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H1N1 flu virus: moderate severity

Experience acquired around the world indicates that the H1N1 flu is of similar clinical severity to that of bad seasonal flu outbreaks.

Large-scale preventive actions have been deployed and still others are in preparation to combat the H1N1 flu. The severity of this epidemic is the first point to be analysed in order to evaluate the risk-benefit balance of such interventions.

The mortality rate of 1 death per 2,000 reported cases of flu sometimes cited for the US and Europe overestimates mortality, since the number of reported flu cases is far lower than the number of actual cases.

In New Zealand, after the Southern Hemisphere's winter flu epidemic, local epidemiologists put the mortality rate at 1 death per 20,000 cases of flu. One person in 330 was hospitalised, and one in 2,700 admitted to intensive care.

The clinical situations associated with an increased risk of hospitalisation are the same as for seasonal flu: third-trimester pregnancy, diabetes, severe respiratory or cardiovascular disease, immunosuppression.

Death is rare in young, healthy individuals. Few deaths have been recorded among persons over 65 years of age, perhaps because of immunity acquired in the 1950s.

In all, the severity of this flu is comparable to that of a bad seasonal flu. Beware, then, of overzealous preventive measures which may have disproportionate consequences: for example unnecessarily broad use of antiviral drugs, indiscriminate use of insufficiently evaluated vaccines, and unjustified coercive measures.

©Prescrire October 2009

Source: "Grippe A/H1N1 de 2009 : gravité clinique modérée" Rev Prescrire 2009; 29 (312): 770-771.