Mortality burden of the A/H1N1 pandemic in Mexico: a comparison of deaths and years of life lost to seasonal influenza

Abstract

Background: The mortality burden of the 2009 A/H1N1 influenza pandemic remains controversial, in part because of delays in reporting of vital statistics that are traditionally used to measure influenza-related excess mortality. Here, we compare excess mortality rates and years of life lost (YLL) for pandemic and seasonal influenza in Mexico and evaluate laboratory-confirmed death reports.

Methods: Monthly age- and cause-specific death rates from January 2000 through April 2010 and population-based surveillance of influenza virus activity were used to estimate excess mortality and YLL in Mexico. Age-stratified laboratory-confirmed A/H1N1 death reports were obtained from an active surveillance system covering 40% of the population.

Results: The A/H1N1 pandemic was associated with 11.1 excess all-cause deaths per 100,000 population and 445,000 YLL during the 3 waves of virus activity in Mexico, April-December 2009. The pandemic mortality burden was 0.6-2.6 times that of a typical influenza season and lower than that of the severe 2003-2004 influenza epidemic. Individuals aged 5-19 and 20-59 years were disproportionately affected relative to their experience with seasonal influenza. Laboratory-confirmed deaths captured 1 of 7 pandemic excess deaths overall but only 1 of 41 deaths in persons >60 years of age in 2009. A recrudescence of excess mortality was observed in older persons during winter 2010, in a period when influenza and respiratory syncytial virus cocirculated.

Conclusions: Mexico experienced higher 2009 A/H1N1 pandemic mortality burden than other countries for which estimates are available. Further analyses of detailed vital statistics are required to assess geographical variation in the mortality patterns of this pandemic.

Figure 1.

Time series of monthly mortality rates coded as pneumonia and influenza (A), respiratory causes (B), respiratory and cardiac causes (C), and all causes (D), from January 2000 through April 2010, Mexico. Blue line: observed mortality rates; black line: baseline rates predicted by a seasonal regression model; red dashed line: upper 95% confidence interval of the baseline. Periods highlighted in gray mark seasonal influenza epidemic periods, while periods highlighted in orange highlight the 4 different waves of the A/H1N1 pandemic (spring 2009, summer 2009, fall 2009, winter 2010). Influenza dominant subtypes are indicated each season. See Supplementary Table 2 and Supplementary Figure 1 (online only) for additional virus surveillance data.

Figure 2.

Monthly pneumonia and influenza (P&I) mortality rates in persons aged <5 (A), 5–59 (B), and 60–99 (C) years, Mexico, January 2000–April 2010. Blue line: observed rate; black line: baseline rates predicted by a seasonal regression model. Gray bars: influenza epidemic periods; orange bar: influenza A/H1N1 pandemic period.

Figure 3.

Monthly respiratory mortality rates in persons aged <5 (A), 5–59 (B), and 60–99 (C) years, Mexico, January 2000–April 2010. Blue line: observed rate; black line: baseline rates predicted by a seasonal regression model. Gray bars: influenza epidemic periods; orange bar: influenza A/H1N1 pandemic period. See Supplementary Figures 2 and 3 (online only) for time series of cardiorespiratory and all-cause deaths by age.

Figure 4.

Age-specific comparison of the impact of pandemic (April–December 2009) and epidemic influenza (top, A and B). Age-specific rates of excess pneumonia and influenza (P&I) and respiratory deaths. Black solid line: 2009 A/H1N1; gray dashed line: 2003–2004 A/H3N2 season; black dashed line: average seasonal influenza, 2000–2008 (bottom, C and D) Relative risk of excess pneumonia and influenza (P&I) and respiratory mortality rates during the pandemic, compared with an average seasonal influenza epidemic (black solid line) and the particularly severe 2003–2004 epidemic (grey dashed line). See Supplementary Figure 5 (online only) for additional causes of death and Supplementary Figures 6 and 7 (online only) for comparisons involving the April 2009–April 2010 period.