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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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Changes in the age distribution of cases during the summer and fall                            pandemic waves in Mexico.(A) Weekly time series of laboratory-confirmed A/H1N1 pandemic cases                            among students (5–20 y, red curve) and other age                            groups (blue curve) and (B) Weekly ratio of student to nonstudent A/H1N1                            cases. The grey shaded area indicates the mandatory school closure                            period (April 24–May 11) and the summer vacation                            period (July 3–August 23) for elementary and                            secondary school students. College students retuned to class on August                            10th(arrow).
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pmed-1000436-g006: Changes in the age distribution of cases during the summer and fall pandemic waves in Mexico.(A) Weekly time series of laboratory-confirmed A/H1N1 pandemic cases among students (5–20 y, red curve) and other age groups (blue curve) and (B) Weekly ratio of student to nonstudent A/H1N1 cases. The grey shaded area indicates the mandatory school closure period (April 24–May 11) and the summer vacation period (July 3–August 23) for elementary and secondary school students. College students retuned to class on August 10th(arrow).

Mentions: To further test the impact of school cycles, we monitored trends in the ratio of incident student to nonstudent influenza A/H1N1 cases. At the national scale, this ratio was low during the summer vacations and increased sharply following the start of school activities in August (Wilcoxon test, p<0.001, Figure 6). At the state level, the ratio of student to nonstudent cases peaked 2–5 wk after schools reopened in the fall of 2009 (Text S1, figures L–M).


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)

Changes in the age distribution of cases during the summer and fall                            pandemic waves in Mexico.(A) Weekly time series of laboratory-confirmed A/H1N1 pandemic cases                            among students (5–20 y, red curve) and other age                            groups (blue curve) and (B) Weekly ratio of student to nonstudent A/H1N1                            cases. The grey shaded area indicates the mandatory school closure                            period (April 24–May 11) and the summer vacation                            period (July 3–August 23) for elementary and                            secondary school students. College students retuned to class on August                            10th(arrow).
© Copyright Policy
Related In: Results  -  Collection

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

pmed-1000436-g006: Changes in the age distribution of cases during the summer and fall pandemic waves in Mexico.(A) Weekly time series of laboratory-confirmed A/H1N1 pandemic cases among students (5–20 y, red curve) and other age groups (blue curve) and (B) Weekly ratio of student to nonstudent A/H1N1 cases. The grey shaded area indicates the mandatory school closure period (April 24–May 11) and the summer vacation period (July 3–August 23) for elementary and secondary school students. College students retuned to class on August 10th(arrow).
Mentions: To further test the impact of school cycles, we monitored trends in the ratio of incident student to nonstudent influenza A/H1N1 cases. At the national scale, this ratio was low during the summer vacations and increased sharply following the start of school activities in August (Wilcoxon test, p<0.001, Figure 6). At the state level, the ratio of student to nonstudent cases peaked 2–5 wk after schools reopened in the fall of 2009 (Text S1, figures L–M).

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