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Covid 19: Where’s the strategy for testing?

BMJ 2020; 369 doi: https://doi.org/10.1136/bmj.m2518 (Published 26 June 2020) Cite this as: BMJ 2020;369:m2518

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Rapid Response:

Re: Covid 19: Following "the science" !

Dear Editor

We read with interest Fiona Godlee's editorial on the UK's approach to testing in this pandemic.

By all accounts accurate, robust testing is now key to containing any localised outbreaks and for trying to determine disease burden and trajectory.

As the UK starts to ease lockdown and some ministers claiming that we are "winning the battle" we have been interested by the use of "R" quoted widely both in the media and press briefings. We are hearing talk of the ‘R’ and the implication is that we can gauge the ‘success’ of lockdown based on R0 (pronounced R naught).

We wanted to see if this is a "scientific" way present the facts, and we decided to read further into "R".

The Politicians and Media seem to imply that where R0 is less than 1, we are ‘winning the fight’ therefore allowing us to ‘ease measures’.
And that where R rises there is a generalised problem.

Triggered by a Tweet by James Gallagher of the BBC
https://twitter.com/JamesTGallagher/status/1271434955172851712?s=20
and a Twitter exchange with several Public Health Consultants (Greg Fell & Kevin Smith) we had an interesting discussion in our common room. It was clear that many of our team were confused by the true meaning of R0, and we wanted to share our learning.

R0 (pronounced R naught) is in fact, in simplest terms, a grade of how infectious a disease is.
R0 stands for Reproduction number.

It simply tells you how many people, previously free of infection, and not vaccinated (the naïve population) will contract an infectious disease if exposed to one person with that disease.

How is the R0 of a disease calculated?
Various factors will be taken into consideration, including:
● Infectious Period – the longer this is, the more people can catch the disease, and this will increase the R0.
● Contact Rate – obviously the better a disease is contained the less contacts will be exposed – and the lower the R0 will be. Hence careful isolation, barrier nursing, PPE, social distancing and quarantine of travellers will all reduce R0.
● Modes of Transmission – we know that SARS-Cov-2 spreads by droplet and surface contact.

The most infectious diseases such as Measles travel easily through the air and will persist in the environment for longer. Ebola – which has much higher Case Fatality Rate, is transmitted through bodily fluids, and hence has a lower R0.

As such the higher the R0 value, the more contagious a disease is. For example, if a disease has an R0 of 5, one person who has the disease will transmit it to an average of 5 other people – in a naïve population. This spread and transmission will continue unless steps are taken to control the spread – such as social distancing, isolation or vaccination.
There are many diseases that can persist with R 0 < 1, while diseases with R 0 > 1 can die out. So there are many factors at play.

COMPARING SOME TYPICAL R0 VALUES
Measles (18)
Whooping cough (18)
SARS-Cov-2 (3-5)
SARS (2-3)
1918 pandemic (H1N1) 1.4-2.8
Ebola (1.8)
H1N1 virus (swine flu 2019) 1.5

For example, the Swine Flu (H1N1) in 2009, had a lower R than Spanish Flu (H1N1) 1918 – presumably due to better hygiene, more advanced treatment options, and possibly some background immunity.

IF R0 IS NOT USEFUL, WHAT SHOULD WE BE MEASURING?
We suggest that the key data needed could be:
● How many new cases have been found ?
● How many patients have died ?
● How many patients have survived ?
● Where the cases occurred (nursing home, schools, hospital, geographical data) ?
● The case fatality rate

SUMMARY:

R0 stands for REPRODUCTIVE number.
R0 simply tells you how infectious a disease is.
R0 will change depending on the incidence of cases, how much immunity there is, how many people have been vaccinated, what protective measures have been taken (social distancing, handwashing, PPE), and treatment options.

R0 strictly speaking should not be used to comment on the ‘successes’ of lockdown, or the ‘safety’ of easing lockdown.

A better guide to deciding on our Coronavirus data would involve localised, accurate data, based on the number of new cases and new deaths.

This will depend on robust test, track and trace.

Dr Simon Hodes Simon is GP Partner, Trainer & Appraiser based in Bridgewater Surgeries, a large group GP Practice in Watford, with a c30,000 practice list size. All views expressed are his own @DrSimonHodes
Dr Kishan Thakrar Kishan is a GP ST, Watford VTS, Currently in GP Post at Bridgewater Surgeries Watford

With thanks to Dr Bharat Pankhania, Senior Clinical Lecturer at University of Exeter Medical School for his input and review. Bharat is an Expert in Communicable Disease Control & Infectious disease management.

References:
https://www.who.int/news-room/fact-sheets/detail/ebola-virus-disease
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3157160/
https://bmcmedicine.biomedcentral.com/articles/10.1186/1741-7015-7-30
https://science.sciencemag.org/content/324/5934/1557.long

Competing interests: No competing interests

26 June 2020
Simon Hodes
General Practitioner
Kishan Thakrar
None
Bridgewater Surgeries, Watford