Remarkably, our understanding of exactly how – and under what conditions – influenza is transmitted from one person to another is still limited.
We know that large droplets expelled by coughing and sneezing, along with smaller aerosolized virus particles that may remain airborne longer, plus contaminated surfaces (fomites) all play a role . . . but exactly how much each contributes has not been determined.
Similarly, it is believed that asymptomatic carriers of the virus may be able to transmit the virus to others despite not showing symptoms themselves, as well as those who are incubating the virus but are still presymptomatic.
But scientific evidence remains scant.
All of this is terribly important – particularly during an influenza pandemic – because if true, calls to isolate those showing symptoms wouldn’t stop its transmission.
Today, in the CDC’s EID Journal we have a dispatch that examines early transmission of the 2009 H1N1 virus in Japan, and finds evidence suggesting transmission of the virus during a carrier’s presymptomatic phase.
Yoshiaki Gu, Nobuhiro Komiya, Hajime Kamiya, Yoshinori Yasui, Kiyosu Taniguchi, and Nobuhiko Okabe
Author affiliation: National Institute of Infectious Diseases, Tokyo, Japan
During an epidemiologic investigation of pandemic influenza (H1N1) 2009 virus infection in May 2009 in Osaka, Japan, we found 3 clusters in which virus transmission occurred during the presymptomatic phase. This finding has public health implications because it indicates that viral transmission in communities cannot be prevented solely by isolating symptomatic case-patients.
Three clusters of pandemic (H1N1) 2009 presymptomatic transmission in May 2009 in Osaka, Japan. All cases were confirmed as pandemic (H1N1) 2009 virus infection by real-time reverse transcription PCR. Squares indicate male case-patients, and circles indicate female case-patients. Colors of the squares and circles denote the similar or different schools the students attended.
During the 2009 H1N1 pandemic Japan, India, and China were among the nations that attempted to identify, interdict, and isolate those who might be carrying the H1N1 virus when they entered their country.
While their efforts may have slowed the introduction of the virus, they certainly didn’t stop it. Earlier blogs on these attempts include:
Their failure was likely due to the large number of presymptomatic, and asymptomatic carriers of the virus that arrived without showing signs of illness.
It has been suggested that areas that receive a small number of arrivals might be able to institute a quarantine system (see Can Island Nations Effectively Quarantine Against Pandemic Flu? ), but even then their ability to interdict infected travelers wouldn’t be 100%.
The authors of today’s study conclude by writing:
Our epidemiologic results provide useful clues for understanding the transmission of pandemic (H1N1) 2009 virus. This finding of presymptomatic transmission has critical implications for public health because it indicates that viral transmission in communities cannot be completely prevented solely by isolating symptomatic case-patients.
Further proof, that with influenza, there are no easy answers.