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H1N1 flu: mild severity confirmed

In mainland France at end November 2009, nearly 4 million people have had the H1N1 flu. There has been approximately 1 death per 40,000 cases of H1N1 flu. 8 out of 10 complications occurred in persons with known risk factors.

As of October 2009, according to epidemiological data from the Southern Hemisphere, the 2009 H1N1 flu (alias H1N1v) appeared to be of mild severity, similar to that of a seasonal flu epidemic (1). Since then, this flu epidemic has struck the Northern Hemisphere: in France, at end November 2009, around 50% of flu-like respiratory infections appear to have been due to the H1N1v flu virus (2).

At 29 November 2009, in mainland France, the network of regional flu observation groups (Groupes régionaux d'observation de la grippe, GROG) estimated that around 3.8 million persons had been infected with the virus since the beginning of August 2009.

At the same date, France's epidemiological institute (Institut de veille sanitaire, www.invs.sante.fr) had recorded some 92 deaths in mainland France since the beginning of the epidemic, due to confirmed or probable H1N1 flu (3). 481 persons had suffered severe flu (hospitalisation in intensive care or death). Around 80% of the deaths and the cases of severe flu were in persons with known risk factors for flu complications: 14 persons with no identified risk factors died. 83% of the deaths and 92% of the cases of severe flu occurred in persons under the age of 65.

According to our calculations, there was an average of around 1 death per 40,000 cases of H1N1 flu and 1 severe flu per 8,000 cases (2,3). The rates of death and of severe flu appear to be 5 to 10 times lower in persons not at risk of complications.

The epidemiological data in Europe (Switzerland, United Kingdom, etc) are consistent with those reported in mainland France.

Overall, severe complications from H1N1 flu are rare (3,4).

©Prescrire 1 January 2010

1- Prescrire Rédaction "Grippe A/H1N1 de 2009" Rev Prescrire 2009 ; 29 (312) : 770-771.
2- Réseau des GROG "France. Semaine n° 2009/48 (23/11/2009 - 29/11/2009). Grippe A(H1N1)2009 : en hausse" 2 décembre 2009 : 1 page.
3- InVS "Point de situation au 1er décembre 2009" Bulletin grippe A (H1N1) 2009 n° 73 : 9 pages.
4- Donaldson L et coll. "Mortality from pandemic A/H1N1 2009 influenza in England : public health surveillance study" BMJ 2009 ; 339 : b5213 (version complète sur le site www.bmj.com : 8 pages).

Source: "Grippe H1N1v : gravité modérée confirmée" Rev Prescrire 2010; 30 (315): 14.

> Click here for Prescrire's other updates at 15 December on the H1N1 flu (in French).