Seasonality and selective trends in viral acute respiratory tract infections

Influenza and many unrelated viruses, including rhinovirus, RSV, adenovirus, coronavirus etc. share the same seasonality, since these viral acute respiratory tract infections (vARIs) are much more common in winter than summer. The lack of a viable explanation is a major problem for microbiology. Unfortunately, early investigations that used recycled “pedigree” virus strains seem to have led microbiologists to dismiss the common folk belief that vARIs often follow chilling, together with the scientific evidence that supports this idea. Today, incontrovertible evidence from polar, tropical and island-based studies, PCR-based surveys, and studies of the effects of outdoor dress and activities, shows that low ambient temperature and host chilling increase the incidence and severity of vARIs. This review considers four possible explanations of this link: (1) increased crowding in winter may enhance viral transmission; (2) lower temperatures may increase the stability of virions outside the body; (3) lower temperatures may increase host susceptibility; (4) chilling may activate dormant virions. There is little evidence for the first two explanations, the second of which is incompatible with tropical observations of vARIs. Explanation 3 is supported by a recent study that found that the immune response of chilled mouse airway cells was diminished. However, tropical observations and epidemiological anomalies such as the simultaneous arrival of vARIs over wide geographical areas, the rapid cessation of influenza epidemics in midwinter, and the low attack rate of influenza within families are compatible with explanation 4, but not 3 (at least not in its simple form). Explanation 4 is also compatible the natural temperature sensitivity of many wild and laboratory strains, and the frequent recovery of temperaturesensitive mutants from persistent infections, because seasonality may be a consequence of natural temperature sensitivity, which is (presumably) essential to viral tropism. The evidence suggests that explanation 4 is the main driver of seasonality, but explanation 3 may also have an important role in seasonality.