Influenza

Influenza is a topic covered in the 5-Minute Clinical Consult.

To view the entire topic, please or .

Medicine Central™ is a quick-consult mobile and web resource that includes diagnosis, treatment, medications, and follow-up information on over 700 diseases and disorders, providing fast answers—anytime, anywhere. Explore these free sample topics:

-- The first section of this topic is shown below --

Basics

Description

  • Acute, typically self-limited, febrile infection caused by orthomyxovirus influenza types A and B
  • Marked by inflammation of nasal mucosa, pharynx, conjunctiva, and respiratory tract
  • Outbreaks have varying degrees of severity and generally peak in winter.
  • Influenza virus can undergo antigenic shift (abrupt change) leading to viral strains with little immunologic resistance in a population. This can result in pandemic outbreaks. Minor seasonal variations are called antigenic drift.
  • Types of influenza include seasonal; pandemic; avian; swine
  • System(s) affected: typical cases: head/eyes/ears/nose/throat, pulmonary; complicated cases: cardiac and CNS involvement
  • Synonym(s): flu; grippe; acute catarrhal fever

Epidemiology

  • Predominant age: children (3 months to 16 years) and young adults
    • Morbidity: seasonal morbidity and rates of hospitalization highest in very young (preschool), elderly (>75 years of age), and individuals with comorbid illness (lung disease, malignancy)
  • Predominant sex: male = female

Incidence
  • Seasonal influenza in preuniversal vaccination: 95 million cases per year, typically fall/winter
  • Attack rates in healthy children: 10–40% each year, prior to routine influenza vaccination
  • Weekly reports: https://www.cdc.gov/flu/weekly

Prevalence
2017–2018 was a high severity influenza season. Disease appeared in November and had high activity through March. The 19 weeks of persistent influenza activity was longer than usual.

Etiology and Pathophysiology

Orthomyxovirus (influenza types A [majority] and B); influenza A virus subtypes HxNx based on hemagglutinin and neuraminidase

  • Incubation is 1 to 4 days; infected persons are most contagious during peak symptoms.
  • Spread by aerosolized droplets or contact with respiratory secretions
  • Hemagglutinin binds to columnar respiratory epithelium where replication occurs, and neuraminidase protein facilitates spread along respiratory epithelium.

Risk Factors

  • For contracting disease
    • Crowded environments such as nursing homes, barracks, schools, and correctional facilities
  • For complications
    • Neonates, infants, elderly
    • Pregnancy, especially in 3rd trimester
    • Chronic pulmonary diseases
    • Cardiovascular diseases, including valvular pathology and congestive heart failure (CHF)
    • Metabolic disease, morbid obesity
    • Hemoglobinopathies
    • Malignancy; immunosuppression
    • Neuromuscular diseases that limit respiratory function and ability to handle secretions

General Prevention

  • Vaccination: All persons >6 months should be vaccinated annually unless contraindication present.
    • Inactivated influenza vaccine (IIV) is available with either 3 (IIV3) or 4 (IIV4) strains of influenza. IIV also is available as high-dose, intradermal, cell culture–based (ccIV3), MF59-adjuvanted (aIIV3), and recombinant hemagglutinin vaccine (RIV3).
    • Live attenuated influenza vaccine (LAIV) is an intranasal quadrivalent vaccine.
  • IIV recommended annually for the following:
    • All persons aged ≥6 months
    • Vaccine should be administered annually as soon as the vaccine is available.
    • Protection occurs 1 to 2 weeks after immunization.
    • Typically, mild side effects include low-grade fever and local reaction at vaccination site.
    • Inactivated IM dose: ≥3 years of age: 0.5 mL; children 6 to 35 months of age: 0.25 mL
    • Intradermal formulation for 18 to 64 year olds uses a short 30-gauge needle in a single-use prefilled syringe with 0.1 mL vaccine; somewhat higher local reactions when given intradermal
    • Single annual dose except for children <9 years of age, who should receive 2 doses (4 weeks apart) the 1st year they receive influenza vaccine
    • Vaccine contraindication: Severe allergy such as anaphylaxis to IIV components, allergies from eggs are not considered a contraindication; observe all patients for 15 minutes after vaccination; no skin testing with influenza vaccine is needed in egg-allergic patients. RIV is safe in patients with an egg allergy.
    • Precaution: Guillain-Barré syndrome within 6 weeks after a previous dose of influenza vaccine
  • LAIV is available for 2019–2020 influenza season.
  • IIV-HD: high-dose quadrivalent IIV
    • Contains 4 times the antigen concentration of IIV
    • Licensed for persons ≥65 years of age
    • Results in higher antibody levels but somewhat higher rates of local reactions
    • Advisory Committee on Immunization Practices does not express a preference for/against IIV-HD.
  • Antiviral prophylaxis depends on current resistance patterns each year; see https://www.cdc.gov/flu/ for patterns or check with local health department.
    • In high-risk groups that have not been vaccinated or need additional control measures during epidemics; not a substitute for vaccination unless vaccine is contraindicated (1)[A]
    • During influenza season, for those with contraindications to vaccine who have been exposed to the virus (1)[A]
    • For staff and residents in nursing home outbreaks
    • For immune-deficient persons who are expected not to respond to vaccination after viral exposure

Pediatric Considerations

  • Vaccinate children 6 months and older annually.
  • Recommend all household members with children <6 months be vaccinated (2)[A].
  • For children who need 2 doses, administer first dose as soon as available for second dose to be given before the end of October (2)[A].
  • For prophylaxis, oseltamivir dosage varies by weight and is recommended by the CDC for prophylaxis for children ≥3 months; zanamivir is approved for prophylaxis for children ≥5 years of age at a dosage of 2 inhalations per day. Prophylaxis treatment duration is 7 days. For prophylaxis, the dosage of amantadine and of rimantadine is 5 mg/kg/day up to 150 mg in 2 divided doses. Currently, amantadine and rimantadine are not recommended due to resistance.

Pregnancy Considerations

  • The CDC recommends vaccinating all women who will be pregnant during influenza season.
  • If unvaccinated at the time of flu season, pregnant women should receive IIV or RIV.
  • Oseltamivir, zanamivir, peramivir, rimantadine, and amantadine are pregnancy Category C.

Commonly Associated Conditions

Bacterial pneumonia

-- To view the remaining sections of this topic, please or --

Basics

Description

  • Acute, typically self-limited, febrile infection caused by orthomyxovirus influenza types A and B
  • Marked by inflammation of nasal mucosa, pharynx, conjunctiva, and respiratory tract
  • Outbreaks have varying degrees of severity and generally peak in winter.
  • Influenza virus can undergo antigenic shift (abrupt change) leading to viral strains with little immunologic resistance in a population. This can result in pandemic outbreaks. Minor seasonal variations are called antigenic drift.
  • Types of influenza include seasonal; pandemic; avian; swine
  • System(s) affected: typical cases: head/eyes/ears/nose/throat, pulmonary; complicated cases: cardiac and CNS involvement
  • Synonym(s): flu; grippe; acute catarrhal fever

Epidemiology

  • Predominant age: children (3 months to 16 years) and young adults
    • Morbidity: seasonal morbidity and rates of hospitalization highest in very young (preschool), elderly (>75 years of age), and individuals with comorbid illness (lung disease, malignancy)
  • Predominant sex: male = female

Incidence
  • Seasonal influenza in preuniversal vaccination: 95 million cases per year, typically fall/winter
  • Attack rates in healthy children: 10–40% each year, prior to routine influenza vaccination
  • Weekly reports: https://www.cdc.gov/flu/weekly

Prevalence
2017–2018 was a high severity influenza season. Disease appeared in November and had high activity through March. The 19 weeks of persistent influenza activity was longer than usual.

Etiology and Pathophysiology

Orthomyxovirus (influenza types A [majority] and B); influenza A virus subtypes HxNx based on hemagglutinin and neuraminidase

  • Incubation is 1 to 4 days; infected persons are most contagious during peak symptoms.
  • Spread by aerosolized droplets or contact with respiratory secretions
  • Hemagglutinin binds to columnar respiratory epithelium where replication occurs, and neuraminidase protein facilitates spread along respiratory epithelium.

Risk Factors

  • For contracting disease
    • Crowded environments such as nursing homes, barracks, schools, and correctional facilities
  • For complications
    • Neonates, infants, elderly
    • Pregnancy, especially in 3rd trimester
    • Chronic pulmonary diseases
    • Cardiovascular diseases, including valvular pathology and congestive heart failure (CHF)
    • Metabolic disease, morbid obesity
    • Hemoglobinopathies
    • Malignancy; immunosuppression
    • Neuromuscular diseases that limit respiratory function and ability to handle secretions

General Prevention

  • Vaccination: All persons >6 months should be vaccinated annually unless contraindication present.
    • Inactivated influenza vaccine (IIV) is available with either 3 (IIV3) or 4 (IIV4) strains of influenza. IIV also is available as high-dose, intradermal, cell culture–based (ccIV3), MF59-adjuvanted (aIIV3), and recombinant hemagglutinin vaccine (RIV3).
    • Live attenuated influenza vaccine (LAIV) is an intranasal quadrivalent vaccine.
  • IIV recommended annually for the following:
    • All persons aged ≥6 months
    • Vaccine should be administered annually as soon as the vaccine is available.
    • Protection occurs 1 to 2 weeks after immunization.
    • Typically, mild side effects include low-grade fever and local reaction at vaccination site.
    • Inactivated IM dose: ≥3 years of age: 0.5 mL; children 6 to 35 months of age: 0.25 mL
    • Intradermal formulation for 18 to 64 year olds uses a short 30-gauge needle in a single-use prefilled syringe with 0.1 mL vaccine; somewhat higher local reactions when given intradermal
    • Single annual dose except for children <9 years of age, who should receive 2 doses (4 weeks apart) the 1st year they receive influenza vaccine
    • Vaccine contraindication: Severe allergy such as anaphylaxis to IIV components, allergies from eggs are not considered a contraindication; observe all patients for 15 minutes after vaccination; no skin testing with influenza vaccine is needed in egg-allergic patients. RIV is safe in patients with an egg allergy.
    • Precaution: Guillain-Barré syndrome within 6 weeks after a previous dose of influenza vaccine
  • LAIV is available for 2019–2020 influenza season.
  • IIV-HD: high-dose quadrivalent IIV
    • Contains 4 times the antigen concentration of IIV
    • Licensed for persons ≥65 years of age
    • Results in higher antibody levels but somewhat higher rates of local reactions
    • Advisory Committee on Immunization Practices does not express a preference for/against IIV-HD.
  • Antiviral prophylaxis depends on current resistance patterns each year; see https://www.cdc.gov/flu/ for patterns or check with local health department.
    • In high-risk groups that have not been vaccinated or need additional control measures during epidemics; not a substitute for vaccination unless vaccine is contraindicated (1)[A]
    • During influenza season, for those with contraindications to vaccine who have been exposed to the virus (1)[A]
    • For staff and residents in nursing home outbreaks
    • For immune-deficient persons who are expected not to respond to vaccination after viral exposure

Pediatric Considerations

  • Vaccinate children 6 months and older annually.
  • Recommend all household members with children <6 months be vaccinated (2)[A].
  • For children who need 2 doses, administer first dose as soon as available for second dose to be given before the end of October (2)[A].
  • For prophylaxis, oseltamivir dosage varies by weight and is recommended by the CDC for prophylaxis for children ≥3 months; zanamivir is approved for prophylaxis for children ≥5 years of age at a dosage of 2 inhalations per day. Prophylaxis treatment duration is 7 days. For prophylaxis, the dosage of amantadine and of rimantadine is 5 mg/kg/day up to 150 mg in 2 divided doses. Currently, amantadine and rimantadine are not recommended due to resistance.

Pregnancy Considerations

  • The CDC recommends vaccinating all women who will be pregnant during influenza season.
  • If unvaccinated at the time of flu season, pregnant women should receive IIV or RIV.
  • Oseltamivir, zanamivir, peramivir, rimantadine, and amantadine are pregnancy Category C.

Commonly Associated Conditions

Bacterial pneumonia

There's more to see -- the rest of this entry is available only to subscribers.