Flu: most cases are without typical symptoms

Flu contagions include many more people whose immune systems are managing the virus without expressing recognizable flu symptoms. How often might patients report transient episodes of increased inflammation, disturbed sleep, anxiety or other manifestations of sympathetic nervous system hyperarousal and increased pro-inflammatory cytokines without a clearly apparent triggering cause? A study just published in The Lancet Respiratory Medicine implies that "fighting of a virus" could indeed be the instigator since their data show that approximately 75% of individuals infected with the flu virus are asymptomatic in the usual sense.The authors' intent was to evaluate traditional public health surveillance methods for seasonal and pandemic influenza by comparing infected individuals identified with DNA amplification (RT-PCR) and symptom profiles. Apparently most peoples' immune systems were holding the virus at bay:

"Based on four-fold titre rises in strain-specific serology, on average influenza infected 18% 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, suggesting most influenza infections are asymptomatic. 25% of all people with serologically confirmed infections had PCR-confirmed disease. 17% of people with PCR-confirmed influenza had medically attended illness. These figures did not differ significantly when comparing pandemic with seasonal influenza."

Clinical note: Simply arousing the immune system to deal with a virus can trigger an a flare-up of autoimmune inflammation or other complaints.An editorial in the same journal offers some interesting observations about transmissibility :

"In view of the undoubtedly high rates of subclinical influenza infection, an important unanswered question is the extent to which mild and asymptomatic influenza infections contribute to transmission. Case-ascertained household transmission studies have shown substantial heterogeneity in the amount and duration of viral shedding and, if the area under the curve of viral shedding is believed to correlate with transmissibility, suggest that 80% of transmission is attributable to 20% of clinically symptomatic cases. However, these estimates are not adjusted for differing contact patterns of sick and well individuals. What is more, serologically defined infection rates are underestimates, because the widely used four-fold or greater rise in haemagglutination inhibiting (HI) antibody titre between paired serum samples is an overly stringent criterion for defining infection in epidemiological studies and a proportion of infected individuals do not produce an appreciable HI antibody response. A large number of well individuals mixing widely in the community might, even if only mildly infectious, make a substantial contribution to onward transmission. This might have important implications for the effectiveness of case isolation and social distancing measures in reducing overall transmission rates."

Observe that people vary significantly in how robustly they produce antibodies of interest, an extremely important factor in autoimmune diagnosis and case management. Also interesting in this study there was about a one in five chance of infection if not vaccinated. The authors conclude:

"Seasonal influenza and the 2009 pandemic strain were characterised by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation."