Thursday, May 12, 2016

EID Journal: Prevalence of Asymptomatic Influenza Virus Infections

Credit NIAID


One of the topics we tend to come back to fairly often is the asymptomatic spread of respiratory diseases like influenza, RSV, or even MERS-CoV.

Last year, in CID: Viral Detection Increases With Family Size, we saw a study that followed 26 households over the course of one year, testing family members for 16 different respiratory viruses, including influenza, rhinovirus, and RSV on a weekly basis.
While the incidence varied widely by household size (no surprise, the more kids in the house, the more sickness), across the board only about 50% of the PRC positive cases reported cold or flu-like symptoms. 

Serological studies have shown that a high percentage of people who are infected with the flu either experience mild, or sub-clinical symptoms.  In 2014 the Lancet: Community Burden & Severity Of Seasonal And Pandemic Influenza found over five years that as many as 75% of those who showed serological evidence of infection reported no significant influenza symptoms.

Not necessarily asymptomatic carriage, but mild illness nonetheless.  

Last week we looked at the possible role of host genetics in influenza severity, and I mentioned a study from 2015 by the University College London that found a high percentage (43%) of test subjects carried a natural T-Cell that targeted the influenza virus nucleoprotein (see Natural T Cell–mediated Protection against Seasonal and Pandemic Influenza) resulting in fewer, and milder flu infections.

Despite these studies (and more), the mystery of why some people are lucky enough to get by with a mild or asymptomatic flu infection, while others are poleaxed by the virus, remains far from answered.

Today we've a research article in the EID Journal - with a pedigree that includes Flublogia's very own Dr. Ian Mackay -  that provides a systemic review and meta-analysis of 55 existing studies to better describe the asymptomatic carriage of influenza.

The authors noted a huge diversity in findings, with asymptomatic rates ranging from 5.2% to 35.5% and subclinical cases (mild illness) ranging from 25.4% to 61.8%.

These vexingly disparate findings could not be explained away by differences in influenza subtypes, the testing methods employed, location, or timing of the tests.  

They do, however, attest to the unpredictability of influenza viruses in general and to public health's enormous challenge containing influenza outbreaks in particular.  

Follow the link to read the article in its entirety. 

Volume 22, Number 6—June 2016 


Heterogeneous and Dynamic Prevalence of Asymptomatic Influenza Virus Infections

Luis Furuya-Kanamori, Mitchell Cox, Gabriel J. Milinovich, Ricardo J. Soares Magalhaes, Ian M. Mackay, and Laith YakobComments to Author


Influenza infection manifests in a wide spectrum of severity, including symptomless pathogen carriers. We conducted a systematic review and meta-analysis of 55 studies to elucidate the proportional representation of these asymptomatic infected persons.

We observed extensive heterogeneity among these studies. The prevalence of asymptomatic carriage (total absence of symptoms) ranged from 5.2% to 35.5% and subclinical cases (illness that did not meet the criteria for acute respiratory or influenza-like illness) from 25.4% to 61.8%. Statistical analysis showed that the heterogeneity could not be explained by the type of influenza, the laboratory tests used to detect the virus, the year of the study, or the location of the study. 

Projections of infection spread and strategies for disease control require that we identify the proportional representation of these insidious spreaders early on in the emergence of new influenza subtypes or strains and track how this rate evolves over time and space.

Our study clearly demonstrates the inappropriateness of a one-size-fits-all approach to mitigating the spread of human influenza viruses. As new subtypes and strains emerge, actively surveying infection status of local populations and tracking any changes in asymptomatic rates of infection should increasingly become a global health priority, possibly necessitating the provision of international resources and the deployment of dedicated rapid-response teams who are guided by standardized protocols.

Mr. Furuya-Kanamori is an infectious disease epidemiologist. He is enrolled in a PhD program at the Australian National University where he uses modern quantitative methods to better understand risk factors associated with different infectious diseases.