Study: Crowding likely spurred H1N1 spread in Mexico City

Oct 11, 2010 (CIDRAP News) – The first wave of pandemic H1N1 influenza cases in Mexico hit hardest in the most crowded parts of Mexico City and may have been spurred by an Easter Week event that drew 2 million visitors to one of those areas, according to Mexican researchers.

Writing in the journal PLoS One, the researchers also report that Mexico saw three waves of cases in 2009: in April and May, in June and July, and from September to December. In contrast, the pandemic in the United States had two major waves, in the spring and fall of 2009.

The Mexican researchers analyzed 751 clinical samples collected in Mexico City early in the pandemic and combined the results with national data from the Mexican Secretariat of Health (MSH) to trace the spread of the disease in Mexico. The researchers were from the National Polytechnic Institute, the National Institute of Respiratory Illnesses, and the MSH.

The 751 clinical samples were nasopharyngeal swabs gathered at 220 outpatient clinics and 28 hospitals throughout Mexico City from May 1 to 5, 2009, about 2 weeks after 2009 H1N1 cases were first described in an Apr 21 report from the US Centers for Disease Control and Prevention.

Of the 751 patients, polymerase chain reaction testing revealed that 202 had 2009 H1N1 infections, the report says. Twelve of the patients required hospitalization, and two of them died of respiratory failure. The patients ranged from 3 months to 70 years old, but most of the cases were in younger people: 32% in 5- to 19-year-olds, 28% in 20- to 39-year-olds, 18% in 40- to 59-year-olds, and 3% in those 60 and over.

The authors found that more than half of the 202 confirmed patients were from neighborhoods on the northeastern side of Mexico City, including Iztapalapa (38 cases), Gustavo A. Madero (28), Iztacalco (19), and Tlahuac (19). The lowest case numbers were found in the southwestern areas of the city. A very similar pattern was seen in 515 H1N1 cases in Mexico City that had been reported to the MSH as of May 5, the article says.

The authors write that the similar patterns seen in their own and the MSH data confirm that densely populated areas are susceptible to flu transmission. The two districts with the most cases, Iztapalapa and Gustavo A Madero, respectively have 2 million and 1.2 millon residents. The city's busy subway system, which carries 4.2 million passengers daily, may have facilitated spread of the virus, the researchers comment.

They also observe that about 2 million visitors came to Iztapalapa during Holy Week, Apr 5 to 11, for the Iztapalapa Passion Play. "This congregation of people may have contributed to the spread of the virus during the initial phase of the first wave," with infected people passing the virus to people attending the play, and the latter spreading it throughout Mexico City and beyond, they speculate. They call for further investigation of this possibility.

The authors sifted national surveillance data to conclude that Mexico saw three waves of 2009 H1N1 cases. The first wave, in April and May, was concentrated in Mexico City, which had the highest attack rate at 106.4 per 100,000 people; the next-highest attack rates were in Jalisco, Tabasco, and San Luis Potosi.

The second wave followed in June and July and hit hardest in southern Mexico, especially Yucatan, Quintana Roo, Tabasco, and Chiapas states. In the third wave, which lasted from September into December, the pandemic again struck hardest in Quintana Roo, Tabasco, and Yucatan, the report says. The highest attack rate found was 365 cases per 100,000 people, during the second wave in Yucatan.

Using the national data, the authors calculated the case-fatality rate at 0.18% in the first wave and 0.11% in the second wave. In the third wave this increased significantly to 1.88%.

Zepeda-Lopez HM, Perea-Araujo L, Millar-Barcia A, et al. Inside the outbreak of the 2009 influenza A (H1N1)v virus in Mexico. PLoS Pathog 2010 Oct 8 [Full text]

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