Seasonality of infectious diseases and severe acute respiratory syndrome-what we don't know can hurt us

Abstract

The novel severe acute respiratory syndrome (SARS) coronavirus caused severe disease and heavy economic losses before apparently coming under complete control. Our understanding of the forces driving seasonal disappearance and recurrence of infectious diseases remains fragmentary, thus limiting any predictions about whether, or when, SARS will recur. It is true that most established respiratory pathogens of human beings recur in wintertime, but a new appreciation for the high burden of disease in tropical areas reinforces questions about explanations resting solely on cold air or low humidity. Seasonal variation in host physiology may also contribute. Newly emergent zoonotic diseases such as ebola or pandemic strains of influenza have recurred in unpredictable patterns. Most established coronaviruses exhibit winter seasonality, with a unique ability to establish persistent infections in a minority of infected animals. Because SARS coronavirus RNA can be detected in the stool of some individuals for at least 9 weeks, recurrence of SARS from persistently shedding human or animal reservoirs is biologically plausible.

Figure 1

Seasonal patterns of three established human respiratory pathogens: percentage of specimens testing positive for influenza viruses among specimens tested by WHO and US National Respiratory and Enteric Virus Surveillance System collaborating laboratories, month of peak incidence; weekly rates of invasive pneumococcal disease in the USA, January 1996–December 1998; and annual cycle of rubella activity, 1963–1969. Adapted from , , .

Figure 2

Seasonal patterns of two presumed zoonotic pathogens: recognised outbreaks of ebola, 1976–2003; and recognised outbreak of the SARS coronavirus. Compiled from , .