achieved by ignoring time completely.

This is because time itself is not a direct factor in the spread of the virus. Essentially, if we consider modelling the spread on a weekly basis rather than for each day (since daily data may fluctuate more) there are two factors influencing the spread: the number of given cases at the beginning of the week being considered, and the aforementioned growth rate. This growth rate is based on the probability of one person infecting another and the average number of people one would be exposed to in a single day.

Plotting a graph like this allows us to more easily see characteristics of the trend common to multiple countries:

Credit to Aatish B (https://aatishb.com/covidtrends/)

As evidenced, the vast majority of countries illustrated on here have all followed the same initial path followed by China: the same straight line, illustrative of exponential growth. A graph like this makes it easier to see where the number of new cases (i.e. the growth rate) has decreased – with China and South Korea most notably plummeting down off the exponential growth line. Italy is the country which is most notably seeming to begin to flatline – corresponding to the infection rate falling for multiple consecutive days – which now lies at around 7%. However, it is clear to see that other countries such as the US are still struggling to cope with reducing the spread of the virus.

Last but not least, we need to consider another really important fact – one which is quite concerning – what is the actual number of cases? Sadly, it is a certainty that there are far more cases than those which have been confirmed – since the virus produces only mild symptoms in many of the infected – and, as a result, the number of actual cases will be far higher. However, this would also have the benefit (if you could refer to it as one) of overestimating the death rate – since the cases that go undetected are likely to not be serious enough to result in deaths.

It is important to consider three things as we brace ourselves for the difficult weeks ahead. For one, whilst a spike in deaths is a terrible, terrible thing, it is important to look at the bigger picture too. For example, it could correspond to a decreasing infection rate, as in Italy. Secondly, in the small percentage of the population for which contracting the illness is extremely serious, there is always going to be a delay between infection and death. So a spike in deaths should not come as a surprise if, a week earlier, the highest infection rate yet was reported. Thirdly, whilst lockdown (and other severe) measures may not initially seem to have an instant impact, in the long term, it will save lives. This is why it is so important for each and every one of us to stay at home, keep aware of social distancing if we must go out, and protect those who are most vulnerable.

More information on how the coronavirus outbreak is affecting the operations of Spyglass Online can be found here.

About the author

Callum Murison

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