Influenza vaccination status is not associated with influenza testing among children: implications for observational studies of vaccine effectiveness.

Estimates of influenza vaccine effectiveness from observational studies that rely on physician-ordered influenza tests may be biased if physician testing behavior is influenced by patient vaccination status. To assess the potential for differential diagnostic testing of children by vaccine status, we examined the association between receipt of a commercial influenza diagnostic test and influenza vaccination among children aged 6-59 months who sought care at the Marshfield Clinic for acute respiratory or febrile illnesses during the 2004-05 through 2007-08 influenza seasons. There was no significant association between prior influenza vaccination and receipt of a diagnostic test for influenza. These findings suggest that estimates of vaccine effectiveness derived from observational studies among children are unlikely to be biased due to differential diagnostic testing.