31 January 2007
WHO H5N1 Flu Investigation Guidelines: 7 Days is the Patient’s Incubation Period Upper Limit, and -1 to +14 Days is the Time Frame for Contact Tracing
On January 29th the World Health Organization posted on their avian flu website a 15-page revision of their October 2006 document titled “WHO Guidelines for investigation of human cases of avian influenza A (H5N1). This update is particularly timely given that WHO announced today on their website that the government of Nigeria has reported their first human infection with H5 avian flu (confirmation pending).
Of note, for essential diagnostic, contact tracing, isolation and quarantine purposes this document states (page 4 last sentence) that “Based on currently available information and for purposes of conducting investigations, 7 days is considered to be the usual upper limit of the incubation period for human cases of A (H5N1)”. For example, this 7 day upper limit for the incubation period explains the WHO guidance to take a “travel history: for the 7 days before the onset of illness symptoms, asking particularly about contact within those 7 days with possibly H5N1-infected animals or humans.
An important distinction should be made, however, between this 7 day incubation period upper limit and the medically unrelated WHO guidance (middle of page 7) that contact tracing“ should focus on persons who had close unprotected (i.e., were not wearing PPE) contact with the case patient in the 1 day before through 14 days after the case patient’s illness onset.”
WHO states that the rationale for choosing the 1-day before until 14 days after illness onset is as follows (footnote on page 6):
(A): “Based on seasonal influenza studies which indicate virus shedding typically begins the day before symptom onset, comparable data are not available for H5N1”;
(B): “H5N1 virus is commonly detected in respiratory specimens during the first 2 weeks after illness onset. Although virus can be detected during the third week of illness for some patients with H5N1-related pneumonia, such patients would likely be hospitalized and isolated”.
Thus, the 7 days upper limit for an individual person’s incubation period from exposure to onset of symptoms, as advised by the WHO, is distinct from, and unrelated to, the 14 days upper limit for the number of days after illness onset in which contact tracing for potentially H5N1-exposed persons should be based. Given that 14 is the next multiple of 7 the risk for confusion exists in distinguishing between the upper limit of time for the incubation period and, separately, the operational upper limit of time recommended for contact tracing.
Lastly, 2 weeks (14 days) appears in the new WHO guidelines in one additional distinct situation, namely “enhanced surveillance” (page 10): “The duration of enhanced surveillance activities will need to be assessed for each investigation but typically would be expected to be undertaken for a minimum of 2 weeks (i.e., two incubation periods) after the last human case is identified.”
Understanding the rationale for the 7 day and 14 day (2 week) guidelines for each of the three specific situations described in this WHO document (on pages 4, 6, and 10) is essential to applying correctly these time periods to the investigation of human cases of H5N1 avian influenza.
Daniel R. Lucey, MD, MPH
Director, Center for Biologic Counterterrorism and Emerging Diseases
Washington Hospital Center EROne Institute, Dept of Emergency Medicine
Co-Director, Georgetown University M.S. Program in
Biohazardous Threat Agents and Emerging Infectious Diseases
Email: Daniel R. Lucey@Medstar.net