Early Occurrence of Influenza A Epidemics Coincided With Changes in Occurrence of Other Respiratory Virus Infections

Liselotte van Asten; Paul Bijkerk; Ewout Fanoy; Annemarijn van Ginkel; Anita Suijkerbuijk; Wim van der Hoek; Adam Meijer; Harry Vennema

Disclosures

Influenza Resp Viruses. 2016;10(1):14-26. 

In This Article

Abstract and Introduction

Abstract

Background Viral interaction in which outbreaks of influenza and other common respiratory viruses might affect each other has been postulated by several short studies. Regarding longer time periods, influenza epidemics occasionally occur very early in the season, as during the 2009 pandemic. Whether early occurrence of influenza epidemics impacts outbreaks of other common seasonal viruses is not clear.

Objectives We investigated whether early occurrence of influenza outbreaks coincides with shifts in the occurrence of other common viruses, including both respiratory and non-respiratory viruses.

Methods We investigated time trends of and the correlation between positive laboratory diagnoses of eight common viruses in the Netherlands over a 10-year time period (2003–2012): influenza viruses types A and B, respiratory syncytial virus (RSV), rhinovirus, coronavirus, norovirus, enterovirus, and rotavirus. We compared trends in viruses between early and late influenza seasons.

Results Between 2003 and 2012, influenza B, RSV, and coronavirus showed shifts in their occurrence when influenza A epidemics occurred earlier than usual (before week 1). Although shifts were not always consistently of the same type, when influenza type A hit early, RSV outbreaks tended to be delayed, coronavirus outbreaks tended to be intensified, and influenza virus type B tended not to occur at all. Occurrence of rhinovirus, norovirus, rotavirus, and enterovirus did not change.

Conclusion When influenza A epidemics occured early, timing of the epidemics of several respiratory winter viruses usually occurring close in time to influenza A was affected, while trends in rhinoviruses (occurring in autumn) and trends in enteral viruses were not.

Introduction

It has been suggested that annual epidemics of different viral infections can interfere with each other, but clear trends over long time periods and underlying mechanisms are not known.[1–4] A few population-level studies in Europe were based on observations in one respiratory season only (the 2009 H1N1 pandemic) in which the annually recurring influenza epidemic occurred relatively early. With the occurrence of several early influenza A seasons in recent years, an exploration of longer time trends of different viruses was considered useful to gain more insight in the suggested relationship between circulating viruses. Understanding viral shifts and potential drivers thereof is relevant for further understanding whether certain viruses might promote or inhibit (pandemic) influenza spread and whether influenza vaccination could potentially affect trends in other respiratory viruses.

In Europe, influenza epidemics generally occur in winter, with the official start of the epidemic when influenza-like illness (ILI) incidence in primary care sentinel surveillance exceeds an epidemic threshold (in combination with influenza A virus detection in clinical specimens collected from a subset of those ILI patients).[5] In the Netherlands, the influenza epidemic threshold has been calculated at an ILI incidence of 5·1/10 000 for minimally two consecutive weeks which is usually not exceeded before the turn of the year,[5–8] but the timing of the first exceedance (i.e., start of the epidemic) can vary between November and March (based on data from 1970 to 2006).[9] An extremely early influenza season was the 2009/2010 season when the influenza A(H1N1)pdm09 pandemic strain appeared and the ILI epidemic threshold was reached by early October (week 41). Such early occurrence may affect the circulation of other seasonal pathogens, and theories on possible interference between outbreaks of different respiratory viruses have been postulated to be a possible cause of delays in expected seasonal outbreaks of other respiratory viruses.[1–3] While those earlier population-level studies focused mainly on the possible interaction between influenza A virus and rhinovirus circulation, there may also be a relationship between influenza A and other prevalent viruses. Therefore, we investigated trends in several common viruses for which laboratory data were available from national surveillance in the Netherlands for a longer time period of up to 10 years including both respiratory and enteral viruses.

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