# 971
Conventional wisdom is that adults remain infectious from influenza for about 5 days after symptoms appear. Children, it is believed, may remain contagious several days longer.
Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick. -CDC Website on Flu
Of course, conventional wisdom has a way of changing over time. What we believe to be true today may turn out to be wrong tomorrow. Sixty years ago, some doctors actually endorsed cigarettes.
Now we get a study strongly suggesting that adults may remain infectious for longer than 5 days. Half of those in the study were shedding the virus after 7 days. How much longer? Well, it doesn't say.
Obviously we need more studies to determine how long people remain infectious after developing symptoms. Right now, we have evidence that it is at least 7 days.
Whether we are dealing with a pandemic, or seasonal influenza, it would be beneficial to know at what point a patient is no longer infectious.
Surbhi Leekha, MBBS, MPH, Nicole L. Zitterkopf, PhD, Mark J. Espy, MS, Thomas F. Smith, PhD, Rodney L. Thompson, MD, and Priya Sampathkumar, MD
Volume 28(2007), pages 000 - 000
DOI: 10.1086/520101Abstract
Objective. To assess the duration of shedding of influenza A virus detected by polymerase chain reaction (PCR) and cell culture among patients hospitalized with influenza A virus infection.
Setting. Mayo Clinic (Rochester, Minnesota) hospitals that cater to both the community and referral populations.
Methods. Patients 18 years old and older who were hospitalized between December 1, 2004, and March 15, 2005, with a laboratory-confirmed (ie, PCR-based) diagnosis of influenza A virus infection were consecutively enrolled. Additional throat swab specimens were collected at 2, 3, 5, and 7 days after the initial specimen (if the patient was still hospitalized). All specimens were tested by PCR and culture (both conventional tube culture and shell vial assay). Information on demographic characteristics, date of symptom onset, comorbidities, immunosuppression, influenza vaccination status, and receipt of antiviral treatment was obtained by interview and medical record review. Patients were excluded if informed consent could not be obtained or if the date of symptom onset could not be ascertained.
Results. Of 149 patients hospitalized with influenza A virus infection, 50 patients were enrolled in the study. Most patients were older (median age, 76 years), and almost all (96%) had underlying chronic medical conditions. Of 41 patients included in the final analysis, influenza A virus was detected in 22 (54%) by PCR and in 12 (29%) by culture methods at or beyond 7 days after symptom onset. All 12 patients identified by culture also had PCR results positive for influenza A virus.
Conclusion. Hospitalized patients with influenza A virus infection can shed detectable virus beyond the 5- to 7-day period traditionally considered the duration of infectivity. Additional research is needed to assess whether prolonging the duration of patient isolation is warranted to prevent nosocomial outbreaks during the influenza season.
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