Influenza C is not included in the annual seasonal influenza vaccine, and has historically been regarded as a minor respiratory pathogen. However, recent work has highlighted its potential role as a cause of pneumonia in infants. We performed nasopharyngeal or nasal swabbing and/or serum sampling (n=148) in Lancaster, UK, over the winter of 2014-2015. Using enzyme-linked immunosorbent assay (ELISA), we estimated a seropositivity of 77%. By contrast, only 2 individuals, both asymptomatic adults, were influenza C-positive by polymerase chain reaction (PCR). Deep sequencing of nasopharyngeal samples produced partial sequences for 4 genome segments in one of these patients. Bayesian phylogenetic analysis demonstrated that the influenza C genome from this individual is evolutionarily distant to those sampled in recent years and represents a novel genome constellation, indicating that it is a product of a decades-old reassortment event. Although we find no evidence that influenza C was a significant respiratory pathogen during the winter of 2014-2015 in Lancaster, we confirm previous observations of seropositivity in the majority of the population. We calculate that this level of herd immunity would be sufficient to suppress epidemics of influenza C and restricts the virus to sporadic endemic spread.

Influenza C incidence and herd immunity in Lancaster, UK, in the winter of 2014-2015