How many weeks before the start of flu season should I get a flu shot (influenza vaccination)?

Optimal Timing for Influenza Vaccination

For most adults and children requiring only one dose, get your flu shot in September or October—ideally by the end of October—to maximize protection throughout the influenza season. 1

General Timing Recommendations

Standard Timeline

• Vaccination should be completed by the end of October for optimal protection, as this balances early immunity with concerns about potential waning later in the season 1
• Adults develop peak antibody protection approximately 2 weeks after vaccination 1
• Vaccination should continue throughout the entire influenza season as long as vaccine supplies are available, even after community influenza activity begins 1

Why October Matters

• In 75% of influenza seasons from 1982-2018, peak influenza activity did not occur until January or later 1
• In 58% of seasons, the peak was in February or later 1
• While localized outbreaks can begin as early as October, most seasonal activity peaks well into winter 1

Age-Specific Timing Strategies

Children Requiring Two Doses (Ages 6 months-8 years)

• Start vaccination as soon as vaccine becomes available, including July and August if accessible 1
• These children need their first dose early to allow the second dose (administered ≥4 weeks later) to be completed by the end of October 1, 2
• Two doses are required for children who have not previously received ≥2 total doses of influenza vaccine (trivalent or quadrivalent) ≥4 weeks apart before July 1st of the current season 2, 1

Children Requiring Only One Dose

• Vaccination during July and August can be considered for children of any age who need only one dose, particularly during back-to-school medical visits 1
• While waning immunity is a concern, capturing vaccination opportunities during routine visits may outweigh theoretical concerns about decreased late-season protection 1

Adults (Especially ≥65 Years)

• Avoid vaccination during July and August unless there is concern that later vaccination will not be possible 1
• Older adults show more pronounced waning of vaccine-induced immunity over the course of the season 1
• September or October vaccination is optimal for this population 1

Pregnant Persons

• Third trimester during July/August: Vaccination can be considered early because it provides protection to the infant during the first months of life when they are too young to be vaccinated 1
• First or second trimester during July/August: Waiting until September or October is preferable unless there is concern about missing the vaccination opportunity later 1
• Vaccination can be administered during any trimester once September arrives 1

Important Caveats

The Waning Immunity Concern

• Multiple observational studies have documented decreases in vaccine effectiveness within a single season as time from vaccination increases 1
• Waning appears more pronounced with influenza A(H3N2) viruses compared to A(H1N1) or influenza B viruses 1
• However, the clinical significance of waning must be balanced against the risk of missed vaccination opportunities if vaccination is delayed 1

When Vaccination After October Is Still Beneficial

• Continue offering vaccination in December and throughout the entire influenza season 1
• Vaccine administered in December or later is likely beneficial in the majority of influenza seasons because peak activity typically occurs in January-March 1
• Never turn away someone seeking vaccination late in the season—protection for the remaining weeks or months of influenza activity is valuable 1

Special Circumstances

• Nursing home residents: Avoid vaccination before October when possible, as antibody levels may decline more rapidly in this population 1
• Healthcare workers and high-risk contacts: Prioritize vaccination in October to ensure protection when caring for vulnerable populations 1

Practical Implementation

Avoid These Common Pitfalls

• Do not defer vaccination in October if someone requests it early—the risk of missing the opportunity outweighs theoretical concerns about early vaccination 1
• Do not revaccinate later in the season for persons who have already been fully vaccinated 1
• Do not delay vaccination to coordinate with other appointments or procedures—timely flu protection takes priority 3

Vaccination Campaign Scheduling

• Schedule organized vaccination campaigns after mid-October to ensure vaccine availability and minimize cancellations 1
• Campaigns should continue into December and January, particularly in settings serving high-risk populations 1