Influenza is a highly contagious upper respiratory illness that can cause serious public health problems around the world. The flu season starts in October and lasts through May; peaking during the months of January or February in the United States.
The 2012-2013 flu season was the worst one in ten years and reached epidemic levels – how do the flu numbers look so far this year?
The Epidemiology and Prevention Branch in the Influenza Division at the Centers for Disease Control and Prevention (CDC) track influenza activity year round and produces a weekly report called FluView. Let’s take a look.
According to the Centers for Disease Control and Prevention (CDC) for the week of December 8-14, 2013, influenza activity continued to increase across the United States. During this week there were 7,294 specimens tested for influenza and 1,301 (17.8 percent) of those tested positive for influenza.
As for the number of hospitalizations associated with influenza, The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts surveillance on children and adults. The data that they gather helps estimate age-specific hospitalization rates on a weekly basis. However, we probably underestimate these rates, because influenza-related hospitalizations can be missed so easily. Health professionals may miss a flu case due to not conducting a influenza test, or attributing the illness to pneumonia or other complications as the main cause, ignoring influenza as an underlying cause of hospitalization. So between October 1, 2013 and December 14, 2013, the rate of influenza-related hospitalization was 3.0 per 100,000.
Health officials track pneumonia and influenza mortality using two systems; the 122 Cities Mortality Reporting System and the Influenza-Associated Pediatric Mortality Reporting System. The 122 Cities Mortality Reporting System reports weekly on the total number of death certificates that listed pneumonia or influenza as a cause or underlying cause of death by age group. The percentage of deaths due to pneumonia and influenza are compared to the seasonal baseline.
During the week of December 8-14th, 6.6 percent of all deaths reported via the 122 Cities Mortality Reporting System were due to pneumonia and influenza. This is below the epidemic threshold of 6.8 percent – but barely.
The second way that health officials gather mortality reports during the flu season is by the Influenza-Associated Pediatric Mortality Surveillance System. They added this surveillance system in 2004 to track flu-related deaths in children ages 18 year and younger. All laboratory-confirmed influenza deaths are reported through this system. Clinical information and demographics are also reported to the CDC.
For the week of December 8-14th there were two reported pediatric deaths, bringing the total to four deaths during this year’s flu season.
Flu and Outpatient Surveillance
The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) tracks the number of visits to the doctor for influenza-like-illness (ILI). The proportion of outpatient visits ILI was 2.3 percent, which is slightly above the national baseline of 2.0 percent. This means that more doctor offices are starting to see an increase in people who are going to the doctor for ILI. Influenza-like-illness is defined as having a fever of 100 degrees Fahrenheit or greater and cough and/or sore throat.
So who is seeing the most influenza-like-illnesses? The ILINET uses the data they collect to give an activity level for each state. These activity levels are based on the percentage of outpatient visits for ILI and compares them to the average percentage during weeks of little to no influenza. The activity ranges from minimal to high. So during the week of December 8-14, 2013, there were four states that have been given an activity rating of “high.” High-flu-activity states are Texas, Mississippi, Louisiana, and Alabama.
Oklahoma was the only state to report moderate activity, while six states (Colorado, Illinois, Missouri, Nevada, Utah, and Virginia) and New York City reported low activity. The District of Columbia, Idaho, and Vermont had insufficient data to be able to calculate and the other thirty-seven states reported minimal activity.
Flu’s Geographic Spread
The geographic spread of influenza is determined by the reporting of state and territorial epidemiologist. This determines the spread of influenza in the United States. Each state is given a rating of either no activity, sporadic(small numbers), local (outbreaks in one region of the state), regional (outbreaks in at least two regions, but less than half of the state), or widespread (outbreaks in at least half of the state).
For the week of December 8-14th widespread influenza was reported in four states, regional influenza was reported in 20 states, local influenza was reported in 17 states, sporadic influenza was reported in the District of Columbia, Guam, Puerto Rico, and eight states. Vermont was the only state to report no activity.
The difference between the activity map and the geographic spread is that the activity map just tracks influenza-like-symptoms. These cases are not confirmed cases of the flu, whereas the geographic spread reports confirmed cases.
Flu Season Hits: Are You Ready?
The CDC works with various partners, including physician offices, state, local, and territorial health departments, laboratories, vital statistic offices, clinic, and emergency departments around the United States. This data is helpful for experts to understand the current flu season and is also used to add into the mathematical equation to determine baseline numbers.