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

Chowell G, Echevarría-Zuno S, Viboud C, Simonsen L, Tamerius J, Miller MA, Borja-Aburto VH - PLoS Med. (2011)

Bottom Line: We also estimated the reproduction number (R) on the basis of the growth rate of daily cases, and used a transmission model to evaluate the effectiveness of mitigation strategies initiated during the spring pandemic wave.We estimate that the 18-day period of mandatory school closures and other social distancing measures implemented in the greater Mexico City area was associated with a 29%-37% reduction in influenza transmission in spring 2009.Please see later in the article for the Editors' Summary.

View Article: PubMed Central - PubMed

Affiliation: Mathematical, Computational & Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, United States of America. gchowell@asu.edu

ABSTRACT

Background: Mexico's local and national authorities initiated an intense public health response during the early stages of the 2009 A/H1N1 pandemic. In this study we analyzed the epidemiological patterns of the pandemic during April-December 2009 in Mexico and evaluated the impact of nonmedical interventions, school cycles, and demographic factors on influenza transmission.

Methods and findings: We used influenza surveillance data compiled by the Mexican Institute for Social Security, representing 40% of the population, to study patterns in influenza-like illness (ILIs) hospitalizations, deaths, and case-fatality rate by pandemic wave and geographical region. We also estimated the reproduction number (R) on the basis of the growth rate of daily cases, and used a transmission model to evaluate the effectiveness of mitigation strategies initiated during the spring pandemic wave. A total of 117,626 ILI cases were identified during April-December 2009, of which 30.6% were tested for influenza, and 23.3% were positive for the influenza A/H1N1 pandemic virus. A three-wave pandemic profile was identified, with an initial wave in April-May (Mexico City area), a second wave in June-July (southeastern states), and a geographically widespread third wave in August-December. The median age of laboratory confirmed ILI cases was ∼ 18 years overall and increased to ∼ 31 years during autumn (p<0.0001). The case-fatality ratio among ILI cases was 1.2% overall, and highest (5.5%) among people over 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. We estimate that the 18-day period of mandatory school closures and other social distancing measures implemented in the greater Mexico City area was associated with a 29%-37% reduction in influenza transmission in spring 2009. In addition, an increase in R was observed in late May and early June in the southeast states, after mandatory school suspension resumed and before summer vacation started. State-specific fall pandemic waves began 2-5 weeks after school reopened for the fall term, coinciding with an age shift in influenza cases.

Conclusions: We documented three spatially heterogeneous waves of the 2009 A/H1N1 pandemic virus in Mexico, which were characterized by a relatively young age distribution of cases. Our study highlights the importance of school cycles on the transmission dynamics of this pandemic influenza strain and suggests that school closure and other mitigation measures could be useful to mitigate future influenza pandemics. Please see later in the article for the Editors' Summary.

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Fit of influenza transmission model to the daily number of H1N1                            pandemic influenza cases in central Mexico, April 1–May 11,                            2009.The grey shaded area indicates the suspension of educational activities                            and other social distancing measures implemented between April 24 and                            May 11, 2009. Black circles represent the observed data. The solid red                            line is the model best-fit, and the blue lines are CIs based on 100                            realizations of the model obtained by parametric bootstrapping (Text                                S1).
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pmed-1000436-g005: Fit of influenza transmission model to the daily number of H1N1 pandemic influenza cases in central Mexico, April 1–May 11, 2009.The grey shaded area indicates the suspension of educational activities and other social distancing measures implemented between April 24 and May 11, 2009. Black circles represent the observed data. The solid red line is the model best-fit, and the blue lines are CIs based on 100 realizations of the model obtained by parametric bootstrapping (Text S1).

Mentions: We identified significant changes in the R during the spring wave according to school activity periods (Figure 4A and 4B). Focusing on central states affected by a substantial spring wave, we estimate that R increased from 1.3 (95% CI 1.2–1.5) to 2.2 (95% CI 1.4, 3.1) after the end of the spring break vacation period. A decrease in R from 2.2 (95% CI 1.4–3.1) to 1.0 (95% CI 0.94–1.06) coincided with the suspension of educational activities and the implementation of other social distancing measures enforced between April 24 and May 11, 2009. To explicitly account for the effects of depletion of susceptible individuals, we fitted a transmission model to daily influenza H1N1 case data and quantified the relative change in mean transmission rate during the intervention period. We estimated that the transmission rate was reduced by 29.6% (95% CI 28.9%–30.2%) during the intervention period (Figure 5). Our model gave a good fit to the spring epidemic curve overall, although it yielded a slightly higher number of cases than observed until the last week of April (chi-square test, bins  =  41, df = 37, p = 0.22, Figure 5). As a sensitivity analysis, we also fitted the model to ILI cases and found a reduction of 36.2% (95% CI 35.9%–36.5%) associated with social distancing measures.


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

Chowell G, Echevarría-Zuno S, Viboud C, Simonsen L, Tamerius J, Miller MA, Borja-Aburto VH - PLoS Med. (2011)

Fit of influenza transmission model to the daily number of H1N1                            pandemic influenza cases in central Mexico, April 1–May 11,                            2009.The grey shaded area indicates the suspension of educational activities                            and other social distancing measures implemented between April 24 and                            May 11, 2009. Black circles represent the observed data. The solid red                            line is the model best-fit, and the blue lines are CIs based on 100                            realizations of the model obtained by parametric bootstrapping (Text                                S1).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3101203&req=5

pmed-1000436-g005: Fit of influenza transmission model to the daily number of H1N1 pandemic influenza cases in central Mexico, April 1–May 11, 2009.The grey shaded area indicates the suspension of educational activities and other social distancing measures implemented between April 24 and May 11, 2009. Black circles represent the observed data. The solid red line is the model best-fit, and the blue lines are CIs based on 100 realizations of the model obtained by parametric bootstrapping (Text S1).
Mentions: We identified significant changes in the R during the spring wave according to school activity periods (Figure 4A and 4B). Focusing on central states affected by a substantial spring wave, we estimate that R increased from 1.3 (95% CI 1.2–1.5) to 2.2 (95% CI 1.4, 3.1) after the end of the spring break vacation period. A decrease in R from 2.2 (95% CI 1.4–3.1) to 1.0 (95% CI 0.94–1.06) coincided with the suspension of educational activities and the implementation of other social distancing measures enforced between April 24 and May 11, 2009. To explicitly account for the effects of depletion of susceptible individuals, we fitted a transmission model to daily influenza H1N1 case data and quantified the relative change in mean transmission rate during the intervention period. We estimated that the transmission rate was reduced by 29.6% (95% CI 28.9%–30.2%) during the intervention period (Figure 5). Our model gave a good fit to the spring epidemic curve overall, although it yielded a slightly higher number of cases than observed until the last week of April (chi-square test, bins  =  41, df = 37, p = 0.22, Figure 5). As a sensitivity analysis, we also fitted the model to ILI cases and found a reduction of 36.2% (95% CI 35.9%–36.5%) associated with social distancing measures.

Bottom Line: We also estimated the reproduction number (R) on the basis of the growth rate of daily cases, and used a transmission model to evaluate the effectiveness of mitigation strategies initiated during the spring pandemic wave.We estimate that the 18-day period of mandatory school closures and other social distancing measures implemented in the greater Mexico City area was associated with a 29%-37% reduction in influenza transmission in spring 2009.Please see later in the article for the Editors' Summary.

View Article: PubMed Central - PubMed

Affiliation: Mathematical, Computational & Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, United States of America. gchowell@asu.edu

ABSTRACT

Background: Mexico's local and national authorities initiated an intense public health response during the early stages of the 2009 A/H1N1 pandemic. In this study we analyzed the epidemiological patterns of the pandemic during April-December 2009 in Mexico and evaluated the impact of nonmedical interventions, school cycles, and demographic factors on influenza transmission.

Methods and findings: We used influenza surveillance data compiled by the Mexican Institute for Social Security, representing 40% of the population, to study patterns in influenza-like illness (ILIs) hospitalizations, deaths, and case-fatality rate by pandemic wave and geographical region. We also estimated the reproduction number (R) on the basis of the growth rate of daily cases, and used a transmission model to evaluate the effectiveness of mitigation strategies initiated during the spring pandemic wave. A total of 117,626 ILI cases were identified during April-December 2009, of which 30.6% were tested for influenza, and 23.3% were positive for the influenza A/H1N1 pandemic virus. A three-wave pandemic profile was identified, with an initial wave in April-May (Mexico City area), a second wave in June-July (southeastern states), and a geographically widespread third wave in August-December. The median age of laboratory confirmed ILI cases was ∼ 18 years overall and increased to ∼ 31 years during autumn (p<0.0001). The case-fatality ratio among ILI cases was 1.2% overall, and highest (5.5%) among people over 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. We estimate that the 18-day period of mandatory school closures and other social distancing measures implemented in the greater Mexico City area was associated with a 29%-37% reduction in influenza transmission in spring 2009. In addition, an increase in R was observed in late May and early June in the southeast states, after mandatory school suspension resumed and before summer vacation started. State-specific fall pandemic waves began 2-5 weeks after school reopened for the fall term, coinciding with an age shift in influenza cases.

Conclusions: We documented three spatially heterogeneous waves of the 2009 A/H1N1 pandemic virus in Mexico, which were characterized by a relatively young age distribution of cases. Our study highlights the importance of school cycles on the transmission dynamics of this pandemic influenza strain and suggests that school closure and other mitigation measures could be useful to mitigate future influenza pandemics. Please see later in the article for the Editors' Summary.

Show MeSH
Related in: MedlinePlus