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Current time:0:00Total duration:9:00

Video transcript

so I put up the definition of r-not and it's kind of a funny spelling in the pronunciation of Nou ght is just n ot not so what it means and the definition is here we're just going to go through what it means is the number of new cases that an existing case generates on average so the way I think about it is just in math terms if you have R naught you can just take literally the new number of cases divided by the existing number and that should give you the average right and then a couple of you know points here over the infectious period and of course some infections can be infectious for let's say a few days and some are going to last for years in fact some can last for decades where you can still spread it even 30 years after you first got it and then in a susceptible population what that refers to is kind of a not vaccinated population so I'm not vaccinated or you can think of it as a group that has never seen this infection before so maybe they are folks that had never naturally gotten this infection either but basically their immune systems are seeing it for the first time and are susceptible to making them sick so for me the the easiest way to kind of think through this is to kind of just come up with some examples and so imagine you have four infections I'm just going to arbitrarily name them ABCD right so four infections and each of these infections let's say we kind of go in to a community and we say how many of you guys have these infections and we find that each of them just to make the numbers easy they all start out with four people having each of these infections okay so we decide to go into the future we say all right we're going to follow you up some point in the future and it's going to depend on which infections so maybe this one will have to come back decades later because this one is infectious for decades so we have to give it an appropriate amount of time this one we can come back to some weeks later maybe some days later over here you can depend on each one right so it's just each one is dependent on how long it's infectious for so you come back at some time point in the future and for a you find out that this person gave it to somebody and this person gave it to somebody and these other two right here they actually didn't end up giving it to anybody this this infection so you'd say all right well what is the or not right so they are not is going to be new cases which is to over old cases which is for or existing cases so R naught equals 0.5 here and what about B well in this one let's say you get two people over here let's say this person gives it to somebody and this person also gives it to nobody so you again you just count up the numbers you say well okay here we have four divided by four equals one so again it's an average so even though some people give it to more and some people gave it to less the average is going to be one and for C you know similar kind of thing I can say well let's say you have a few people getting it you know maybe two over there maybe one over here and this person gives it to let's say three people right so in total I've got four five six seven eight actually let me make these numbers even nicer so let's just say we've got eight people total so R naught equals 8 divided by four we started with four and in this case R naught equals two and D I'm just going to make this one almost ridiculous sounding but let's say we have tons of people that get sick so this one for whatever reason a lot of people when we come back you know a few weeks later we see that just many many many people have gotten sick and of course on average we're going to take the total number of people that we see here and it's divided by the number four so I'm just going to quickly scroll this out let's see what we get here I'm going to get up to trying to get to 72 let's see if I just I don't want to overshoot that number so we've got 30 we've got 40 50 60 and then I've got to do Stute 72 so this one whoops this 1/5 what do you get you get 72 sorry I divided by four rather 72 divided by four is going to give us 18 right so this one has an R nought of 18 and I wanted to visually show that to you just so you can see kind of what that would look like and so so far you know this is all making sense and we can say okay well you know we don't have to worry about these these first folks anymore because we already followed them for as long as we need - right so they're not going to spread it anymore because we we made sure that we follow them for weeks or days or decades whatever you know whatever the infection requires but now we're interested in these blue folks right so we want to know well how how are they going to spread it let's start with this one right here let's say this person spreads it to a single person this person spreads it to one let's say one one and two and this person sends it to nobody let's actually just quickly make sure I do the math here so this is a future time point even further in the future I guess and now our R nought is again you have to think about new versus existing right so existing we had two blue ones so we had two down here and we have one new person here so our R nought value is going to stay fixed that's kind of the idea I want to want to express the R naught is not going to change it's kind of a measure that we we think is unique to a particular infection so 4 over 4 remains 1 R naught over here I just said is not going to change so we can we can already kind of assume what's going to happen it's going to be okay well on average it's going to be 2 so maybe some less some more but overall it's going to be that number right so we're going to get something like this and it also kind of easily tells you how quickly it's going to spread it's going to double every time you come back so every few days if that's if that's what this is every set number of days it's going to double so you're going to say okay instead of we started with 8 and now we have 16 and over here with this one over here is just going to be an enormous number right it's going to be 72 times 18 which equals I actually did the the math here is 1296 cases that are new and we started with 72 so that's R naught of 18 so this is I'm not not going to draw all that in but just a lot right a lot of people so what you can already kind of see happening here is that when you have an R naught less than 1 which is kind of the case here you basically see that this is going to start dying out so this this infection starts to die out and it's it's almost gone already and we started with 4 we only have really one person left with this in action over here when R naught equals 1 exactly 1 it's going to be stable so kind of moving over time you see that the same number of people in this case 4 are going to have the infection and over here when R naught is greater than 1 and depends on you know how much greater it's going to spread the infection is spreading and this is why this number is so important it's it's helpful and to add a little bit of context here Ebola for example has gotten R naught of about 2 so that was a the example I had in mind when I wrote that down and infection D this is a little bit like measles so measles you can see is much much more inclined to spread than Ebola for example all right now so before I leave you feeling completely hopeless about this situation because this this looks pretty scary the way that these infections are spreading let me point out something so I don't want to seem like you can't actually intervene and in fact there's a few things we can do to reduce the spread of these infections that have these high are not numbers one of them would be to block transmission so you can make sure that if you have a sick patient they are isolated if you have a healthy person has to be around them for example let's say a nurse or doctor or something you can make sure that they're wearing protective clothing so you could isolate the patient you can wear protective clothing all these kinds of things help to block transmission right and in addition you can also think okay well what if what if a person does actually get exposed to an infection for example let's say measles this is actually a great example and an Ebola doesn't fit this second category unfortunately not yet today being October 12th but we have vaccines against measles right so you can actually vaccinate people and in the case of measles it would be mmm are so measles is MMR vaccine and if you actually use this vaccine and it has a certain effectiveness that effectiveness prevents a lot of folks that have had the vaccine from getting sick and so these are a couple ways to reduce the spread of these very very infectious agents