Comparing Pandemic to Seasonal Influenza Mortality: Moderate Impact Overall but High Mortality in Young Children

Abstract

BackgroundWe 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.Methods and FindingsWe 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.ConclusionThe 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.

Mortality burden from seasonal influenza and 2009 H1N1 pandemic influenza in Beijing, China, 2007‐2013

“…Consistent with studies conducted in other countries, our study found that, compared with seasonal epidemics, there was an “age shift” in age‐specific mortality burden toward persons <65 years of age during the influenza A(H1N1)pdm09 pandemic period 4, 20, 29, 33, 34, 35, 36. A previous meta‐analysis from 27 studies showed that the influenza A(H1N1)pdm09 cumulative incidence varied significantly by age with much higher incidence in younger people 37.…”

Influenza-Associated Excess Mortality in South Korea

“…The overall estimate of excess mortality for all ages was higher in the second wave in 2010–2011 than the first wave in 2009–2010 (Table 1), despite the prolonged duration of the first wave that lasted for 40 weeks in total (Appendix Figure 1). The low impact of pH1N1 was also reported in other studies, 10,13,14,18,19 and could be attributed in part to wide-scale use of antivirals 38 and monovalent pH1N1 vaccines (14.3 million people or 84% of the vaccinated “priority groups”) 39 during the first wave, when it should be noted that deaths in young adults during the pandemic still represented a substantial impact even without the high number of excess deaths in the elderly seen in other years. For example, Viboud and colleagues 40 estimated the total years of life lost attributable to influenza during the 2009 pandemic in the U.S. to be comparable to that from the 1957 and 1968 pandemics, despite a relatively smaller number of excess deaths.…”

“…6–12 The direct approach tends to underestimate the total impact of influenza on mortality because it neglects deaths caused by influenza but not diagnosed or laboratory confirmed, 13,14 as well as deaths from exacerbation of pre-existing chronic diseases triggered by influenza virus infection and therefore not coded as having been caused by “influenza.” 15 Statistical models can provide more complete assessment of the impact of influenza on mortality by estimating the number of deaths that would have occurred if influenza had not circulated, accounting for seasonality and other relevant factors. 11,13,14,16–21 This measurement of influenza mortality burden from these statistical models is generally referred to as the “excess mortality” estimate.…”

“…6–11,13,14,16,18–25 However, there are relatively few published estimates of excess mortality in Asian countries, and no study has yet quantified the excess mortality impact of influenza in South Korea (hereafter Korea). The objectives of this study were to estimate influenza-associated excess mortality in Korea, from 2003 through 2013, and to examine patterns in excess mortality by age, influenza type/subtype, and geographic area.…”

Mortality burden of the 2009‐10 influenza pandemic in the United States: improving the timeliness of influenza severity estimates using inpatient mortality records

“…Our estimates of pandemic mortality in the United States are higher than those reported by similar methods for Australia (−6·0/100 000), France (1·0/100 000), Hong‐Kong (1·6/100 000), the United Kingdom (2·7/100 000), and the Netherlands (3·7/100 000) but lower than those for Mexico (11/100 000) 15, 16, 17, 18, 19, 20. Some of these differences could reflect estimation errors due to use of non‐specific outcomes16 or true variation in disease burden, as suggested by the unusual clinical severity of the disease reported in Mexico during the pandemic 18, 21.…”