What Internists Need to Know about Influenza
Key takeaways:
- Influenza (ssRNA virus) is a major cause of morbidity and mortality, especially in patients > 55 years of age.
- Influenzas A and B cause the yearly epidemics of respiratory illnesses.
- Not every patient with influenza needs antiviral therapy.
- Give treatment in 3 settings: Those at high risk of complications, hospitalized patients, and severe, progressive, or complicated illness.
- All individuals > 6 months old should receive annual influenza vaccination with one exception: previous severe allergic reaction (i.e., anaphylaxis) to influenza vaccine.
According to the U.S. Centers for Disease Control and Prevention, the flu has caused at least 14 million illnesses, 150,000 hospitalizations and 9,400 deaths so far this season. As healthcare professionals, you know those numbers are likely to climb.
In this blog, we'll review what you need to know about Influenza for boards and daily practice. This information has been pulled from our 20th Edition Internal Medicine Core.
Influenza Overview
Influenza (ssRNA virus) is a major cause of morbidity and mortality, especially in patients > 55 years of age with COPD. Annual vaccination decreases mortality.
There are 3 types of influenza viruses: influenzas A, B, and C. Subtypes of influenza A exist based on their specific neuraminidase and hemagglutinin antigens. Influenzas A and B cause the yearly epidemics of respiratory illnesses. Influenza C causes very mild symptoms, if any.
Diagnosis of Influenza
Influenza presents as an acute febrile respiratory illness with fever, cough, and myalgias. Serious complications include viral pneumonia, secondary bacterial pneumonia, rhabdomyolysis, encephalitis, and myocarditis.
A positive antigen test or PCR for influenzas A and B from nasopharyngeal swab or respiratory secretions is diagnostic. Not every patient with influenza needs antiviral therapy.
Treatment of Influenza
Give treatment in 3 settings:
- Those at high risk of complications (e.g., immunocompromised; pregnancy; underlying heart, lung, liver, or kidney disease; > 65 years of age; residents of chronic care facilities; active malignancy; diabetes; morbid obesity; hemoglobinopathies; neurologic conditions causing inability to handle respiratory secretions; Native American patients; Alaska Native patients)
- Hospitalized patients
- Severe, progressive, or complicated illness
In addition, anyone outside of these high-risk groups who presents with influenza within 48 hours of onset benefits from treatment with respect to the duration of illness.
There are 3 classes of anti-flu drugs: adamantanes, neuraminidase inhibitors, and cap snatch inhibitors. Oseltamivir, a neuraminidase inhibitor, is the most commonly used. For a complete discussion of these medications, see Antiviral Agents on page 4-90 of the Internal Medicine Core.
The prevalence of antiviral resistance to any specific agent is dependent on the circulating strains. Look to CDC guidance for up-to-date information on flu resistance to antiviral drugs.
Influenza Vaccine Information
All individuals > 6 months old should receive annual influenza vaccination with one exception: previous severe allergic reaction (i.e., anaphylaxis) to influenza vaccine. A history of Guillain-Barré syndrome is a precaution to vaccination, but not necessarily a contraindication. Egg allergy is not a contraindication to vaccination. If your patient reports hives only, give influenza vaccine. If your patient reports more than urticaria (e.g., angioedema, swelling, recurrent vomiting, lightheadedness), give influenza vaccine in monitored healthcare setting with basic life support capability.
Influenza vaccines are targeted to the serotypes that are most likely to be present when the influenza season occurs. The better the antigenic match to the currently circulating virus, the better the vaccine works. The types of influenza vaccines available in the U.S. are inactivated, recombinant, and intranasal live attenuated vaccine. The inactivated vaccine also comes in a high-dose form that is more immunogenic and recommended for individuals > 65 years old. Avoid the intranasal live attenuated vaccine in immunocompromised patients, pregnant patients, and health care workers who work with immunosuppressed populations. Regardless of which is used, administer vaccines when they become available each year, which is usually in September or October.
Ready to check out the rest of the Core? Start your board review with this source of truth.
