Please provide the dates on which Elly Koss was immunized with the influenza seasonal injectable preservative-free vaccine.
date of immunization refers to immunizations.DATE; immunized with the influenza seasonal injectable preservative-free vaccine refers to immunizations.DESCRIPTION = 'Influenza seasonal injectable preservative free';
SELECT T2.DATE FROM patients AS T1 INNER JOIN immunizations AS T2 ON T1.patient = T2.PATIENT WHERE T2.DESCRIPTION = 'Influenza  seasonal  injectable  preservative free' AND T1.first = 'Elly' AND T1.last = 'Koss'