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