Epidemiology of swine flu deaths

Research describing the characteristics of 574 swine-flu related deaths up to July 16 2009 has been published online. The general themes from this research have already been communicated by policy makers across the world. However, this study gives some new data to back up the claims and helps build a more accurate picture of the underlying conditions that contributed to risk of death in the first 10 weeks of the pandemic.

This French study of swine-flu related deaths looked at worldwide data and found that:

• The elderly may be protected from infection (except in Australia and Canada).
• There was underlying disease in at least half of the fatal cases. Taking data recording issues into account this could be as high as 90%.
• Pregnancy and having an underlying metabolic condition are particular risk factors.
• The researchers looked at the role of obesity and diabetes as risk factors for death from swine flu, despite the fact that these are not usually considered as factors for previous pandemics or for seasonal influenza. Their results showed that 13 cases with either or both of these risk factors died (from 241 recorded deaths). More research will be needed to confirm whether this rate is more than could be expected. 

Where was the article published?

This research was carried out by L Vaillant and colleagues from the French Institute for Public Health Surveillance in St Maurice, France.

The study was published in Eurosurveillance , a scientific journal devoted to the epidemiology, surveillance, prevention and control of communicable diseases.

What kind of study was this?

In this study, the researchers investigated risk factors and severe cases and deaths associated with pandemic H1N1 influenza 2009. The say that estimating the case fatality ratio (CFR), which represents the proportion of infected people who die, is a challenge during an evolving pandemic. As many countries have abandoned individual case counts and systematic testing of all suspect cases, it has become difficult to get accurate numbers.

Data from the start of the epidemic until July 16 2009 were collected from official sources such as the Ministries of Health, local or national public health authorities, the European Centre for Disease Prevention and Control, the United States Centers for Disease Control and Prevention and the World Health Organization.

What does the research say?

The first person to die from swine flu lived in Oaxaca State, Mexico, and developed symptoms on April 4 2009. By July 16 2009, 684 confirmed deaths had been reported worldwide for a total of 126,168 reported cases. This gave an overall 'computed CFR' of 0.6%, which varied from 0.1% to 5.1% depending on the country (and the accurate quantification of deaths and overall case counts).

The researchers note that, at this stage, no deaths had been reported and scarce data were available from African countries.

During the same period, 16 pregnant or recently pregnant women died. This comprised 10% of all women who died and 30% of 20-39-year-old women who died. Eight of these had underlying health risks (obesity, heart disease or a respiratory disease such as asthma or tuberculosis). The researchers could not get information on the underlying health status of the other eight pregnant women who died.

The main underlying diseases that were looked at included respiratory disease, heart disease, kidney disease, other metabolic conditions or liver conditions (including obesity and diabetes) and immunosupression. In 241 of 449 deaths the records were complete and included individual data (53%). Of these, 218 (90%) had documented underlying disease. The other 23 (10%) had documented absence of underlying disease.

Diabetes and obesity (mostly defined as BMI over 30) were the most frequently identified underlying conditions in adults. Among the 13 fatal cases with individual detailed data on metabolic conditions, seven cases had obesity, five cases had diabetes and one case had both. The available data for the other cases with an underlying metabolic condition did not say if this was obesity, diabetes or both.

What is the implication and importance of this?

In previous pandemics, most deaths have been in the very young, the elderly or those with underlying disease, and most cases involve short illness not requiring hospitalisation. The exception is the 1918-1919 pandemic, which had a high mortality rate in healthy young adults and an estimated CFR of 2-3%.

The authors note that even with a low CFR, seasonal influenza epidemics can cause a large number of complications as the absolute number of cases can be high. Previous estimates for the current pandemic have been for three to five million cases of severe illness and 250,000 to 500,000 deaths worldwide.

• It is important to point out that the data were collected only 10 weeks after the first international alert, when the pandemic was still in its early stages. The researchers say a more reliable CFR could be estimated through specific surveys, mathematical modelling, syndromic surveillance of influenza-like illness and of reported deaths in the population. All this is currently underway in the UK.
• The researchers encourage reporting of data in a common international format and also note that they found missing data, meaning that the proportion of deaths with documented underlying disease must be interpreted with care.
• There may be an information bias that overestimates the proportion of underlying disease since the presence of disease may be reported more readily than its absence.
• The number of people with diabetes and obesity was small and as there were problems with data quality it is too soon to say whether these are truly risk factors for developing complications from swine flu. The researchers say, "The role of obesity... remains to be further analysed in order to ascertain whether the risk is linked with complications of obesity during intensive care or with a severe course of disease due to diabetes frequently associated with obesity, or whether obesity plays a specific role in the pathogenesis of severe influenza A(H1N1)v infection, for example by interfering with the host's immune responses.”

Overall, this study adds to the emerging research into the virus and the pandemic, but should be taken in the context of other data from sampling, so that accurate and up-to-date complication and death rates can be calculated.