Haematological changes associated with influenza vaccination in people aged over 65: case report and prospective study.

Blood count abnormalities are a recognized feature of many viral infections and immunizations but little is known about the haematological effects of influenza vaccination. We report a 67-year-old patient who developed thrombocytopenia and severe neutropenia 3 weeks after she was vaccinated against influenza. The case led us to study prospectively the blood counts of 70 people aged over 65 before and after they received influenza vaccine. There were no significant changes in the levels of haemoglobin, neutrophils, monocytes, eosinophils or platelets after vaccination, but the total WBC counts (mean +/- SD, 6.86 +/- 1.52) and lymphocyte counts (1.69 +/- 0.61) were significantly lower at 4 weeks than at baseline (7.22 +/- 1.60 [P = 0.02] and 1.86 +/- 0.62 [P = 0.001] respectively) and in four subjects the lymphocyte count fell to below 0.7 x 10(9)/l. Since influenza vaccine does not contain live virus, its haematological effects presumably relate to the host immune response rather than to viral replication.