A(H1N1)2009 pandemic in France: epidemiological features based on virological surveillance

The French national pandemic plan includes an early containment phase followed by a limitation phase. The efficacy of such a plan depends on pre-existing surveillance and laboratory networks. The GROG community surveillance network and the hospital lab networks organized by the two French NICs carried out the virological monitoring of the A(H1N1)2009 pandemic. The NICs set up and distributed the RT-PCR tools to the lab networks early May 2009. During the containment phase, all suspected and virologically confirmed cases were hospitalized and declared to InVS. During the limitation phase, the clustered cases were monitored, and GROG swabs were collected by practitioners in general population. During the pandemic, the NICs carried out additional testing for the monitoring of antiviral resistance and of genetic changes involved in virus adaptation (PB2) and virulence (HA). The first imported A(H1N1)2009 influenza cases were detected at the end of April 2009. Local transmission was observed at the end of May. Clusters were observed in schools in June and in summer camps during summer. Sporadic cases were reported up until October when the pandemic wave started. One single 10 week-long pandemic wave was observed between mid-October and the end of December. Overall, 103 352 samples have been tested with 24 279 positives. The weekly positive rate ranged from 0% to 48% with a peak week 48 (3877 positives). Phylogenetic and antigenic analyses did not show any emerging genetic or antigenic variants. Eight cases had a D222G mutation in the HA. Eleven cases had an oseltamivir-resistant virus (H275Y); one harboured a reduced sensitivity to zanamivir (additional I223R mutation). All but one resistant virus were detected in treated immunocompromised patients. According to the profile of hospitalized cases, A(H1N1) 2009 was more virulent than seasonal viruses. Even if the mortality was limited (312 cases), the age distribution of the deceased patients was different as compared to seasonal influenza (75% mortality in < 65 years of age). The virological monitoring of the pandemic was achieved by the preexisting seasonal influenza networks.