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When fever's gone, HCWs still shed virus
Outbreak shows course of H1N1 pandemic
Even when health care workers return to work after being ill with influenza, they still may be shedding viable virus. That is a finding from an analysis of a small outbreak of pandemic H1N1 in the fall of 2009.1
Viral shedding is unlikely to lead to transmission if the infected person does not have any symptoms, says Tim Uyeki, MD, MPH, MPP, a medical epidemiologist with the influenza division at the Centers for Disease Control and Prevention in Atlanta, who was not an author of the paper. But evidence of shedding underscores the importance of maintaining infection control measures, he says. "If you have no signs and symptoms, you're unlikely to be transmitting," he says.
A retreat for medical residents in Seattle in September 2009 offered an opportunity to investigate the transmissibility of H1N1. Thirty-two participants gathered for the five-day retreat and stayed in a cabin, joined at various times by 14 facilitators. On the last day of the retreat, September 25, one participant developed a cough.
By the next day, the participant was diagnosed with H1N1 pandemic influenza. Two days later, 19 participants and one facilitator had developed respiratory symptoms; influenza was confirmed in 17 of them.
Per the CDC recommendations at the time, the ill health care workers received anti-viral medication and were excluded from work for seven days. Sixteen of the 17 ill participants agreed to keep a daily symptom log, including oral temperature, complete a questionnaire, and perform a nasal wash three times a week until they had two consecutive negative results.
Fever, which is often used to determine when someone can return to work, was not associated with viral shedding, according to the study. In fact, four of the ill participants never reported a fever. (Cough, myalgias, and headache were the most common symptoms.)
While those with fever only reported having the fever for a day or two, the study found that viral shedding lasted 3 to 13 days after the onset of symptoms. (The shedding was longer when tested with real-time RT-PCR than with rapid culture.) Based on a return-to-work policy of being afebrile for 24 hours, 75% of the health care workers still had virus detected by PCR and 56% had virus detected by culture.
"These results raise essential considerations regarding exclusion policies for infected health care personnel," the authors concluded. "Because febrile and afebrile health care personnel had similar virologic shedding durations and viral loads, the absence of influenza by real-time RT-PCR or culture might be preferable to the absence of fever as a criterion for health care personnel who are returning to work in settings where they place others at high risk."
The authors note that viral shedding doesn't necessarily equate to transmissibility. "Persons who are not symptomatic are unlikely to be transmission risks," comments Uyeki. "But we need more data to understand the risk of transmission from someone who is asymptomatic but infected."
The lessons from this outbreak also might have limited application to seasonal influenza. In this case, no one had pre-existing immunity to the pandemic strain and the vaccine was not yet available.
But it still underscores the need to be vigilant about infection control, says Uyeki. "Annual influenza vaccination is the best way to prevent influenza and is recommended for all health care personnel," he says. "Nevertheless, influenza vaccine effectiveness varies from season to season and is not 100%. Even vaccinated persons may still develop influenza illness from influenza virus infection. That highlights the importance of infection control measures to prevent and control nosocomial transmission."
1. Kay M, Zerr DM, Englund JA, et al. Shedding of pandemic (H1N1) 2009 virus among health care personnel, Seattle, Washington, USA. Emerging Infectious Diseases 2011; [Epub ahead of print] Available at: http://1.usa.gov/dOwKdf.