What are the recommendations and perceptions regarding influenza vaccination among vulnerable populations, including the elderly, young children, and individuals with chronic health conditions?

Influenza Vaccination: Knowledge, Attitudes, and Perceptions

Universal Vaccination Recommendation

Annual influenza vaccination is recommended for all persons aged 6 months and older, representing a fundamental shift from earlier risk-based approaches to comprehensive population coverage. 1 This universal policy reflects the recognition that influenza affects all age groups and that broader vaccination strategies reduce community transmission and protect vulnerable populations indirectly. 2

Priority Vulnerable Populations

While vaccination is universal, certain groups require intensified vaccination efforts due to elevated risk of severe complications, hospitalizations, and death:

Elderly Adults (≥65 years)

• Adults aged 65 years and older should receive high-dose or adjuvanted influenza vaccines preferentially over standard-dose formulations. 3 These enhanced vaccines were specifically developed to overcome immunosenescence and provide superior protection in this age group. 4
• This population experiences the highest rates of influenza-related hospitalizations (26.5 cases per 100,000 population) and mortality. 5
• Portugal has achieved vaccination coverage exceeding the WHO-recommended 75% target in adults ≥65 years since 2019-2020, contrasting with most other European Union countries. 5

Young Children

• All children aged 6-23 months should receive annual influenza vaccination, as this age group has elevated hospitalization risk comparable to elderly adults. 1
• Children aged 6 months through 8 years who are receiving influenza vaccine for the first time require two doses administered at least 4 weeks apart for adequate immune priming. 3 After the initial two-dose series, only one dose annually is needed in subsequent seasons.
• Inactivated influenza vaccine (IIV) is preferred over live attenuated vaccine for all children. 1

Pregnant Women

• All pregnant women should receive influenza vaccination at any time during pregnancy to protect both mother and infant through transplacental antibody transfer. 1
• Vaccination during pregnancy provides passive immunity to infants during their first months of life when they are too young to be vaccinated directly. 6

Persons with Chronic Medical Conditions

• Individuals with chronic pulmonary diseases (including asthma and COPD), cardiovascular disease, diabetes mellitus, renal dysfunction, hemoglobinopathies, or immunosuppression require annual vaccination regardless of age. 5, 1
• These patients should receive only inactivated vaccines, never live attenuated vaccines, as the latter can cause influenza illness in high-risk populations. 3
• Patients with cardiovascular disease benefit from vaccination as secondary prevention, with Class I, Level B recommendation strength. 3

Healthcare Personnel and Caregivers

All healthcare workers, nursing home employees, home care providers, and household members of high-risk persons must be vaccinated annually. 5, 1 This recommendation serves dual purposes:

• Prevents healthcare-associated influenza transmission to vulnerable patients who may have suboptimal vaccine responses due to immunosenescence or immunosuppression. 5
• Reduces community transmission, as healthcare workers serve as vectors between high-risk populations. 5

Vaccine Selection and Timing

Optimal Vaccination Window

• Vaccination should occur between September and November for optimal protection, ideally by the end of October. 1, 3
• Vaccination efforts should continue throughout the influenza season as long as vaccine is available, as late vaccination remains beneficial. 5, 3

Annual Vaccination Necessity

• Annual revaccination is mandatory because immunity declines within one year following vaccination. 5, 1, 3
• Each season's vaccine is reformulated to match predicted circulating strains, making previous year's vaccines ineffective for the current season. 5

Single Dose Per Season

• Most individuals require only one dose per influenza season; multiple doses of the same seasonal formulation provide no additional benefit. 3
• This applies even to high-risk patients with diabetes, COPD, or cardiovascular disease, despite some studies showing greater antibody responses to repeat immunization. 3

Common Pitfalls and Misconceptions

Vaccine Interchangeability

• Different brands and formulations (trivalent vs. quadrivalent) are interchangeable within the same season for age-appropriate recipients. 3
• Vaccination should never be delayed waiting for a specific brand or formulation, as timely vaccination is more important than brand consistency. 3

Safety Concerns

• Influenza vaccines have a robust safety profile with acceptable reactogenicity across all age groups. 5
• The most common adverse events are injection-site pain, redness, and swelling for inactivated vaccines, with systemic symptoms (fatigue, headache, myalgia) occurring less frequently. 5
• The association between influenza vaccination and Guillain-Barré Syndrome remains uncertain, with risk estimated at most 1-2 cases per million vaccinees if any association exists. 5

Live Attenuated Vaccine Contraindications

• Live attenuated influenza vaccine (LAIV/nasal spray) is contraindicated in immunosuppressed individuals, pregnant women, children under 2 years, adults over 50 years, and close contacts of severely immunosuppressed persons. 5
• Healthcare workers who receive LAIV should refrain from contact with severely immunosuppressed patients for 7 days post-vaccination. 5

Suboptimal Coverage Challenges

Despite widespread vaccine availability, coverage remains inadequate in several key groups:

• Adults aged 60-65 years with comorbidities. 5
• Pregnant women. 5
• Healthcare professionals. 5
• Children aged 6-23 months. 5

Strategies to improve coverage include implementing reminder/recall systems, standing orders programs, and targeting organized vaccination programs toward high-risk persons and their close contacts. 5