Seasonal Influenza Epidemics and El Niños

Abstract

Seasonal influenza epidemics occur annually during the winter in the northern and southern hemispheres, but timing of peaks and severity vary seasonally. Low humidity, which enhances survival and transmission of influenza virus, is the major risk factor. Both El Niño and La Niña phases of El Niño-southern oscillation (ENSO), which determine inter-annual variation of precipitation, are putative risk factors. This study was done to determine if seasonality, timing of peak, and severity of influenza epidemics are coupled to phases of ENSO. Monthly time series of positive specimens for influenza viruses and of multivariate El Niño-Southern Oscillation Index from January 2000 to August 2015 were analyzed. Seasonality, wavelet spectra, and cross-wavelet spectra analyses were performed. Of 31 countries in the dataset, 21 were in the northern hemisphere and 10 in the southern hemisphere. The highest number of influenza cases occurred in January in the northern hemisphere, but in July in the southern hemisphere, p < 0.0001. Seasonal influenza epidemic was coupled to El Niño, while low occurrence was coupled to La Niña. The moderate La Niña of 2010–2011 was followed by weak seasonal influenza epidemic. The influenza pandemic of 2009–2010 followed the moderate El Niño of 2009–2010, which had three peaks. Spectrograms showed time-varying periodicities of 6–48 months for ENSO, 6–24 months for influenza in the northern hemisphere, and 6–12 months for influenza in the southern hemisphere. Cross spectrograms showed time-varying periodicities at 6–36 months for ENSO and influenza in both hemispheres, p < 0.0001. Phase plots showed that influenza time series lagged ENSO in both hemispheres. Severity of seasonal influenza increases during El Niño, but decreases during La Niña. Coupling of seasonality, timing, and severity of influenza epidemics to the strength and waveform of ENSO indicate that forecast models of El Niño should be integrated into surveillance programs for influenza epidemics.

Evaluation of the ECMWF Sub-seasonal to Seasonal Precipitation Forecasts during the Peak of West Africa Monsoon in Nigeria

“…(Null, 2013;Oluwole, 2015) during the months of August which is the peak of the boreal summer monsoon. Here, 10 ensemble members from the hind-cast dataset are utilized.…”

Deterministic Chaos, El Niño Southern Oscillation, and Seasonal Influenza Epidemics

“…Influenza epidemics occur annually during the winter in northern and southern hemispheres (Oluwole, 2015). Occurrence is attributed to seasonal changes in virulence, transmission, and survival of influenza viruses, as well as seasonal changes in host immunity and behavior like overcrowding (Furhmann, 2010).…”

“…Occurrence is attributed to seasonal changes in virulence, transmission, and survival of influenza viruses, as well as seasonal changes in host immunity and behavior like overcrowding (Furhmann, 2010). Spectra coherence of El Niño southern oscillation (ENSO) and influenza time series (Oluwole, 2015) show, however, that climate is a major determinant of seasonality. ENSO modulates global precipitation on multiple timescales which range from subannual to multi-decadal (An and Wang, 2000), but it is the most dominant modulator of interannual changes in precipitation (Liu et al, 2014).…”

Waves of El Niño-Southern Oscillation and Influenza Pandemics

“…Influenza pandemics have been documented for over 500 years, but there is consensus that influenza pandemics that occurred since 1700 were less likely to be mistaken for other febrile illnesses (Potter, 2001;Lina, 2008). The intervals between historical influenza pandemics have been estimated to vary from 10 to 50 years (Potter, 2001) and 6-149 years , unlike seasonal influenza epidemics, which occur annually during the winter of northern and southern hemispheres (Oluwole, 2015). Further, influenza pandemics peak multiple times, but the peaks are not restricted to winter seasons (Taubenberger and Morens, 2006).…”

“…El Niño southern oscillation (ENSO), however, modulates interannual and interdecadal changes in precipitation of all parts of the earth through teleconnection (Wang and Ropelewski, 1985;An and Wang, 2000). Changes in occurrence of seasonal influenza epidemics have been correlated with interannual changes in indices of ENSO (Oluwole, 2015). Occurrence of seasonal influenza epidemics was highest during El Niño, the warm phase of ENSO, but lowest during La Niña, its cool phase (Oluwole, 2015).…”