Most Flu Asymptomatic?

Tuesday, March 18, 2014 // Uncategorized

This is interesting. Many people who have the flu have no symptoms and might be infecting other people who then develop symptoms.  this is the summary from Journal Watch followed by the abstract from Lancet.

Influenza, whether of the seasonal or pandemic variety, is asymptomatic in most people with serologically confirmed infection, according to a study in the Lancet Respiratory Medicine.


Researchers studied five successive cohorts of people in England during the 2006-2011 flu seasons. The cohorts ranged in size from 600 to 3500, and all members provided blood samples before and after each flu season. Their households were contacted weekly to identify flu-like illness and symptoms.


On average, roughly 20% of the unvaccinated had serologic evidence of influenza infection, but up to three quarters of the infected were asymptomatic. The proportions did not vary significantly between seasonal and pandemic influenzas. The pandemic H1N1 strain was associated with less severe symptoms than the seasonal H3N2 strain.


A commentator says an important unanswered question is how much the asymptomatic cases contribute to flu transmission

Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study



Assessment of the effect of influenza on populations, including risk of infection, illness if infected, illness severity, and consultation rates, is essential to inform future control and prevention. We aimed to compare the community burden and severity of seasonal and pandemic influenza across different age groups and study years and gain insight into the extent to which traditional surveillance underestimates this burden.


Using preseason and postseason serology, weekly illness reporting, and RT-PCR identification of influenza from nasal swabs, we tracked the course of seasonal and pandemic influenza over five successive cohorts (England 2006—11; 5448 person-seasons’ follow-up). We compared burden and severity of seasonal and pandemic strains. We weighted analyses to the age and regional structure of England to give nationally representative estimates. We compared symptom profiles over the first week of illness for different strains of PCR-confirmed influenza and non-influenza viruses using ordinal logistic regression with symptom severity grade as the outcome variable.


Based on four-fold titre rises in strain-specific serology, on average influenza infected 18% (95% CI 16—22) of unvaccinated people each winter. Of those infected there were 69 respiratory illnesses per 100 person-influenza-seasons compared with 44 per 100 in those not infected with influenza. The age-adjusted attributable rate of illness if infected was 23 illnesses per 100 person-seasons (13—34), suggesting most influenza infections are asymptomatic. 25% (18—35) of all people with serologically confirmed infections had PCR-confirmed disease. 17% (10—26) of people with PCR-confirmed influenza had medically attended illness. These figures did not differ significantly when comparing pandemic with seasonal influenza. Of PCR-confirmed cases, people infected with the 2009 pandemic strain had markedly less severe symptoms than those infected with seasonal H3N2.


Seasonal influenza and the 2009 pandemic strain were characterized by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation. In the community the 2009 pandemic strain caused milder symptoms than seasonal H3N2.


Medical Research Council and the Wellcome Trust.

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