Introduction: Influenza infection is common post-transplant. However, virologic characteristics such as viral load (VL) and duration of shedding have not been well studied in organ transplants. We conducted a prospective, multicenter study of influenza infection in transplant patients and characterized influenza virus kinetics.

Methods: Transplant recipients with laboratory-confirmed influenza infection were enrolled in a multicenter prospective clinical study from 2010-2012. A subset of consenting patients had serial nasopharyngeal swabs at baseline, day 3, 6, 11, 18 and 28. All swabs underwent quantitative RT-PCR for Influenza A and qualitative RT-PCR for influenza B at a central lab using a real-time assay. The lower limit of detection for the influenza A assay was 30 copies/5 ΜL of input specimen. Data including duration of shedding, peak VL, and viral clearance kinetics were analyzed.

Results: We enrolled 172 patients with influenza in the clinical study. Of these, 62 had serial NP swabs performed for virologic analysis (50 influenza A; 12 influenza B). Type of transplant were kidney (27), liver (9), lung (7), heart (11), and HSCT (8). Median time post-transplant to infection was 2.8 (0.04-23.8) years. The majority of patients (98.4%) received antiviral therapy. For patients with influenza A, day 0 VL was 9710 copies/input (range 30-22,700,000); the median peak VL was the same since most patients had the highest VL on day 0. Median time to clearance was 6 days (range 3-28). Detailed viral kinetic analysis was performed in 32 patients. Of these, in 26 patients (81.3%) there was a rapid clearance with a single phase exponential decline in VL with a median half life of 0.69 days (range 0.3-3). In 6 patients, there was an initial increase in viral load followed by a logarithmic decline. Patients requiring hospital admission had significantly higher initial viral loads (median 26850 vs 1430 copies; p=0.05). For influenza B, median time to clearance was 4.5 days (range 3-18).

Conclusion: Our study provides novel insights into influenza virus kinetics in transplant recipients treated with antiviral therapy. Viral clearance is highly dynamic. Higher viral loads were associated with need for hospitalization.

Kumar, D.: Grant/Research Support, Roche. Humar, A.: Grant/Research Support, Roche.

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