Disproportional effects in populations of concern for pandemic influenza: insights from seasonal epidemics in Wisconsin, 1967-2004.
BACKGROUND
Influenza infections pose a serious burden of illness in the United States. We explored age, influenza strains, and seasonal epidemic curves in relation to influenza-associated mortality.


METHODS
The state of Wisconsin death records for the years 1967-2004 were analyzed for three distinct populations: children, general population, and elderly. Yearly parameters of duration, intensity, and peak timing were obtained from Annual Harmonic Regression coefficients.


RESULTS
Overall, elderly had the highest rate and intensity of influenza mortality. The children and infant subpopulations showed an earlier and wider range in duration of peak timing than elderly. During A/Hong Kong/1/68 pandemic years, the elderly subpopulation showed no change in mortality rates while a sharp increase was observed for the children and infant subpopulations. In epidemic years such as 1966-1969, children and infants showed a dramatic decrease in the severity of influenza outbreaks over time. The elderly had increased baseline mortality in years (1986-1987) where predominant strain was characterized as A/Singapore/6/86.


CONCLUSIONS
Our findings indicate that the younger populations may have benefited from the lack of a major shift in viral strains for a number of decades. Furthermore, we demonstrate considerable heterogeneity in the spread of seasonal influenza across age categories, with implications both for the modeling of influenza seasonality, risk assessment, and effective distribution and timing of vaccine and prophylactic interventions.
Lofgren
Eric T
ET
UNC Gillings School of Global Public Health, Department of Epidemiology, Chapel Hill, NC, USA.
Wenger
Julia B
JB
Fefferman
Nina H
NH
Bina
David
D
Gradus
Steve
S
Bhattacharyya
Sanjib
S
Naumov
Yuri N
YN
Gorski
Jack
J
Naumova
Elena N
EN
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D058872:Epidemics; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D009980:Influenza A virus / Q000145:classification / Q000235:genetics* / Q000302:isolation & purification; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality* / Q000503:physiopathology / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012621:Seasons; D014922:Wisconsin / Q000453:epidemiology; D055815:Young Adult