12 December 2005
Early online publication of two papers from the WHO slated for the January 2006 edition of the Emerging Infectious Diseases (EID) CDC journal (www.cdc.gov home page) provide useful literature and policy reviews for “nonpharmaceutical interventions for pandemic influenza”. The first paper addresses international measures and the second national and community measures.
A selection of key points emphasized relevant to data review on influenza virus transmission in these WHO documents follows. Later discussion will focus on the extrapolation of these data to recommendations by WHO regarding nonpharmaceutical interventions.
During the incubation period of influenza persons with asymptomatic (or “presymptomatic”) infection “shed virus at lower titers than persons with symptoms; however, the infectiousness of those with presymptomatic infection has not been studied”. They cite one study of adults that occurred in 1991 as “apparently the only published report implicating transmission during the incubation period”; it involved 26 adults in New Zealand and was published in the journal Communicable Disease New Zealand 1992; 92:18-19.
In healthy adults, titers of infectious influenza virus peak during the first 24-72 hours of illness and decline within several days with low or undetectable titers by day 5. However, shedding in severely immunocompromised adults “may last weeks to months”. Children also shed the highest amount of virus for 1-3 days after symptoms start and the “median duration of virus detection is typically 7-8 days after illness onset, but shedding up to 21 days has been noted”. “In both children and adults, shedding does not usually continue once illness has resolved. Serologic testing indicates that ~ 30%-50% of seasonal influenza infections may not result in illness”.
The predominant mode of influenza transmission is via virus-laden large particles when persons cough or sneeze, but evidence also exists for aerosol transmission by smaller particles. “The precise proportion of infections transmitted by large droplets versus aerosols is difficult to assess and likely depends on the setting but is relevant when developing recommendations on mask use. Data do not exist to quantify the relative efficacy of surgical masks versus respirators in preventing influenza infections in exposed persons, but surgical masks should protect against large droplets, believed to be the major mode of transmission.
“Transmission of influenza viruses by contaminated hands, other surfaces, or fomites has not been extensively documented but is believed to occur…In an environmental study, influenza virus placed on hard, nonporous surfaces (steel and plastic) could be cultured from the surfaces at diminishing titer for < 24 to 48 hours and from cloth, paper, and tissue for < 8 to 12 hours”…Virus on nonporous surfaces could be transferred to hands 24 hours after the surface was contaminated, while tissues could transfer virus to hands for 15 minutes after the tissue was contaminated. On hands, virus concentration fell by 100 to 1000-fold within 5 minutes after transfer.”
“The mean incubation period for influenza averages 2 days (range 1-4 days), and the serial interval (the mean interval between onset of illness in 2 successive patients in a chain of transmission) is 2-4 days. Also, viral excretion peaks early in illness. These factors enable influenza to spread rapidly through communities. By contrast, SARS has a serial interval of 8 to 10 days, and peak infectivity does not occur until week 2 of the illness, which allows more time to effectively implement isolation and quarantine measures.”
Daniel R. Lucey, MD, MPH
Director, Center for Biologic Counterterrorism and Emerging Diseases
ER One Institutes, Washington Hospital Center
Co-Director, Masters of Science Program in Biohazardous Threat Agents and
Emerging Infectious Diseases, Georgetown Medical School, Washington, DC.
Website: www.BePast.org. E-mail: Daniel.R.Lucey@Medstar.net