I don't understand about the R number

[Noworrieshere]

I know it's the number of people one person can infect, so is it just lower because that one person comes into contact with less people? The virus itself doesn't become less infectious?

So how else does it become lower apart from people staying apart from each other? More people become immune?

Is that basically it? Physical distance or immunity?

Sometimes things get talked about so much and I just go "oh yea, the R number" when I'm not really sure I understand how it works.

Now I'm in Scotland and our R number is supposedly higher than everyone else's, I can't wrap my brain around that at all.

I have found this link helpful noworrieshere.

www.nytimes.com/2020/04/23/world/europe/coronavirus-R0-explainer.html

I think it's meant to be higher in Scotland because the cases are concentrated in some care homes and around Glasgow. it also arrived in Scotland later than London for example.

Yes the rate at which one person infects other people is reduced by

• some people already having immunity (or already currently ill)
• the infected person seeing fewer people
• the infected person and everybody else washing their hands a lot, not touching faces, keeping physical distance etc.

It is the risk of infecting others.

If we all reduce who we mix with the rate will reduce.

If we spread it by mixing with people it will rise.

There is no evidence that you become immune to it.
There is no evidence you can't get it twice.
We just don't know.

Kate Winslet’s character actually explains it really well in Contagion . Viruses have their own inherent R number, but you can also manipulate the “in practise” number down by public safety measures;

m.youtube.com/watch?v=VrATMF_FB9M

The R number is partly about the virus itself and partly about environment and behaviour. Anything which reduces the number of virus particles picked up by vulnerable people will reduce R. This includes:-

• physical distance
• avoiding confined spaces
• limiting the time you are near other people
• limiting the number of people you interact with
• handwashing and surface cleaning
• more people being immune
• facecoverings and PPE
• not recirculating air
• not singing or shouting. Or even speaking, actually.

And how do we calculate the R number?

By the number of diagnosed patients?

So the more we test, the more we find? And we don't know how long you stay positive for so no way to know who's new?

Yes - so they think the R0 (the rate at which an infected personally naturally infects others) is about 3. The tighter we lock down the lower we get it.

If 100 people get it, with its natural R0, and they infect 3 people, who each infect 3 people, who each infect 3 people then 3 more then within a fairly short period you have 100 x 3 X 3 X 3 X 3 = 8100 people sick.

If we were able to keep it to 0.5 (an average of less than one infection per infected person) then it would be 100 x 0.5 x 0.5 x 0.5 x 0.... Which is around 6 people! And that number is small enough to operate a meaningful 'track and trace' approach where as soon as the cases were detected they could alert people who'd been in close contact to go into lockdown, stay away from the vulnerable and potentially shut down a workplace for a couple of weeks etc.

The virus doesn’t change. Our behaviour means it is less able to transmit and infect. Also, the more people that are immune (either from having had it and acquiring antibodies or having a vaccine when and if one becomes available) the less able the virus is able to infect others. However, current estimates are the only 4-5% have had it.

And how do we calculate the R number?

Good question... I think the number of hospitalisations would be a reasonable, though imperfect, way of measuring it.

So forgive me if I’m being thick, it’s been a long day, but does that mean as soon as people stop distancing the R number goes back up? If so, how do we ever un-social distance again?

There is no evidence that you become immune to it. There is no evidence you can't get it twice.

Whereas that’s true, it’s largely because there’s not enough evidence to conclusively prove this. It would be virtually unknown for antibodies created after infection not to provide a reasonable level of immunity. Given that Covid-19 is apparently more stable than influenza, it is less likely to mutate into something sufficiently different in a short period to circumvent existing antibodies as flu does.

That’s correct. Businesses need to start planning for how they can operate with social distancing in place. Until a vaccine is found.

does that mean as soon as people stop distancing the R number goes back up?

Yes, this is the crux of the problem. I think this is why they’ve created the alert levels, if it creeps up above 1 measures will have to be tightened, once it’s below 1 again measures can be lifted slightly. That, combined with social distancing measures in day to day life, should slow the spread until there’s a reliable treatment or a vaccine.

It's all very complicated. I didn't even like science at school and now my whole life is controlled by it.
Thanks for your explanations, they help.

But if there is no vaccine, as there isn’t for SARS etc, is this it? Or does the R number mean that when it gets low enough only a few unlucky people get affected?

Well it's possible the R is higher than they estimate and the number of asymptomatic cases is also significantly higher and thus a lot more people have been exposed. So as the vulnerable keep lying low and the same already infected people only are in circulation the virus has less and less places to go and gradually fades......

There are several important things to know about R.

1. If it is below 1 that means that the number of infections is falling.
2. Anything above 1 and the growth is exponential
3. The number of current infections (c) x R = the number of new infections (n). So the number of current infections is as important as R. Currently, this is far too high.
4. The more testing there is, the more accurate the numbers in the equation.
5. It appears that both R and c are falling because the numbers in intensive care are falling, but they really have no accurate numbers.
6. The natural R for coronavirus is very high, so lockdown is needed to reduce it. (Ebola is much lower, so it didn't spread as easily, but obviously much more deadly)

@Clemmieandareallybigbunfight

The higher the number of asymptomatic people the better, and there are no reliable estimates on that yet I believe. However, unfortunately the sheer number of deaths in Bergamo and NYC mean the earlier hope that a substantial proportion are asymptomatic is unlikely.

Or does the R number mean that when it gets low enough only a few unlucky people get affected?

Not exactly... It depends a lot on how many have it as well as the R.

For instance, if 1,000,000 were infected, even with a R of 0.2, 200,000 more people would get it.

If 20,000 were infected, and the R was 3, “just” 60,000 more people would get it.

The problem with the latter is that the 60,000 quickly becomes 180,000 then 540,000 etc. if nothing is done.

By my calculations, based on what we know about R, had we waited just another week to social distance and lockdown, 4 to 6 times more people would have been infected, and the death toll would have been c. 150,000.