Inside the outbreak of the 2009 influenza A (H1N1)v virus in Mexico

Abstract

Background: Influenza viruses pose a threat to human health because of their potential to cause global disease. Between mid March and mid April a pandemic influenza A virus emerged in Mexico. This report details 202 cases of infection of humans with the 2009 influenza A virus (H1N1)v which occurred in Mexico City as well as the spread of the virus throughout the entire country.

Methodology and findings: From May 1st to May 5th nasopharyngeal swabs, derived from 751 patients, were collected at 220 outpatient clinics and 28 hospitals distributed throughout Mexico City. Analysis of samples using real time RT-PCR revealed that 202 patients out of the 751 subjects (26.9%) were confirmed to be infected with the new virus. All confirmed cases of human infection with the strain influenza (H1N1)v suffered respiratory symptoms. The greatest number of confirmed cases during the outbreak of the 2009 influenza A (H1N1)v were seen in neighbourhoods on the northeast side of Mexico City including Iztapalapa, Gustavo A. Madero, Iztacalco, and Tlahuac which are the most populated areas in Mexico City. Using these data, together with data reported by the Mexican Secretariat of Health (MSH) to date, we plot the course of influenza (H1N1)v activity throughout Mexico.

Conclusions: Our data, which is backed up by MSH data, show that the greatest numbers of the 2009 influenza A (H1N1) cases were seen in the most populated areas. We speculate on conditions in Mexico which may have sparked this flu pandemic, the first in 41 years. We accept the hypothesis that high population density and a mass gathering which took in Iztapalapa contributed to the rapid spread of the disease which developed in three peaks of activity throughout the Country.

Figure 1. Destribution of cases.

A) distribution of 202 confirmed cases of human infection with 2009 influenza A(H1N1)v in the Federal District, B) distribution of 515 cases reported by the Mexican Secretariat of Health as May the 5^th. (http://www.dgepi.salud.gob.mx/influenza/AH1N12009/boletines/052009/Influenza_H1N1_Mexico_06may09.pdf, C) distribution of population in the Federal District per neighbourhood according to the 2005 census. 0 to 20% in the bottom represents the percentage of people infected with the novel virus in each district. C represents the population density in the Federal District (range 73625 [Cuajimalpa] to 1,820888 [Iztapalapa]).

Figure 2. Epidemiological curve of confirmed cases of human infection during the outbreak of the 2009 influeza A(H1N1)v.

Three waves of influenza activity were seen: the first wave occurred between April and May, the second wave took place between June and July and the third, between September and December. Bars in red show the days when samples were collected. Iztapalapa Play passion (Bar in white) and return to schools (Blue arrow). Text in white boxes describes each event.

Figure 3. Distribution of human infection with 2009 influenza A(H1N1)v throughout México during the third wave.

A shows absolute numbers of confirmed cases infected with the novel virus while B gives the estimated attack rate throughout México during the third wave. Data was obtained from the Sistema Nacional de Vigilancia Epidemiológica/Dirección General Adjunta de Epidemiología/Secretaría de Salud. www.dgepi.salud.gob.mx/influenza/AH1N12009/boletines_index.html, accessed 1 March 2010 (last time).

Figure 4. Estimated attack rate of influenza A (H1N1)v virus infection in the different Mexican States during the 3^rd wave.

A. Map shows geographic distribution of cases throughout Mexico. B. Table shows distribution of cases quantitatively (lanes represent the mean [95% confidence interval]). Data was obtained from the Mexican Secretariat of Health: www.dgepi.salud.gob.mx/influenza/AH1N12009/boletines_index.html, accessed 1 March 2010 (last time).