Researchers link school closings to decrease in H1N1 transmission in Mexico

Chowell G. PloS Med. 2011;doi:10.1371/journal.pmed.1000436.

Issue: June 2011

School closings and other social distancing measures were linked to a nearly 30% reduction in influenza A/H1N1 transmission in Mexico during 2009, according to study results.

The findings are based on surveillance data compiled by the Mexican Institute for Social Security. Hospitalizations, deaths and case-fatality ratios for 117,626 influenza-like illness (ILI) cases were stratified by pandemic wave and geographical region. Influenza tests were performed on 30.6% of these ILI cases, and 23.3% were positive for the pandemic strain.

Three waves of the 2009 pandemic were identified. The first wave was in the Mexico City area during April and May, the second was in the southeastern states in June and July and the third wave spread throughout the country between August and December.

Laboratory confirmed ILI cases occurred in those with a median age of about 18 years. During the autumn months, the median age increased to about 31 years (P<.0001).>

An 18-day mandatory school closing period in Mexico City — along with other social distancing measures carried out in the Mexico City metropolis — was linked to a reduction in transmission of 29% to 37% during the spring wave.

During late May and early June, the southeastern states experienced an increase in the reproduction number after mandatory school suspension resumed and before summer vacation began. A shift in the age of cases occurred along with a state-specific autumn wave that began 2 to 5 weeks after schools reopened.

A 1.2% case-fatality ratio was observed. This ratio was 5.5% among those aged older than 60 years.

The growth rate of daily cases was used to estimate a reproduction number. The researchers used a transmission model to assess the effectiveness of strategies used to stem the infection during the spring pandemic wave.

During spring, the reproduction number was 1.8 to 2.1. This number was 1.6 to 1.9 during summer and 1.2 to 1.3 during fall.

In the early stages of the 2009 influenza A/H1N1 pandemic, local and national authorities in Mexico initiated an intense public health response. The current study is an analysis of epidemiological patterns of the pandemic involving 40% of the Mexican population between April and December 2009. The analysis focuses on the effect of nonmedical interventions, school cycles and demographic factors on transmission of the infection.

“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,” the researchers wrote.

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