Influenza vaccine
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Here is a comprehensive overview of the influenza vaccine based on authoritative medical textbooks:
Influenza Vaccine
Types of Influenza Vaccines
Three main categories are available for seasonal influenza:
| Type | Abbreviation | Route | Notes |
|---|---|---|---|
| Inactivated influenza vaccine (standard-dose) | IIV4 (quadrivalent) | Intramuscular | Egg-based; covers 2A + 2B strains |
| High-dose inactivated vaccine | HD-IIV4 | Intramuscular | 4× hemagglutinin per strain; preferred ≥65 yr |
| Adjuvanted inactivated vaccine | aIIV4 | Intramuscular | MF59 adjuvant; preferred ≥65 yr |
| Recombinant influenza vaccine | RIV4 | Intramuscular | Not egg-based; approved ≥18 yr |
| Cell culture–based inactivated vaccine | ccIIV4 | Intramuscular | Not egg-based; approved ≥18 yr |
| Live attenuated influenza vaccine | LAIV4 | Intranasal | Cold-adapted; 2–49 yr, non-pregnant, healthy only |
LAIV consists of four cold-adapted, temperature-sensitive, attenuated viruses reassorted to contain the surface hemagglutinin and neuraminidase of circulating A(H3N2), A(H1N1), and two B strains. — Goldman-Cecil Medicine, p. 176
Who Should Be Vaccinated
Universal recommendation: Annual vaccination is recommended for all persons ≥6 months of age.
Priority populations when vaccine supply is limited:
- All children aged 6–59 months
- All persons aged ≥50 years
- Adults/children with chronic pulmonary (including asthma), cardiovascular (except isolated HTN), renal, hepatic, neurologic, hematologic, or metabolic disorders
- Immunocompromised persons (any cause, including HIV)
- Pregnant women (or those who will be pregnant during influenza season)
- Children/adolescents on long-term aspirin/salicylate therapy (Reye syndrome risk)
- Nursing home and long-term care facility residents
- American Indians/Alaska Natives
- Persons with BMI ≥40
- Healthcare personnel and household contacts of high-risk individuals
— Goldman-Cecil Medicine, Table 394
Dosing by Age
| Age Group | Dose |
|---|---|
| 6 months–8 years | 2 doses (4 weeks apart) if not previously vaccinated with 2 doses since July 2010; otherwise 1 dose |
| ≥9 years | 1 dose annually |
| ≥65 years | Prefer HD-IIV4, RIV4, or aIIV4; use any age-appropriate vaccine if these unavailable |
Vaccine Efficacy
- Efficacy varies with how well the vaccine matches circulating strains.
- In persons ≥50 years: ~30% effective at preventing laboratory-confirmed outpatient illness.
- In long-term care facility residents: ~45% effective at preventing influenza-like illness.
- High-dose and adjuvanted vaccines confer greater protection, especially in older adults.
- Influenza vaccination also reduces cardiovascular events.
— Goldman-Cecil Medicine, p. 176
Contraindications and Special Situations
LAIV4 should NOT be used in:
- Pregnant individuals
- Persons with asthma
- Immunocompromised patients
- Close contacts of severely immunosuppressed patients (or if used, avoid contact for 7 days)
- Adults ≥50 years or those with high-risk underlying conditions
Egg Allergy
- Hives only: Any age-appropriate influenza vaccine may be used.
- Severe reactions (angioedema, respiratory distress, required epinephrine): Any vaccine may be given, but administer in a medical setting with a provider able to manage severe allergic reactions.
- True contraindication: Prior severe allergic reaction (anaphylaxis) to any influenza vaccine component.
- Cell culture–based (ccIIV4) and recombinant (RIV4) vaccines are not egg-based and are preferred in these patients.
Guillain-Barré Syndrome (GBS)
- History of GBS within 6 weeks of a prior influenza vaccine dose is a precaution (not absolute contraindication).
- For non-high-risk persons, generally avoid vaccination; for high-risk persons, benefits may outweigh risks.
- Estimated rate: ~1 case per 1 million doses.
Adverse Reactions
- IIV: Injection-site soreness most common. Fever, malaise, myalgia may occur 6–12 hours post-vaccination, lasting 1–2 days (more common in first-time vaccinees, especially children).
- LAIV4: Runny nose, headache, sore throat.
- GBS: Rare (~1/million doses).
Timing
- Vaccination should begin as soon as vaccine is available each fall, ideally by end of October.
- Pregnant persons in the third trimester should be vaccinated as soon as available.
- Influenza season peaks January–February and continues through March.
Sources: Goldman-Cecil Medicine (International Ed.), Textbook of Family Medicine 9e
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