Characterizing the epidemiology of the 2009 influenza A/H1N1 pandemic in Mexico

Background

Mexico''s local and national authorities initiated an intense publichealth response during the early stages of the 2009 A/H1N1 pandemic. In thisstudy we analyzed the epidemiological patterns of the pandemic duringApril–December 2009 in Mexico and evaluated the impact of nonmedicalinterventions, school cycles, and demographic factors on influenzatransmission.

Methods and Findings

We used influenza surveillance data compiled by the Mexican Institute forSocial Security, representing 40% of the population, to studypatterns in influenza-like illness (ILIs) hospitalizations, deaths, andcase-fatality rate by pandemic wave and geographical region. We alsoestimated the reproduction number (R) on the basis of the growth rate ofdaily cases, and used a transmission model to evaluate the effectiveness ofmitigation strategies initiated during the spring pandemic wave. A total of117,626 ILI cases were identified during April–December 2009, of which30.6% were tested for influenza, and 23.3% were positive forthe influenza A/H1N1 pandemic virus. A three-wave pandemic profile wasidentified, with an initial wave in April–May (Mexico City area), asecond wave in June–July (southeastern states), and a geographicallywidespread third wave in August–December. The median age of laboratoryconfirmed ILI cases was ∼18 years overall and increased to ∼31 yearsduring autumn (p<0.0001). The case-fatality ratio amongILI cases was 1.2% overall, and highest (5.5%) among peopleover 60 years. The regional R estimates were 1.8–2.1, 1.6–1.9,and 1.2–1.3 for the spring, summer, and fall waves, respectively. Weestimate that the 18-day period of mandatory school closures and othersocial distancing measures implemented in the greater Mexico City area wasassociated with a 29%–37% reduction in influenzatransmission in spring 2009. In addition, an increase in R was observed inlate May and early June in the southeast states, after mandatory schoolsuspension resumed and before summer vacation started. State-specific fallpandemic waves began 2–5 weeks after school reopened for the fallterm, coinciding with an age shift in influenza cases.

Conclusions

We documented three spatially heterogeneous waves of the 2009 A/H1N1 pandemicvirus in Mexico, which were characterized by a relatively young agedistribution of cases. Our study highlights the importance of school cycleson the transmission dynamics of this pandemic influenza strain and suggeststhat school closure and other mitigation measures could be useful tomitigate future influenza pandemics.Please see later in the article for the Editors'' Summary