Comparing pandemic to seasonal influenza mortality: moderate impact overall but high mortality in young children.
BACKGROUND
We assessed the severity of the 2009 influenza pandemic by comparing pandemic mortality to seasonal influenza mortality. However, reported pandemic deaths were laboratory-confirmed - and thus an underestimation - whereas seasonal influenza mortality is often more inclusively estimated. For a valid comparison, our study used the same statistical methodology and data types to estimate pandemic and seasonal influenza mortality.


RESULTS
We used data on all-cause mortality (1999-2010, 100% coverage, 16.5 million Dutch population) and influenza-like-illness (ILI) incidence (0.8% coverage). Data was aggregated by week and age category. Using generalized estimating equation regression models, we attributed mortality to influenza by associating mortality with ILI-incidence, while adjusting for annual shifts in association. We also adjusted for respiratory syncytial virus, hot/cold weather, other seasonal factors and autocorrelation. For the 2009 pandemic season, we estimated 612 (range 266-958) influenza-attributed deaths; for seasonal influenza 1,956 (range 0-3,990). 15,845 years-of-life-lost were estimated for the pandemic; for an average seasonal epidemic 17,908. For 0-4 yrs of age the number of influenza-attributed deaths during the pandemic were higher than in any seasonal epidemic; 77 deaths (range 61-93) compared to 16 deaths (range 0-45). The ≥75 yrs of age showed a far below average number of deaths. Using pneumonia/influenza and respiratory/cardiovascular instead of all-cause deaths consistently resulted in relatively low total pandemic mortality, combined with high impact in the youngest age category.


CONCLUSIONS
The pandemic had an overall moderate impact on mortality compared to 10 preceding seasonal epidemics, with higher mortality in young children and low mortality in the elderly. This resulted in a total number of pandemic deaths far below the average for seasonal influenza, and a total number of years-of-life-lost somewhat below average. Comparing pandemic and seasonal influenza mortality as in our study will help assessing the worldwide impact of the 2009 pandemic.
National Institute for Public Health and the Environment, Center for Infectious Disease Control, Bilthoven, The Netherlands.
Cees C van den
CC
Wijngaard
Liselotte van
Lv
Asten
Marion P G
MP
Koopmans
Wilfrid van
Wv
Pelt
Nico J D
NJ
Nagelkerke
Cornelia C H
CC
Wielders
Alies van
Av
Lier
Wim van der
Wv
Hoek
Adam
A
Meijer
Gé A
GA
Donker
Frederika
F
Dijkstra
Carel
C
Harmsen
Marianne A B van der
MA
Sande
Mirjam
M
Kretzschmar
D017677:Age Distribution; D002675:Child, Preschool; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality*; D058873:Pandemics; D012621:Seasons