Yes, a 6-year-old boy can receive a trivalent influenza vaccine as the second dose after receiving a quadrivalent vaccine as the first dose.
Vaccination should not be delayed to obtain a specific product for either dose, and any available age-appropriate trivalent or quadrivalent vaccine can be used interchangeably. 1
Evidence-Based Rationale
Explicit Guideline Support for Mixing Formulations
The American Academy of Pediatrics and CDC explicitly state that different brands and formulations (trivalent vs. quadrivalent) can be used for different doses in the same season. 2
Both the Advisory Committee on Immunization Practices (ACIP) and American Academy of Pediatrics recommend administering whichever formulation is available in the community, with neither vaccine formulation preferred over the other. 1
The guidelines emphasize that vaccination should not be delayed to obtain a specific product, as timely vaccination is more important than brand or formulation consistency. 1, 2
Dosing Requirements for This 6-Year-Old
Since this child is 6 years old (within the 6 months through 8 years age group), the number of doses needed depends on vaccination history, not formulation matching. 1
If this child has previously received ≥2 total doses of any trivalent or quadrivalent influenza vaccine ≥4 weeks apart before July 1 of the current season, only 1 dose is needed for this season. 1
If this child has received fewer than 2 doses of any influenza vaccine before July 1 of the current season, 2 doses are required this season (administered ≥4 weeks apart), and these doses can be different formulations. 1
Important Caveat About Influenza B Coverage
A child who receives only 1 of the 2 doses as a quadrivalent formulation is likely to be less primed against the additional influenza B virus (the fourth strain present only in quadrivalent vaccines). 1
However, this does not contraindicate mixing formulations—it simply means the child will have received priming for three strains instead of four. 1
The trivalent vaccine still provides protection against the most critical influenza strains (two influenza A strains and one influenza B strain). 1
Why Formulations Are Interchangeable
All influenza vaccines contain the same antigenic composition each season for the shared strains, making them functionally interchangeable within age-appropriate products. 2
The key consideration is age-appropriateness, not brand or formulation consistency—any licensed, age-appropriate vaccine is acceptable. 2
Previous doses from any season and any formulation count toward completion of the recommended series, and brand/formulation switching does not require restarting the series. 2
Common Pitfalls to Avoid
Do not delay vaccination waiting for a specific formulation—timely protection is more important than receiving identical products for both doses. 1, 2
Do not assume that mixing formulations invalidates the vaccination series—both trivalent and quadrivalent vaccines are considered valid doses. 1
Do not confuse the formulation issue with the age-based dosing algorithm—the critical factor is whether the child has received ≥2 total doses of any influenza vaccine in their lifetime (before July 1 of the current season), not whether those doses were the same formulation. 1