Case Presentation: A 54-year-old woman with no past medical history who received her influenza vaccine 2 weeks prior presented with a sudden onset of profuse gum bleeding upon brushing her teeth. In the ED, the patient was found to have a platelet count of 3 K/μL which was 285 K/μL 3 months ago. She denied any family history or bleeding disorders. No other abnormalities in her lab test. Additionally, peripheral blood smear was negative for schistocytes and clumping. Her physical exam was only significant for some small hematomas in the oral cavity. One unit of platelets was transfused and a dose of prednisone was administered for her critically low platelet count. She was then started on 2 days of intravenous immunoglobulin (IVIG) drip for suspected immune thrombocytopenic purpura (ITP). HIV and hepatitis C viral tests were both negative. Her platelets improved to 26 K/μL on the second day, 61 K/μL the third, and 116 K/μL on the last. Oral prednisone 1mg/kg was started on day 2 as IVIG finished. Throughout the hospital course she remained asymptomatic and stable other than the appearance of some petechiae on the extremities. She was discharged with oral prednisone and referral for hematology follow-up. 

Discussion: ITP is a thrombocytopenic disorder in which autoantibodies directed against platelet antigens cause accelerated platelet destruction. The disorder may manifest itself with new-onset purpura or petechiae in the extremities, bleeding of the oral mucosa, epistaxis, or menorrhagia. Secondary ITP in the setting of viral infections likely occurs due to a mechanism of molecular mimicry in which antibodies that are directed toward epitopes of viral antigens cross-react with antigens present on the surface of platelets. There has been significant evidence of ITP in the setting of recent vaccination, most notably the live-attenuated measles-mumps-rubella vaccination for which there are as many as 4 cases per 100,000 doses. There have been case reports of ITP following influenza vaccination. A review of autoimmune disorders after Influenza A/H1N1 vaccination during the 2009-2010 pandemic identified 28 cases of ITP, making it the third most common associated autoimmune disease (after GBS and Rheumatoid arthritis).  It has been proposed that the hemagglutinin contained in influenza vaccine, the major glycoprotein on the surface of influenza virus, binds to the platelets which can trigger the complement cascade and platelet lysis. 

Conclusions: Our patient presented with new-onset bleeding and isolated thrombocytopenia following a recent influenza vaccination. We are reporting this case to remind providers to consider the association of recent influenza vaccination with ITP. The destruction of the platelets is possibly due to the binding of hemagglutinin in the vaccine to the surface of platelets. However, the benefit of influenza vaccination in preventing disease far outweighs the potential low risk of ITP as an adverse event.