SPEAKER 1: COVID-19 is a new disease. And so stopping transmission really is the best way we have to mitigate the impact of this pandemic. Let's review again what happens when someone is infected. They develop signs and symptoms of disease usually about five days after they're infected, but up to 14 days after their infection. And then they themselves are able to infect others two days before they get sick, particularly on the day that they get sick. And they're still able to infect people as long as they have signs and symptoms, which is at least ten days. So let's imagine that they have contact with someone else on the day that they're sick, really as they begin to feel ill. That person, let's assume, will have a five day incubation period. So after they're infected, they will develop signs and symptoms five days later. Here you can see that after they develop signs and symptoms, they'll have disease for tens days. We can also see in the yellow highlighted area here, when they are infectious. So you can see that they are infectious two days before they become ill, which is really only three days after they were infected. So if we want to interrupt transmission, if we want to stop transmission of COVID-19. We have to find people who have been infected. We have to limit the number of people that they have contact with, so that the transmission chain won't keep going. But you can see in this schematic why that's very difficult to do. Because very quickly after their infection, they will themselves become infectious and able to transmit to other people. So that means that, as you can see here, the window of opportunity for us to act and find those people who've been exposed, find the people who are infected but aren't sick yet, and ask them to change their behavior, to limit their contact with other people, to stop the transmission, that window of opportunity is very short. It happens very quickly. So our public health systems have to respond very quickly when we find cases of COVID-19 and try to find everybody who's been exposed to those cases, so that we can take some action and stop transmission. Next I want to introduce you to another concept called the reproductive number. You may also hear it referred to as R naught. The reproductive number is the number of people that one infectious person will go on to infect if everyone they have contact with is susceptible to the disease. It's a good way to think about how fast a disease can spread in the population. And the higher the reproductive number, the more people who will be infected over the course of any outbreak. You can see in this figure some important numbers for R naught and how the number of infected people would change over time, depending on what the reproductive number is. So in the blue line, you can see the number of infected people looks like if the reproductive number is 1. If the reproductive number is 1, this means that every infected person infects just 1 other person. So over time, the number of infected people will not change. It will stay constant. Now what happens if the reproductive number is less than 1? That's what's shown in the purple line here. So over time, if each person infects less than 1 person-- so sometimes, an infected person will infect someone else. But sometimes, infected people won't infect anyone at all. And on average, it's less than 1 person. Over time, the total number of infected people will come down. What happens if the reproductive number is 2? That means each infected person will infect 2 other people. That line is shown in red here. And we can see that if the reproductive number is 2, the number of infected people will increase very quickly over time. Although typically, we think about the difference between 1 and 2 as not being very much, the difference between a reproductive number of 1 and a reproductive number of 2 is huge in terms of the number of people who will ultimately be infected. So everything we can do to try to get the reproductive number closer to 1 will help us control the spread of a disease. So what is the reproductive number for SARS coronavirus 2? Or to say it another way, on average, how many other people will 1 person infected with this virus infect? Let's look at some examples. So here in this figure, you can see that there's an infected person, represented by a purple dot in the middle. And there are gray dots all around. Those represent the close contacts of this infected person. Measles is another virus that you may have heard of before. Measles is one of the most infectious viruses that we know of. The reproductive number for measles is 15. That means that each person with measles can infect up to 15 other close contacts, as you can see in this figure. It's very infectious. In this figure, we're looking at MERS, or Middle Eastern Respiratory Syndrome. That's another type of coronavirus. MERS is not very infectious. On average, each infected person-- which is shown here in the purple dot-- will infect less than one other person. You can see that in the dot just above that's partially covered in red. So MERS usually doesn't cause very large outbreaks. Because on average, each case will only infect 1 other person or less. Now let's look at the reproductive number of COVID-19. On average, each person with COVID-19 will infect 2 to 3 other people. And you can see that represented here. There are 2 and 1/2 close contact dots that are covered in red, showing that those are the people who are going to be infected. This reproductive number of 2 to 3 is very similar to the reproductive number for influenza virus. And you can see that here for comparison. People infected with influenza virus, on average, will infect two to three other people, if everyone they have contact with is susceptible. So a reproductive number of 2 may sound small. But even a reproductive number of 2 can create a large outbreak if each infected person infects just two other people, the size of the outbreak doubles quickly, as you can see in the figure on the slide. If we can prevent just 1 infection, so that each person infects only 1 person instead of 2, we can have a big reduction in the number of cases over time. So you can see on this figure, if the first person infects only 1 person, and that person infects only 1 person, and that person infects only 1 person, you end up with only 4 cases in this outbreak, instead of the 15 that would have otherwise occurred if you hadn't have stopped some transmission. Just because you can't stop all transmission doesn't mean that you can't have a big impact on slowing down the outbreak.