Research Article: Characterizing the Epidemiology of the 2009 Influenza A/H1N1 Pandemic in Mexico

Date Published: May 24, 2011

Publisher: Public Library of Science

Author(s): Gerardo Chowell, Santiago Echevarría-Zuno, Cécile Viboud, Lone Simonsen, James Tamerius, Mark A. Miller, Víctor H. Borja-Aburto, J.S. Malik Peiris

Abstract: Gerardo Chowell and colleagues address whether school closures and other social
distancing strategies were successful in reducing pandemic flu transmission in
Mexico by analyzing the age- and state-specific incidence of influenza morbidity
and mortality in 32 Mexican states.

Partial Text: In late March and early April 2009, reports of respiratory hospitalizations and
deaths among young adults in Mexico alerted local health officials to the occurrence
of atypical rates of respiratory illness at a time when influenza was not expected
to reach epidemic levels [1]–[3]. Infections with novel swine-origin influenza A/H1N1 virus
were confirmed in California, (United States), on April 21 [4] and in Mexico on April 23 [5]. The Ministry
of Health cancelled educational activities in the greater Mexico City area on April
24 and expanded these measures to the rest of the country on April 27 [6].
Additional social distancing interventions were implemented in the greater Mexico
City area, including the closure of movie theaters and restaurants and the
cancellation of large public gatherings (Table 1) [6]. Schools reopened on
May 11 and remained in session until the scheduled summer vacation period, which
began in July 2009. Whether these intense interventions were successful in reducing
disease transmission has yet to be evaluated, which is important for the control of
future pandemics [7].

This is, to our knowledge, the first study to explore spatio-temporal variation in
the dynamics and age patterns of the 2009 A/H1N1 pandemic in Mexico, relying on a
large sample of laboratory-confirmed and ILI data collected by a private medical
system representing a population of over 100 million people. Our findings support
the effectiveness of early mitigation efforts in the greater Mexico City area in the
spring of 2009, including mandatory school closures and cancellation of large public
gatherings. In addition, the onset of the fall pandemic wave in Mexico coincided
with the start of the fall term in schools and universities, reinforcing the
importance of school cycles in the transmission of pandemic influenza. Our data also
reveal substantial geographical variation in pandemic patterns across Mexico, in
part related to population size, with three consecutive waves of varying amplitude
occurring over an 8-mo period. In line with previous studies [30]–[32], we
note that the age distribution of pandemic influenza morbidity was highly skewed
towards younger age groups (median 18 y), while the risk of severe disease was
skewed towards older age groups. Of note was the particularly high CFR reported in
these Mexican data (CFR≈1% based on the ratio of ILI deaths to ILI



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