Information in five categories is collected from nine data sources in order to:. It is important to maintain a comprehensive system for influenza surveillance for the following reasons:. Influenza testing practices differ between public health and clinical laboratories and each network provides valuable information for monitoring influenza activity.
Clinical laboratories primarily test respiratory specimens for diagnostic purposes, and data from these laboratories provide useful information on the timing and intensity of influenza activity. Public health laboratories primarily test specimens for surveillance purposes to understand what influenza virus types, subtypes, and lineages are circulating and the ages of people that are infected.
All public health and clinical laboratories report each week to CDC the total number of respiratory specimens tested for influenza and the number positive for influenza viruses, along with age or age group of the person, if available. CDC presents data from clinical laboratories that includes the weekly total number of specimens tested, the number of positive influenza tests, and the percent positive by influenza virus type.
In order to obtain specimens in an efficient manner, public health laboratories often receive samples that have already tested positive for influenza at a clinical laboratory. As a result, monitoring the percent of specimens testing positive for influenza in a public health laboratory is less useful i.
In order to use each data source most appropriately and to avoid duplication, reports from public health and clinical laboratories are presented separately in both FluView and FluView Interactive.
The age distribution of people who have tested positive for influenza reported from public health laboratories can be visualized in FluView Interactive.
Additional laboratory data for current and past seasons and by geographic level national, Department of Health and Human Services HHS region, and state are available on FluView Interactive.
Virus Characterization — This includes genetic and antigenic characterization. Most U. The goals of genetic and antigenic characterization are to assess how similar the currently circulating influenza viruses are to viruses used to produce current influenza vaccines and to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. For genetic characterization, all influenza-positive surveillance samples received at CDC undergo next-generation sequencing to determine the genetic identity of circulating influenza viruses and to monitor the evolutionary trajectory of viruses circulating in the human population.
CDC also analyzes influenza viruses collected by public health laboratories for susceptibility to influenza antivirals, including neuraminidase inhibitors oseltamivir, zanamivir, and peramivir and a PA cap-dependent endonuclease inhibitor baloxavir. Susceptibility to the neuraminidase inhibitors is assessed using next-generation sequencing analysis. Neuraminidase sequences of viruses are inspected to detect the presence of amino acid substitutions pdf icon external icon previously associated with reduced or highly reduced inhibition by any of the three neuraminidase inhibitors.
In addition, a subset of viruses is tested using a neuraminidase inhibition assay. Susceptibility to baloxavir is assessed using next-generation sequencing analysis to identify amino acid substitutions in the PA protein that are associated with reduced susceptibility to this antiviral. A subset of representative viruses is also tested phenotypically using a high-content imaging neutralization test. For surveillance purposes, antiviral susceptibility is typically conducted on viruses that are collected from patients not treated with influenza antivirals or before initiation of treatment.
Results of genetic and antigenic characterization and antiviral susceptibility testing are presented in the virus characterization and antiviral resistance sections of the FluView report. Surveillance for Novel Influenza A Viruses — In , human infection with a novel influenza A virus became a nationally notifiable condition. Novel influenza A virus infections include all human infections with influenza A viruses that are different from currently circulating human seasonal influenza H1 and H3 viruses.
These viruses include those that are subtyped as nonhuman in origin and those that cannot be subtyped with standard laboratory methods and reagents. Rapid detection and reporting of human infections with novel influenza A viruses — viruses against which there is often little to no pre-existing immunity — is important to facilitate prompt awareness and characterization of influenza A viruses with pandemic potential and accelerate the implementation of public health responses to limit the transmission and impact of these viruses.
Newly reported cases of human infections with novel influenza A viruses in the United States are reported in FluView and additional information, including case counts by geographic location, virus subtype, and calendar year, are available on FluView Interactive.
Outpatient Illness Surveillance. Information on outpatient visits to health care providers for influenza-like illness ILI is collected through the U. Virgin Islands. Approximately 85 million patient visits were reported during the season. A subset of providers also reports total visits by age group. Sites with electronic health records use an equivalent definition as determined by public health authorities.
These data should be evaluated in the context of other surveillance data to obtain a complete and accurate picture of influenza virus activity. Additional data on medically attended visits for ILI for current and past seasons and by geographic level national, HHS region, and state are available on FluView Interactive.
The national percentage of patient visits to healthcare providers for ILI reported each week is calculated by combining state-specific data weighted by state population. This percentage is compared each week with the national baseline of 2.
The baseline is developed by calculating the mean percentage of patient visits for ILI during non-influenza weeks for the most recent three seasons excluding the COVID pandemic and adding two standard deviations As of October the time period excluded for the pandemic is March through September A non-influenza time period e. Region-specific baselines are calculated using the same methodology. Due to the wide variability in regional level data, it is not appropriate to apply the national baseline to regional data.
Region 1 — 2. Region 2 — 3. Region 3 — 2. Region 4 — 3. Region 5 — 2. Region 6 — 3. Region 8 — 2. ILI Activity Indicator Map: — Activity levels are based on the percent of outpatient visits due to ILI in a jurisdiction compared with the average percent of ILI visits that occur during weeks with little or no influenza virus circulation non-influenza weeks in that jurisdiction.
The number of sites reporting each week is variable; therefore, baselines are adjusted each week based on which sites within each jurisdiction provide data. To perform this adjustment, provider level baseline ILI ratios are calculated for those that have a sufficient reporting history.
Providers that do not have the required reporting history to calculate a provider-specific baseline are assigned the baseline ratio for their practice type. The jurisdiction level baseline is then calculated using a weighted sum of the baseline ratios for each contributing provider.
The activity levels compare the mean reported percent of visits due to ILI during the current week to the mean reported percent of visits due to ILI during non-influenza weeks. The 13 activity levels correspond to the number of standard deviations below, at, or above the mean for the current week compared with the mean during non-influenza weeks.
Activity levels are classified as minimal levels , low levels , moderate levels , high levels , and very high levels An activity level of 1 corresponds to an ILI percentage below the mean, level 2 corresponds to an ILI percentage less than 1 standard deviation above the mean, level 3 corresponds to an ILI percentage more than 1 but less than 2 standard deviations above the mean, and so on, with an activity level of 10 corresponding to an ILI percentage 8 to 11 standard deviations above the mean.
All data are preliminary and may change as more reports are received. Influenza surveillance data the way you want it. To receive weekly email updates about Seasonal Flu, enter your email address:. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Influenza Flu. Section Navigation. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. Weekly U. Influenza Surveillance Report FluView.
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