Now, let's turn our attention to a public health surveillance system that's within the United States. The US Centers for Disease Control and Prevention is the public health agency of the United States that's primarily tasked with the conduct of public health surveillance. Influenza virus is a seasonal infectious disease in the United States. It can cause outbreaks of mild to severe disease every winter, and it can be prevented by vaccination importantly. So, that's our primary public health prevention measure. There's a link here at the bottom of the slide that will take you to the CDC website, and you can learn all about the influenza surveillance that happens in the United States. So, CDC conducts influenza surveillance because of the public health threat that it poses. They use eight different data sources and they collaborate with health providers, health departments, as well as public health laboratories, and states all over the United States to correlate these data sources and to track influenza. The objectives for CDC's influenza surveillance system are to find out when and where influenza activity is occurring, to track influenza-related illness, to determine what influenza viruses are circulating. So, there are different types of influenza viruses A and B, and among influenza A viruses, there are also different subtypes. So, we want to know specifically which viruses are causing disease from year-to-year. Another objective is to detect changes in those influenza viruses that are circulating, as well as to measure the impact influenza is having on hospitalizations and deaths. So, it's important to know where we're seeing a lot of influenza transmission and infection, but we also want to know and have a signal if we're seeing that the strain of flu that's circulating in one particular year is causing more severe disease than usual. That will be important for the public health system to know and for the medical community to know so that we can prepare. So, influenza surveillance in the United States is continuous, it happens all year round. If you go to the website, you can see data that they have showing where influenza activity is ongoing in any given week, and it's updated on a weekly basis. So, for the week ending of October 13th, 2018, which is week 41, you can see here a map of the United States, and that there's some local activity of influenza going on. So, it looks like Massachusetts, out in Guam there's some local activity of influenza. There are some sporadic activity across the United States, that's the hatched line that you see. But in many places there's been no activity reported yet. Let's just compare and contrast this a bit with a different time period which was January 2018. So again, on the right side of the map you see a legend showing what the different colors mean and in January 2018, very first week of 2018, there was widespread influenza activity throughout the continental United States. So, you can track the flu epidemic each year over time week-by-week through the US CDC site. This is an invaluable source of information for practitioners, both of public health and of medicine. So, you can imagine that if you are in, for example, the state of Georgia and you see activity increasing in the states around you, you might expect that it's coming your way next and it will help you to prepare. As I mentioned, one of the objectives of CDC's influenza surveillance is to identify when flu infections are more severe than usual. This is important to track because it could give us some indication that there's a new influenza strain that we haven't seen before. So, what this figure shows is the proportion of all deaths due to pneumonia and influenza that are reported each week year-by-year. So again, let's break this figure down so we're sure we understand what we're seeing. So first, let's look at the x-axis. So, you see 40, 50, 10, 20, 30, along the bottom. Those are weeks of the year. So, there are 52 weeks in every year, and what we're seeing here are four different flu years, if you will. Influenza tends to peak in the wintertime. So, each of these years starts in the summer of one year and ends in the summer of the next year. So, the first white column on the figure on the left side at the top says 2014 to 2015. So, this is showing us that flu year which began in the summer of 2014 and ended in the summer of 2015, and then the next flu year began in the summer of 2015 and goes to the summer of 2016 and so on. On the y-axis, we see the proportion of all deaths due to pneumonia and influenza. So, we have reporting of cause of death in the United States, and we track those, the proportion of all the deaths that are caused by pneumonia and influenza. The red line on this graph shows how the proportion of all deaths due to pneumonia and influenza changes week-by-week across these four years, so that's the red line. The black lines show what we would expect in a normal year. I'm not going to get into details about how we get those expectations or the black lines, but there's a mathematical algorithm we use to guess what would be within the normal range based on what we've seen in the past. So, essentially, what we want to do is look to see when the red line goes above our expectation, and that tells us we're having a very bad influenza season and we're having an outbreak, if you will, of influenza and pneumonia deaths. So, you can see here we had one in 2014-2015 season. Somewhere in 2016-2017 and the 2017-2018 year, we also saw an increase that was way higher than what we would expect. So again, these are important data for us to track as a nation. So nationally, we're tracking these data, but there are also important for regional responses within the United States and helping us to prepare. Finally, I want to talk about another way that you can look at data from influenza surveillance that's gathered and analyzed by the US CDC. One of the questions is, who. Who is suffering most from influenza? We do see year-to-year variations in who is at highest risk for influenza based on the strengths that are circulating. These data come from the week of October 13th, 2018, and they represent the rates of laboratory-confirmed influenza hospitalizations by age group. So, in the previous graph I showed you, influenza and pneumonia deaths not all of those deaths are laboratory-confirmed with influenza, they're just diagnosed as have had influenza like illness or pneumonia when they died. So, this graph is showing us something that's a bit different that is laboratory-confirmed flu hospitalizations. So, let's break this down again along the x-axis. On the bottom, we are seeing the weeks. Again, starting from week 40 of 2017, and we're now through week 17 of 2018. What you'll see is different lines over time showing rates per 100,000 population of influenza. So, along the y-axis, you see it goes from zero all the way up to 450, and each of the colored lines represents a different age group. So, if you look at the legend on the right side at the very bottom you have a green line, and that indicates persons who are 65 years or older. If you look back at the figure that's the top line in the figure, so that indicates the cumulative rates of laboratory-confirmed influenza hospitalizations throughout the year in that age group. So, how would we interpret this? As you can see here, persons aged 65 years or older suffer disproportionately and are hospitalized with laboratory-confirmed influenza much more than any other age group in the United States. So, that tells us that this is an important age group for severe infections, and may prompt us to think about ways we can prevent infections in this age group who might be at high risk for this severe disease outcome. So, we've shown you now a few different ways that the US CDC tracks influenza across geography, tracks severity over time to find outbreaks, and as in this example, looks to see who's at highest risk for influenza. Hopefully, this gives you a sense of the types of data that can come out of public health surveillance and how they might be used at a national level.
