Longitudinal clearance of seasonal influenza A viral RNA measured by real-time polymerase chain reaction in patients identified at a hospital emergency department

To study influenza virus shedding during acute infection, viral load was longitudinally measured by quantitative PCR in nasal flocked swabs from patients with seasonal H3N2 influenza at a Swedish emergency department, including both hospitalized patients and outpatients. Influenza A was detected in 65/184 patients. Sampling was repeated every 3-4 days in 45 patients, with the aim of continuing sampling until day 12 after disease onset. Home visits were offered. Antibodies were measured on paired sera in 95/184 patients. Fifty percent of the patients remained polymerase chain reaction (PCR)positive 8 days after disease onset in a Kaplan-Meier survival curve. The longest observed duration of viral shedding was 12 days. The average viral load was initially low, peaked on days 2-3 of disease and then declined. Viral decline results remained similar when all 15 (25%) oseltamivir-treated patients were excluded. Significant antibody titre changes were seen in all the 35 PCR verified cases with available paired sera and in 8 of the 58 patients with negative PCR tests on acute phase nasal samples. In conclusion, quantitative PCR testing indicated the presence of influenza virus for up to 12 days, which could have implications for disease transmission and infection control.