Santiago Echevarría-Zuno MD a, Juan Manuel Mejía-Aranguré PhD b, Alvaro J Mar-Obeso MD b, Concepción Grajales-Muñiz MD b, Eduardo Robles-Pérez MSc b, Margot González-León MD b, Manuel Carlos Ortega-Alvarez MSc b, Cesar Gonzalez-Bonilla PhD b, Ramón Alberto Rascón-Pacheco MSc b, Dr Víctor Hugo Borja-Aburto PhD b Summary. The Lancet, Early Online Publication, 12 November 2009
Background
In April, 2009, the first cases of influenza A H1N1 were registered in Mexico and associated with an unexpected number of deaths. We report the timing and spread of H1N1 in cases, and explore protective and risk factors for infection, severe disease, and death.
Methods
We analysed information gathered by the influenza surveillance system from April 28 to July 31, 2009, for patients with influenza-like illness who attended clinics that were part of the Mexican Institute for Social Security network. We calculated odds ratios (ORs) to compare risks of testing positive for H1N1 in those with influenza-like illness at clinic visits, the risk of admission for laboratory-confirmed cases of H1N1, and of death for inpatients according to demographic characteristics, clinical symptoms, seasonal influenza vaccine status, and elapsed time from symptom onset to admission.
Findings
By July 31, 63 479 cases of influenza-like illness were reported; 6945 (11%) cases of H1N1 were confirmed, 6407 (92%) were outpatients, 475 (7%) were admitted and survived, and 63 (<1%) died. Those aged 10—39 years were most affected (3922 [56%]). Mortality rates showed a J-shaped curve, with greatest risk in those aged 70 years and older (10·3%). Risk of infection was lowered in those who had been vaccinated for seasonal influenza (OR 0·65 [95% CI 0·55—0·77]). Delayed admission (1·19 [1·11—1·28] per day) and presence of chronic diseases (6·1 [2·37—15·99]) were associated with increased risk of dying.
Interpretation
Risk communication and hospital preparedness are key factors to reduce mortality from H1N1 infection. Protective effects of seasonal influenza vaccination for the virus need to be investigated.
Funding
None.
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