# COVID-19: Basic Reproduction Number

by Sean Elliott, MD

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00:07 COVID-19 statistics.

00:10 In this session, we're going to talk about the common numbers that you will have seen to describe and predict the behavior of the COVID-19 pandemic caused by the SARS-Coronavirus II.

00:19 The most familiar number probably is the basic reproduction number, known as the “R naught.” This is the average number of secondary infections produced by an index case or a typical first case of an infection in a population, where everyone is susceptible.

00:36 It is used to measure the transmission potential of the disease, in this case of COVID-19.

00:42 If R naught, is greater than 1, then the number of infected people will likely increase exponentially and an epidemic or pandemic can ensue.

00:52 If the R naught is less than one, then the number of those secondary cases from the index case, will not be sufficient to sustain an outbreak and it will likely recede on its own.

01:03 The problem of course as with any of these indices, is that the R naught, is not sufficiently valid, to definitively forecast an outbreak.

01:13 It certainly can be affected by other factors, but it is very important as an early warning system, sort of, giving the indication of the possibility, of an epidemic or a pandemic.

01:25 The R naught then can be used even to compare and contrast, different types of epidemics or pandemics, even such as we see here, comparing the current COVID-19 pandemic, with an annual influenza epidemic.

01:40 So, on the left COVID-19 pandemic, R naught values have been from 2 to 2.5, certainly, in the early days of the pandemic, and up to 10 with the Omicron variant, compared to a typical year with an influenza, where the on R naught has ranged anywhere from 0.9 up to 2.1.

01:59 Those years where the R naught was 0.9 for influenza, were largely fizzles, that there were not significant numbers of secondary cases and the epidemic was not extreme.

02:10 Versus those years of influenza where the R naught was 2.1, where there was a rip-roaring epidemic with tons of secondary cases.

02:18 So, the R naught, can vary widely and in fact, those ranges you see both for COVID-19 and for influenza, the R naught varies widely because there are other host and viral factors, which can affect it.

02:31 Certainly, if you think about it are not can be affected by interventions, right, so, as we have seen with COVID-19 and as we hope to see every year with influenza intervening with public health interventions such as, wearing masks, social distancing, physical distancing, closing the schools, avoiding social gatherings, all those can have a major impact, on the exposures for the virus, both for SARS-Coronavirus II and for influenza and therefore on the numbers of secondary cases.

03:05 After those public health measures are initiated, then we can look and see what happens and that number is called, the “Effective reproductive number.” As you would hope this is typically much lower than the R naught, meaning that we've had an effective cluster of interventions.

03:24 So, the effective reproductive number, if it's greater than 1, means that we were not very good at our interventions and that the outbreak is likely going to overwhelm further healthcare resources, to overwhelm the healthcare system.

03:42 In if for example you can see this playing out in lifetime in February of 2021, the R naught for COVID-19 in the United Kingdom was 0.6 to 0.9, so less than 1, meaning that there was going to be a successful or predicted successful diminishing of the outbreak numbers in the United Kingdom and that in fact was what was seen after a very strict lockdown measures were implemented.

04:09 So again, diminishing spread with the R naught.

04:13 Unfortunately, things change and variance and in this case, we're talking about virus factors now.

04:20 Variants of the SARS-Coronavirus II, have shown increasing, differing levels of infectivity and are of major concern.

04:29 So, despite the United Kingdom having an initial response, it's both R naught and its effective reproductive numbers below 1, unfortunately, the delta variant the B.1.617.2, first reported in India in late 2020, entered into the United Kingdom and changed its epidemic picture completely.

04:53 The R naught and the effective reproductive number both rose, to be above 1.

04:58 The later Omicron variants presented with markedly higher R naughts, so they spread very rapidly, even among vaccinated people.

05:05 Omicron mostly escaped vaccine immunity because the first vaccines were made using different strains of virus.

05:12 Fortunately, Omicron causes less severe disease in most people when compared to the earlier major variants.

05:19 This is played out in other parts of the world as well.

05:23 So, here's where viral factors can negatively affect, the R naught and the effective reproductive number and how does that happen? Well, it likely has to do with variants or mutations, which occur in the SARS-Coronavirus II itself.

05:42 This virus itself has shown itself quite able to undergo mutations, especially in the spike protein, which will increase its infectivity or its transmissibility.

05:53 Of course, it's very difficult to know exactly what effect a single mutation or a cluster of mutations might have, in the transmissibility of the virus.

06:03 But, by using the R naught and the effective reproductive number, then we can at least follow the small changes and predict, that there's going to be a change in the behavior of the pandemic.

The lecture COVID-19: Basic Reproduction Number by Sean Elliott, MD is from the course Coronavirus.

### Included Quiz Questions

1. The average number of secondary infections produced by one infectious individual in a population where everyone is susceptible
2. The number of cases generated in the current state of a population
3. The average number of individuals who are likely to get infected
4. The doubling time of the number of active cases
5. The average number of secondary infections produced by one infectious individual in a population where some of the population is already immune
1. It is intended to be an indicator of the contagiousness or transmissibility of infectious agents.
2. It is not dependent on the host or viral factors.
3. It is a constant during an outbreak of an infectious disease.
4. The basic reproductive number R0 will increase as more cases are diagnosed.
5. It is intended to predict how long an outbreak will last.
1. < 1
2. 1
3. > 1
4. < 0.5
5. > 2

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