Does a 6-month-old baby, born to a mother vaccinated for the current influenza (flu) season, have sufficient passive immunity to last through the remainder of the flu season, or should they receive the influenza vaccine this season?

Vaccinate the Baby Now—Do Not Wait Until Next Season

The 6-month-old should receive influenza vaccine immediately this season (January) and cannot rely on passive immunity from maternal vaccination to provide adequate protection through the remainder of flu season. 1

Why Passive Immunity Is Insufficient

While maternal influenza vaccination during pregnancy does provide some transplacental antibody transfer to infants, this protection is limited and wanes over the first months of life. 1 The practice of maternal vaccination is intended to help protect infants younger than 6 months who are too young to be vaccinated themselves, but once the infant reaches 6 months of age, they become eligible for direct vaccination and should receive it. 1

Infants cannot rely solely on maternal antibodies beyond 6 months of age because:

• Maternal antibodies transferred during pregnancy decline over time and may not provide adequate protection through an entire flu season 2
• Influenza seasons are unpredictable and can extend into March, April, or even late May, meaning protection is needed for several more months 1
• The infant is now at the age where direct vaccination is both licensed and recommended 1

Vaccination Schedule for This 6-Month-Old

This child requires 2 doses of influenza vaccine this season, administered at least 4 weeks apart. 1, 3

Since this is the infant's first influenza vaccination season, the two-dose schedule is essential:

• First dose: Administer immediately (now in January) 1, 3
• Second dose: Administer at least 4 weeks after the first dose 1, 3, 4
• Both doses should ideally be completed as soon as possible, though vaccination should continue even late in the season 1

The two-dose requirement exists because vaccine-naïve children aged 6 months through 8 years do not produce adequate protective antibody responses from a single dose. 3, 4

Why Vaccination Cannot Wait Until Next Season

Delaying vaccination until next season leaves the infant unprotected during the current flu season, which poses significant risk:

• Infants under 12 months have hospitalization rates from influenza comparable to or exceeding those of elderly adults 5, 6
• Previously healthy children with influenza in their first year of life have substantial risk for developing serious disease 6, 7
• The current flu season can extend through spring (March-May), meaning 4-6 more months of potential exposure 1
• Peak influenza activity in the United States typically occurs January through March, meaning the infant is entering the highest-risk period now 1

Dosing Considerations

For a 6-month-old infant, either 0.25 mL or 0.5 mL dose of age-appropriate inactivated influenza vaccine (IIV) may be used, depending on the specific product available. 3 The vaccine should be administered intramuscularly into the anterolateral thigh. 3

Common Pitfalls to Avoid

• Do not assume maternal vaccination provides sufficient protection once the infant reaches 6 months—direct vaccination is now indicated 1
• Do not delay the first dose—begin immediately even though it's already January, as flu season extends for months 1
• Do not fail to complete the two-dose series—a single dose provides inadequate protection in vaccine-naïve children 3, 4
• Do not wait until next season—this leaves the infant vulnerable during 4-6 months of ongoing flu circulation 1

Additional Protection Strategy

While vaccinating the infant directly, ensure all household contacts, siblings, and caregivers are also vaccinated to provide additional "cocooning" protection. 1 However, this does not substitute for direct infant vaccination once the child reaches 6 months of age.