How Does the CDC conduct influenza surveillance?
Are we seeing an increase in influenza activity yet?
Is it time to get my flu shot?
How Does the CDC conduct influenza surveillance?
First, there is virologic surveillance, i.e., testing by a network of laboratories for the influenza virus and reporting to the CDC of those results. This network of laboratories consists of the U.S. World Health Organization (WHO) Collaborating Laboratories System and the National Respiratory and Enteric Virus Surveillance System (NREVSS). Together, this accounts for approximately 100 public health and approximately 300 clinical laboratories located throughout all 50 states, Puerto Rico, Guam, and the District of Columbia that participate in virologic surveillance for influenza throughout the year.
The public health laboratories primarily help determine the specific strains of influenza virus in circulation (A and B) and for influenza A the subtypes and for influenza B the lineages, while the clinical laboratories provide the CDC with data as to the timing of increases and decreases in seasonal influenza activity and the amount of influenza activity. Both kinds of laboratories report the age of the person who was tested, if known, so that the CDC can also track and report influenza activity by age group (0-4 years, 5-24 years, 25-64 years, and ≥65 years).
In addition, the state and public health laboratories send samples of the influenza A and B viruses they identify so that the CDC can do further genetic and antigenic analysis of the viruses to determine how closely they are aligned to the viruses used in preparation of that season’s flu vaccine, as well as to identify how the viruses are evolving, as they continually are doing so.
Another use of these samples is for the CDC to test the ongoing effectiveness of our antiviral treatment options in order for the CDC to advise physicians if the virus is developing resistance to one or more of our treatment options.
Human seasonal influenza A viruses are of the H1 or H3 type (in my prior posts, I have explained that we often refer to specific influenza A viruses by the antigenic characterization of their two primary proteins – neuraminidase and hemagglutinin, which results in a designation of H_N_, where the blanks are filled in with a number that correlates to the specific antigen identification. For our seasonal influenza outbreaks, we expect to see influenza A viruses of the H1N1 and the H3N2 types. Thus, an additional goal of antigenic characterization of circulating influenza A viruses is to identify any novel viruses (e.g., an H5, H7 or H9 virus) that might emerge such as swine or avian influenza A viruses that might have pandemic potential (in large part because there would not be preexisting population immunity) so that public health measures can be put in place as early as possible. The avian (bird) influenza virus currently circulating in dairy cattle is of the H5N1 type.
The second component to the surveillance system is outpatient illness monitoring. Data on outpatient visits to health care providers for respiratory illness [fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat] referred to as influenza-like illness (ILI) is collected through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet). ILINet consists of outpatient healthcare providers in all 50 states, Puerto Rico, the District of Columbia, and the U.S. Virgin Islands. These providers also report the total number of visits so that the CDC can monitor for increases or decreases in the percentage of visits for influenza-like illness. Note that this surveillance component is not specific for influenza, since it does not require confirmatory testing to validate that the respiratory illness is caused by influenza; nevertheless, this has proved to be a valuable indicator for the onset and scale of illness during the respiratory virus season. We consider an increase in influenza-like illnesses to be occurring when visits for ILI are >2.9% nationally for a reporting week. For Idaho (Region 10), the CDC considers an increase to be occurring when these visits account for more than 1.9% of visits (due to a lower baseline level of influenza compared to nationally).
The CDC then assigns one of 13 levels of influenza activity for each reporting week in comparison to the mean level during non-seasonal weeks. Activity levels are classified as minimal (levels 1-3), low (levels 4-5), moderate (levels 6-7), high (levels 8-10), and very high (levels 11-13). 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. The very high levels correspond to an ILI percentage 12 to 15 standard deviations above the mean for level 11, 16 to 19 standard deviations above the mean for level 12, and 20 or more standard deviations above the mean for level 13.
The third part of the surveillance system is the hospitalization monitoring. Laboratory-confirmed influenza-associated hospitalizations are monitored through the Influenza Hospitalization Surveillance Network (FluSurv-NET). The current network of hospitals covers over 90 counties or county equivalents in the 10 Emerging Infections Program (EIP) states (CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN) and four additional states through the Influenza Hospitalization Surveillance Project (MI, NC, OH, and UT). The network represents approximately 9% of US population (~30 million people). New hospital admissions are defined as patients who were admitted to an inpatient bed on the previous calendar day and had a positive influenza test at admission or during the 14 days prior. Laboratory confirmation includes detection of influenza virus infection through molecular tests (e.g., polymerase chain reaction [PCR], nucleic acid amplification [NAAT]), antigen detection tests, immunofluorescence tests, and virus culture.
During the COVID-19 pandemic, all hospitals were required to report COVID-19 and influenza information on laboratory testing, capacity and utilization, and patient flows to facilitate the public health response to the pandemic. As of December 15, 2022, these data are required to be reported to CDC’s National Healthcare Safety Network (NHSN), which monitors national and local trends in healthcare system stress, capacity, and community disease levels for approximately 6,000 hospitals in the United States.
The final component of the surveillance system is mortality monitoring. The National Center for Health Statistics (NCHS) collects death certificate data from state vital statistics offices for all deaths occurring in the United States. Deaths included in this component of the U.S. Influenza Surveillance System are those which are classified based on the various ICD-10 (this is the set of codes that doctors and hospitals use to characterize a patients’ illness) codes used to for cause of death such as those associated with influenza. Data are aggregated by the week of death occurrence.
Pediatric influenza-associated deaths are also tracked. For surveillance purposes, an influenza-associated pediatric death is defined as a death in a person less than 18 years of age, resulting from a clinically compatible illness that was confirmed to be influenza by an appropriate laboratory diagnostic test. There should be no period of complete recovery between the illness and death.
Together, all of these components of influenza surveillance give us a general understanding of when the flu season is beginning, when it is ending, how much transmission we are seeing and how severe the season seems to be.
The beginning of the annual “Flu season” — as determined by elevated flu activity – varies from season to season. During most seasons, activity begins to increase in October, most often peaks between December and February and can remain elevated into May. The flu season is said to have started after consecutive weeks of elevated flu activity are registered in the various CDC influenza surveillance systems.
It can be very confusing for the uninitiated as to how to identify specific dates of the influenza season reporting because the reporting period for each influenza season begins during Morbidity and Mortality Weekly Report (MMWR) week 40 and ends week 39 of the following year.
Are we seeing an increase in influenza activity yet?
The short answer is no. We have data as of week 42 (the third week of flu season reporting)
From the virologic surveillance standpoint, the percent positivity for influenza testing is only 0.7% from the reporting clinical laboratories and remaining stable. (This means that of those presenting with influenza-like illnesses for which diagnostic testing was performed, less than 1 percent of the tests are returning positive for influenza.) The public health labs have identified that two influenza A strains are predominantly circulating – influenza A(H1N1)pdm2009 (we’ll discuss this in greater detail in subsequent blog posts, but basically, this is the designation for the current influenza A virus that has the antigenic designation of H1N1 based upon the two main proteins [H for hemagglutinin and N for neuraminidase]and the pdm2009 indicates that this particular strain is a descendent of the 2009 pandemic strain) and influenza A(H3N2) (which is also an influenza A virus, but with different proteins (H3 and N2). Both of these viruses are included in this year’s seasonal flu vaccine.
Influenza-like illness surveillance also reflects low activity. Currently, only 2.1% of visits are for influenza-like illness, and this rate is not on the increase, so we have not yet seen an increase over baseline.
Further, weekly hospitalizations for influenza-associated illness are very low at 0.1 per 100,000 people.
Finally, influenza-associated mortality is also low currently (0.05%), meaning that 0.05% of all reported deaths for that week were attributed to influenza. Fortunately, none of those deaths were in children less than age 18.
As expected, the majority of influenza viruses identified that are currently circulating are A, but there were some B viruses isolated, and all of these are of the Victoria lineage. We previously had influenza B/Yamagata circulating until it appears to have been eliminated with the respiratory precautions taken early in the COVID-19 pandemic, though we can’t rule out its eventual return just yet.
Is it time to get your flu shot?
Not unless you are travelling internationally or you have an opportunity to get your shot now and know that if you don’t get it now, you won’t.
On the other hand, if you are willing to wait a bit and know that you can go in and get your shot with little notice, it may be advisable to wait until we see that increase because the flu vaccine effectiveness does wane a bit each month, and as mentioned above, the flu activity often peaks between December and February and can remain at elevated levels through May, so we want to have people maximally protected at the end of this year and beginning of next, as well as some protection persisting through the spring.
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