R0 (pronounced R-nought) or
Basic Reproduction Number
#15,653
In the 10 days since the `more transmissible' UK COVID variant was first announced (see UK Health Secretary Announces A New COVID Variant), there has been a scramble to quantify exactly what `more transmissible' really means.
The yardstick by which disease transmissibility is measured is by its R0 (pronounced R-nought) or Basic Reproductive Number. Essentially, the number of new cases in a susceptible population likely to arise from a single infection.We've seen estimates that this variant may be 50%, even 70% more transmissible than the previous strain, and that it could increase the R0 (basic reproduction number by .4%).
In the simplest of terms, when its R0 drops below 1.0, a virus (as an outbreak) begins to sputter and dies out. Above 1.0, and an outbreak can have `legs’.
Calculating the R0 is notoriously difficult, however since much hinges upon the existence and subtle differences between viral strains, the accuracy of surveillance and reporting, `seasonality’,individual host responses to the virus (i.e. number of `super spreaders’), and how diligent people are in following social distancing, mask wearing, and hand hygiene recommendations.
In an attempt to better quantify the increased transmissibility of the new UK variant, the Centre for Mathematical Modeling of Infectious Diseases (CMMID) at the London School of Hygiene and Tropical Medicine (LSHTM), has produced a 35-page mathematical analysis of the new strain.
With the caveat made famous by the late George E. P. Box (18 October 1919 – 28 March 2013) - Professor Emeritus of Statistics at the University of Wisconsin - that . . .While they cite no evidence of increased severity among patients contracting this new variant, they do find it to be considerably more transmissible, which means the pandemic will become harder to control as number of cases, and serious illnesses continue to rise.
“All models are wrong, but some models are useful.”
Estimated transmissibility and severity of novel SARS-CoV-2 Variant of Concern 202012/01 in England
Status: report | First online: 23-12-2020 | Last update: 23-12-2020
Authors: Nicholas Davies*, Rosanna C Barnard1, Christopher I Jarvis1, Adam J Kucharski1, James D Munday1, Carl A.B. Pearson1, Timothy W Russell1, Damien C Tully1, Sam Abbott, Amy Gimma, William Waites, Kerry LM Wong, Kevin van Zandvoort, CMMID COVID-19 working group, Rosalind M Eggo, Sebastian Funk, Mark Jit, Katherine E Atkins & W John Edmunds.* corresponding author1 contributed equally
This study has not yet been peer reviewed.
A novel SARS-CoV-2 variant, VOC 202012/01, emerged in southeast England in November 2020 and appears to be rapidly spreading towards fixation. We fitted a two-strain mathematical model of SARS-CoV-2 transmission to observed COVID-19 hospital admissions, hospital and ICU bed occupancy, and deaths; SARS-CoV-2 PCR prevalence and seroprevalence; and the relative frequency of VOC 202012/01 in the three most heavily affected NHS England regions (South East, East of England, and London).
We estimate that VOC 202012/01 is 56% more transmissible (95% credible interval across three regions 50-74%) than preexisting variants of SARS-CoV-2. We were unable to find clear evidence that VOC 202012/01 results in greater or lesser severity of disease than preexisting variants.Nevertheless, the increase in transmissibility is likely to lead to a large increase in incidence, with COVID-19 hospitalisations and deaths projected to reach higher levels in 2021 than were observed in 2020, even if regional tiered restrictions implemented before 19 December are maintained.
Our estimates suggest that control measures of a similar stringency to the national lockdown implemented in England in November 2020 are unlikely to reduce the effective reproduction number Rt to less than 1, unless primary schools, secondary schools, and universities are also closed. We project that large resurgences of the virus are likely to occur following easing of control measures. It may be necessary to greatly accelerate vaccine roll-out to have an appreciable impact in suppressing the resulting disease burden.
(SNIP)DiscussionCombining multiple behavioural and epidemiological data sources with mathematical models, we estimated that the novel SARS-CoV-2 variant VOC 202012/01 is more transmissible than existing circulating SARS-CoV-2 viruses. As a result of this increased transmissibility, existing control measures are likely to be less effective, and countries may require stronger proactive interventions to achieve the same level of control. We found no evidence that the new variant is associated with higher disease severity, but without strengthened controls, there is a clear risk that future epidemic waves may be larger – and hence associated with greater burden – than previous waves.
