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Duration of Influenza A Virus Shedding Longer than Expected

By Luciana L. Borio, M.D., July 17, 2007

Adults infected with influenza A are generally considered infectious from the day before the onset of symptoms until about 5 days after onset.1 Longer periods of shedding have been described in children and immunocompromised patients. The estimated period of infectivity in adults guides current infection control guidelines for the prevention of influenza in acute care facilities, which recommend that patients hospitalized with suspected or confirmed influenza be placed under standard and droplet isolation precautions for 5 days after onset of symptoms. In addition, infected health care workers are encouraged to abstain from patient care for 5 days after onset of their symptoms.2 A recent study suggests that more prolonged isolation precautions may be necessary to prevent nosocomial outbreaks.

Surbhi Leekha and colleagues from the Mayo Clinic College of Medicine assessed the duration of viral shedding in 50 adult patients hospitalized with laboratory-confirmed influenza A virus infection at the Mayo Clinic hospitals during the 2004-2005 influenza season.3 Respiratory tract specimens from patients with symptoms of influenza-like illness were tested for infection by PCR. If the initial PCR assay was positive, throat swab samples were assayed at 2, 3, 5, and/or 7 days from the time the initial specimen was obtained, but only for patients who remained in the hospital.

Study patients tended to be older (a mean age of 72 years, and a median age of 76 years), and 48 (96%) had at least one underlying chronic medical illness. For the 47 patients for whom influenza vaccine status was ascertained, 36 (77%) had been vaccinated more than 2 weeks prior to influenza infection diagnosis. Twenty-seven patients (54%) received antiviral therapy, although the antiviral used was not specified. Forty-one patients were included in the final analysis.

Prolonged viral shedding was defined as “the detection of influenza A virus RNA by PCR at or beyond 7 days after symptoms onset.” Influenza A virus was detected in 22 (54%) by PCR, and in 12 (29%) by culture methods (both tube culture and a shell vial assay) at or beyond 7 days after onset of symptoms. Neither prior vaccination nor use of antiviral therapy was significantly associated with prolonged viral shedding. Mean duration of viral shedding was 5.7 days (median of 6 days) for those treated with antiviral therapy, and 7.5 days (median of 7 days) for those not treated with antivirals; these differences were not statistically significant.


Patients hospitalized with influenza A infection tend to be elderly, with underlying chronic medical illnesses, and could shed virus beyond the period traditionally considered for infectivity. If this is the case, then current CDC infection control guidelines may be insufficient to prevent nosocomial transmission during influenza season. What is not known is whether patients with influenza A virus detected by PCR are able to transmit the virus. Thus, additional studies, including surveillance of contacts, should be undertaken to assess whether prolonged viral shedding as determined by PCR testing is associated with continued infectivity.


  1. Harper SA, Fukuda K, Uyeki TM, Cox NJ, Bridges CB; Advisory Committee on Immunization Practices (ACIP), Centers for Disease Control and Prevention (CDC).Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2005;54:1-40.

  2. Tablan OC, Anderson LJ, Besser R, Bridges C, Hajjeh R; CDC; Healthcare Infection Control Practices Advisory Committee. Guidelines for preventing health-care--associated pneumonia, 2003: recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee. MMWR Recomm Rep 2004;53:1-36.

  3. Leekha S, Zitterkopf NL, Espy MJ, et al. Duration of influenza A virus shedding in hospitalized patients and implications for infection control. Infect Control Hosp Epidemiol 2007;28:000-000. DOI: 10.1086/520101