A. Background. Influenza A(H1N1) causes serious complications in immunocompromised patients. The efficacy of seasonal influenza vaccination in transplant recipients and patients receiving chemotherapy has been questioned. Our aim is to describe two outbreaks of influenza A(H1N1) in immunocompromised patients.

B. Methods. Two outbreaks of influenza A(H1N1) occurred in our institution: on the kidney transplant ward in 2014 among patients early after kidney or simultaneous pancreas-kidney transplantation, and on the oncology ward in 2016 among patients receiving chemotherapy for malignant tumors, including recipients of high dose chemotherapy with autologous stem cell transplantation. Factors leading to these outbreaks and the clinical efficacy of seasonal influenza vaccination were analyzed.

C. Results. Altogether 86 patients were exposed to influenza A(H1N1) during the outbreaks, among whom the seasonal influenza vaccination status was unknown in ten. Only 3/38 (8%) vaccinated patients were infected with influenza A(H1N1), compared to 20/38 (53%) unvaccinated patients (P=0.02). None of the vaccinated patients died directly due to influenza, whereas 6/38 of the unvaccinated patients died due to pneumonia caused by influenza (P=0.07). In addition, one vaccinated and one unvaccinated patient had to stop chemotherapy due to influenza and died due to progressive lymphoma. Shared factors behind the two outbreaks included old fashioned facilities not designed for the treatment of immunosuppressed patients, with lack of adequate isolation facilities and limited number of single rooms, and short distance between beds (90 – 120 cm) in other rooms. Vaccination coverage among patients was low, between 40-70% despite vaccination being offered to all patients free of charge. Vaccination coverage of health care workers on the transplant ward was low (46%), but, despite high coverage on the oncology ward (92%) the outbreak occurred.

D. Conclusions. Seasonal influenza vaccination was clinically effective with both a reduced risk of influenza infection and a trend towards reduced mortality in these immunocompromised patients. Several possible causes were identified behind these two outbreaks that require continuous awareness in health care professionals to prevent further outbreaks.

CITATION INFORMATION: Helanterä I., Janes R., Anttila V-.J. The Clinical Efficacy of Seasonal Influenza Vaccination – Characteristics of Two Outbreaks of Influenza A(H1N1) in Immunocompromised Patients Am J Transplant. 2017;17 (suppl 3).

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