Tuesday, 02 June 2020

RO and R may not be reliable but they are the best we have right now

THIS week we have reached a new stage in the covid-19 pandemic and, with government advice now switching from “stay at home” to “stay alert”, many of us will be trying to work out the new normal.

While we are not advised to go back to our normal lives quite yet, the message to ease complete lockdown will come as a relief to many.

Now is a critical time. however.

Among the new words and phrases added to our lexicon courtesy of covid-19 (“coronference call” and “covidiot” for example), the R0 number has returned to the fore. It would seem we are now placing huge importance on this hitherto fairly unknown statistical figure.

Most people will have read or heard about the R0 number.

R0 (or R “nought”), the basic reproduction number, is essentially a metric by which epidemiologists estimate the number of further cases that will be caught, on average, from each case of the disease.

In other words, how many additional cases arise per case. Above 1 and an outbreak will expand. Below 1 and infection rates should decrease.

Importantly, for the calculation of R0 the whole population is regarded as susceptible to infection.

The key to the use of R0 is its variability depending on certain factors and there will be an inevitable shift as people begin to come out of their houses a little more.

No one is quite sure what factors helped most during lockdown to reduce the number, whether it was the social distancing element, or the extra hygiene measures, or the closure of schools etc. What is generally agreed is that a combination of all of the above helped drive the R0 value below 1.

Importantly, people need to understand that the R0 value is an estimate. Broadly, it is based on three different parameters.

Firstly, it depends on the duration of contagiousness or transmissibility after a person becomes infected. This is not known for certain and there have been reports of some people testing positive for the virus for several weeks after their initial positive test.

Secondly, it depends on the likelihood of infection per contact with another person. The susceptibility aspect here is key and I will come onto that. Suffice to say, however. that we are not sure exactly what this likelihood of infection is. People who are asymptomatic but test positive may still be contagious (although the consensus is that they are probably less so).

There is also no strong evidence on levels of immunity once you have had the disease. Without testing data on this, we don’t have a firm grasp of how many people have had it and therefore don’t have too good an idea of the susceptible population. Lots of uncertainty then.

Returning briefly to the subject of susceptibility, the R0 number takes no account of immunity gained through infection or vaccination as it assumes that everyone is susceptible. To estimate transmissibility for an entire population where some immunity has been established, a different statistical value is needed. Rather confusingly, this is known as simply the R number (which is what a lot of people and publications are erroneously using interchangeably with R0). The R number, or effective reproduction number, takes account of changes in susceptibility in a population and is a good way to track vaccination programmes for example.

The third parameter affecting the R0 is the contact rate. Arguably, this is the factor we can most influence with our behaviour, although hygiene measures such as hand washing and avoiding touching our faces are important for likelihood of infection on contact.

Contact rates are likely to be increasing as of this week and this is why we must all act responsibly. Due to so much potential variation, it will be a difficult task to produce accurate estimates of the R0 value, a fact that is striking as it will seemingly be used as a basis for much of the strategy as we emerge from the lockdown.

We need to treat this cautiously as there is potential for huge variability. Measles has previously been identified as having feasible R0 values ranging from 3.7 to 203.3.

Factors for the spread of any disease include population density, social organisation and engagement with official advice, all of which vary from region to region and between different socio-economic groups so the R0 value will not be a one-size-fits-all for the whole country.

In many ways, and for the reasons I have explained above, it is not a good tool on which to base decisions, particularly if advice is to be broad and generalised over a large population.

Most commonly, the R0 value is an estimate reached retrospectively once all the correct data has been collated, validated and rationalised, something that unfortunately we don’t have the luxury of at the moment.

To place any worth on the R0 value therefore, we need better data collection in the short term. This brings me to both testing (vitally important) and the proposed NHS app that tracks infection and contacts.

No doubt you will have your own thoughts on this and will have noted the criticisms. One of the biggest of these is the potential for large proportions of the population to ignore it. However, at the moment, it is the best we have.

Critically, whatever the R0 value is, it must be interpreted and used correctly. Notwithstanding its reliability based on the data collected from around the country, in my opinion it will be most effective when managed at a local level by public health bodies that exist rather than via a standardised top down approach which ignores regional and socio-economic variations.

When it comes to the current advice therefore, it’s a bit hazy and raises lots of questions. But then this is an unprecedented situation. With such fluid and uncertain circumstances, it is difficult to be too precise. Whatever your view on the matter, that is the route that has been taken and, in order for it to work, we must all contribute to the effort.

Once again, common sense will remain vital here. The NHS app is there and if it is rolled out then please use it — likewise any other app if there is a switch. It is no excuse if you can’t work your phone, for example! Make an effort to figure it out. If not, the efforts of many may be wasted and (as far as I am aware) there is no alternative plan in place.

As always, if you are concerned you may have symptoms of covid-19, follow the NHS 111 website advice and remember, if you have any other health problems that you are really concerned about, contact either 111 or your GP. Good luck and keep up the social distancing.

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