Is it too late to get a flu shot, especially for vulnerable populations such as the elderly, young children, pregnant women, and individuals with chronic health conditions like heart disease, lung disease, or diabetes?

It Is Not Too Late to Get a Flu Shot

Vaccination efforts should continue throughout the entire influenza season, and vaccine administered in December or later—even after influenza activity has begun—is likely to be beneficial in the majority of influenza seasons. 1

Timing Recommendations

The Advisory Committee on Immunization Practices (ACIP) explicitly states that providers should offer influenza vaccine routinely and organized vaccination campaigns should continue throughout the influenza season, including after influenza activity has already begun in the community. 1

Key Timing Principles:

• Influenza season duration varies significantly: Activity might not occur in certain communities until February, March, or even later, making late vaccination still protective. 1

• October is optimal but not mandatory: While vaccination by the end of October is recommended for ideal timing, this represents a target rather than a deadline. 1

• Late vaccination remains beneficial: Vaccine administered in December or later provides meaningful protection even when influenza is already circulating. 1

Special Considerations for Vulnerable Populations

High-Risk Groups Who Should Prioritize Vaccination Regardless of Timing:

All persons aged ≥6 months without contraindications should receive annual influenza vaccination, with particular emphasis on those at increased risk for severe complications: 1

• Children aged 6-59 months (all young children) 1
• Adults aged ≥50 years 1
• Pregnant women at any stage of pregnancy during influenza season 1
• Persons with chronic conditions including:
  • Pulmonary disease (including asthma) 1
  • Cardiovascular disease (excluding isolated hypertension) 1
  • Diabetes mellitus and other metabolic disorders 1
  • Renal, hepatic, neurologic, or hematologic disorders 1
• Immunocompromised persons from any cause, including medication-induced or HIV infection 1
• Nursing home and long-term care facility residents 1

Vaccine Selection for Elderly Patients:

For adults aged ≥65 years, preferentially use one of the following higher-dose or adjuvanted vaccines: 1

• Quadrivalent high-dose inactivated influenza vaccine (HD-IIV4)
• Quadrivalent recombinant influenza vaccine (RIV4)
• Quadrivalent adjuvanted inactivated influenza vaccine (aIIV4)

If none of these three enhanced vaccines is available, any other age-appropriate influenza vaccine should be used rather than delaying vaccination. 1

Protection Against Multiple Circulating Strains

Providers should offer influenza vaccination even to persons who have already become ill with influenza during the current season, because the vaccine might protect them against other circulating influenza viruses. 1

This is particularly important because seasonal vaccines protect against four different influenza viruses (A[H1N1]pdm09, A[H3N2], B/Victoria lineage, and B/Yamagata lineage), and having illness from one strain does not confer protection against the others. 2

Clinical Pitfalls to Avoid

Common mistake: Telling patients it's "too late" for vaccination once December arrives or after local influenza activity begins. The evidence clearly contradicts this practice. 1

Waning immunity concern: While some studies suggest vaccine effectiveness may decline by approximately 6-11% per month for certain strains, vaccine effectiveness remains greater than zero for at least 5-6 months after vaccination. 1 This waning effect should not deter late-season vaccination, as protection during the remaining weeks or months of influenza season is still clinically meaningful.

Variable effectiveness: Even in seasons with suboptimal vaccine-virus matching (such as the 2021-22 season when effectiveness against A[H3N2] was not statistically significant at 16%), vaccination can still prevent serious outcomes including hospitalization, ICU admission, and death. 2

Practical Implementation

Continue offering and actively recommending influenza vaccination to all eligible patients throughout the entire influenza season, which can extend through May in some years. 1 Do not use calendar date or presence of local influenza activity as reasons to withhold vaccination from unvaccinated individuals.