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Infection and death from influenza A H1N1 virus in Mexico: a retrospective analysis
Santiago Echevarría-Zuno1, Juan Manuel Mejía-Aranguré2, Alvaro J Mar-Obeso2
1Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
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.
Insights
The 2009 H1N1 influenza pandemic in Mexico disproportionately affected young adults but had the highest mortality in older adults. Seasonal flu vaccination may offer some protection against H1N1 infection.
Area of Science:
- Epidemiology
- Public Health
- Infectious Diseases
Background:
- The 2009 influenza A H1N1 pandemic emerged in Mexico with significant mortality.
- Understanding the epidemiology and risk factors is crucial for public health response.
Purpose of the Study:
- To analyze the timing, spread, and demographic impact of the H1N1 influenza A outbreak in Mexico.
- To identify protective and risk factors for H1N1 infection, severe disease, and mortality.
Main Methods:
- Analysis of influenza surveillance data from April 28 to July 31, 2009.
- Inclusion of patients with influenza-like illness from the Mexican Institute for Social Security network.
- Calculation of odds ratios to assess risks related to demographics, symptoms, vaccination status, and time to admission.
Main Results:
- 6,945 confirmed H1N1 cases out of 63,479 influenza-like illness reports.
- The 10-39 age group was most affected (56%), while mortality risk was highest in those aged 70+ (10.3%).
- Seasonal influenza vaccination was associated with reduced infection risk (OR 0.65), while delayed admission and chronic diseases increased mortality risk.
Conclusions:
- Effective risk communication and hospital preparedness are vital for reducing H1N1 mortality.
- Further investigation into the protective effects of seasonal influenza vaccination against H1N1 is warranted.
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