A paper reporting on information gathered by the influenza surveillance system in Mexico on the timing and spread of H1N1, and exploring protective and risk factors for infection, severe disease, and death has been published early online in the Lancet. 

Researchers 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 (Instituto Mexicano del Seguro Social; IMSS). IMSS is a public institution that provides health-care services to a population of nearly 40 million people in Mexico, which has reported the largest number of cases and deaths of H1N1 within the country. 

The main findings were as follows (taken from the abstract):

• By July 31, 63,479 cases of influenza-like illness were reported; 11%) were confirmed cases of H1N1, 92% were outpatients, 7% were admitted and survived, and <1% died.
• Those aged 10—39 years were most affected (56%)
• Mortality rates showed a J-shaped curve, with greatest risk in those aged 70 years and older (10.3%)
• Risk of infection was lower 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.
• Fever, cough, headache, muscular pain, and rhinorrhoea were the main symptoms of H1N1.  Additionally, prostration, dyspnoea, cyanosis, and tachypnoea reported at the first visit were most strongly related to risk of admission and death.

The authors note that they had difficulty assessing the effectiveness of single control measures taken during the first outbreak, e.g. school suspension and restriction of mass gatherings, as other measures taken at the same time to reduce transmissibility could have had an effect, such as frequent handwashing, barrier measures (wearing of masks), and isolation of people with suspected respiratory-tract infections.  Although a possible protective effect of seasonal influenza vaccination was seen, the authors note this is controversial and that others have not identified such an effect.  However, the Mexican population who has received seasonal influenza vaccination since 1977, including H1N1 components, could have benefited from cross immunity. 

The authors note the various limitations to their study, and that “a major difficulty is that this pandemic is still active and surveillance systems are still evolving.”