Seasonal Influenza For Health Care Professionals and Laboratorians
Every year in the United States, millions of people get sick with influenza (the flu). Influenza epidemics in the United States usually occur during the winter months. According to the Centers for Disease Control and Prevention (CDC), an estimated 23,607 (range 3,349-48,614) influenza-associated deaths and over 200,000 influenza-associated hospitalizations occur every year in the United States. The highest rates of influenza infection occur among children, but the risks for serious health problems, hospitalizations, and deaths from influenza are higher among people 65 years of age or older, young children, pregnant women, and people of any age who have medical conditions that place them at increased risk for complications from influenza. Anyone though, including healthy people, can get influenza, and serious health problems from influenza can occur at any age. The severity of an influenza season varies from year to year and depends on many things, including the strains of circulating influenza viruses, how much flu vaccine is available, when the vaccine is available, how well the flu vaccine is matched to flu viruses that are causing illness, and the levels of protective antibody in the population.
A primary feature of the influenza virus is that it regularly undergoes genetic and/or recombination changes, which if dramatic enough, can result in the creation of an influenza virus never seen before in humans. Since the population would not have antibody protection against this new form of influenza virus, and if it were highly contagious and infectious, the potential for a worldwide epidemic (pandemic) would be increased. During most pandemics in the past, the rates of illnesses and deaths from influenza-related health problems have increased dramatically worldwide. During the 1918-19 "Spanish Flu" pandemic, it is estimated that ≈50 million deaths occurred worldwide, including over a half-million Americans. Influenza can have a very serious and severe impact on public health.
Influenza is a contagious respiratory illness caused by the influenza virus. There are three types of influenza viruses: A, B, and C. Influenza type A viruses can infect people, birds, pigs, horses, seals, cats, whales, and other animals, but wild birds are the natural hosts for these viruses. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus. Only some influenza A subtypes (i.e., H1N1 and H3N2) are currently in general circulation among people. Other subtypes are found more commonly in other animal species. Influenza B viruses are normally found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Although influenza B viruses can cause human epidemics, they have not caused pandemics. Influenza type C viruses cause mild illness in humans and are not thought to cause epidemics.
Influenza is not the same illness as a cold. Different viruses cause colds. Influenza tends to be worse than the common cold, and symptoms such as fever and body aches are more common and intense. Colds are usually milder than the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia. Influenza attacks the respiratory tract of the nose, throat, and lungs. Cold viruses attack the mucous linings of the nose and throat.
Strengthening Respiratory Surveillance in Texas
Texas Department of State Health Services (DSHS) continues to implement enhanced respiratory virus surveillance to ensure early detection of seasonal influenza, novel influenza A viruses, and other respiratory threats. This strengthened approach aligns with national CDC and Association of Public Health Laboratories (APHL) guidance and focuses on building statewide capacity to monitor, detect, and respond to respiratory disease activity.
Expanded Laboratory Testing
- Increasing the number of influenza-positive specimens tested and subtyped at public health laboratories to meet the Influenza Right Size detection goals (1 in 200-700 events detection, depending on the season).
- Performing whole genome sequencing (WGS) on additional influenza viruses to detect genetic changes, antiviral resistance, and potential novel strains.
- Ensuring rapid referral of unsubtypable or unusual influenza A results to state public health laboratories and national reference centers.
Clinical and Sentinel Surveillance
- Strengthening partnerships with sentinel providers, hospitals, and clinical labs to increase specimen submission for influenza and other respiratory viruses.
- Encouraging testing of severely ill or hospitalized patients with respiratory symptoms to ensure subtyping of all influenza A positives.
- Expanding use of ILINet and syndromic surveillance systems to capture outpatient and emergency department visits for influenza-like illness (ILI).
Partner Engagement
- Conducting outreach and education with healthcare providers, laboratories, and public health partners to improve awareness of novel influenza risks and testing protocols.
This strengthened approach, sometimes referred to as “enhanced surveillance”, ensures that Texas remains prepared to identify unusual respiratory events early, limit transmission, and protect public health through timely action.
Request for Faster Subtyping of In-Patient Influenza A Positive Specimens
In a recent CDC Health Alert released January 16, 2025, the CDC recommended all influenza A positive specimens from hospitalized patients be subtyped as soon as possible, ideally within 24 hours of admission.
Recommendations for Clinicians
Subtyping should be carried out in the hospital laboratory, if available, or at a commercial laboratory. If subtyping is not available through a hospital or commercial lab, DSHS urges clinicians to promptly contact their local health department or public health region to coordinate shipping specimens to the appropriate public health laboratory for subtyping.
Recommendations for Clinical Laboratories
Clinical laboratories that subtype should submit influenza A positive specimens that are negative for seasonal influenza A virus subtypes A(h) and A(H3) to their local public health laboratory within 24 hours of subtyping results.
Submission Recommendations
If you are going to submit influenza specimens to the DSHS Austin Laboratory, please complete a G-2V specimen submission form. Indicate in Section 4 of the G-2V form whether:
- The patient was exposed to infected wild or domestic animals with confirmed or suspected influenza A(H5N1),
- The patient experienced a high-risk exposure to H5N1, or
- Subtyping results were negative for seasonal influenza A virus subtypes.
Subtyping specimens as soon as possible will help to quickly identify human infections with avian influenza A(H5) viruses and routinely characterize seasonal influenza viruses.