The molecular epidemiology of influenza viruses: a lesson from a highly epidemic season

Abstract

Aims: To analyse the epidemiological and molecular features of a long-lasting epidemic (12 weeks) in North-Eastern Italy during season 2004/05.

Methods: Morbidity rates were analysed by time and age. Influenza virus isolates (93 strains) were submitted to antigenic evaluation by HI test and to molecular assessment by sequencing.

Results: The incidence peak (14.9‰) was the highest recorded over the last six years in North-Eastern Italy. The epidemic was sustained by two subsequent waves of circulating viruses, namely a H3N2 variant and two type B variants, respectively. In addition, scattered isolation of a H1N1 variant occurred. Antigenic and molecular characterization showed the emergence of a H3N2 virus drifted with respect to vaccine strain, entailing substantial impact on morbidity in vaccinated subjects, too. Moreover, a single K145N substitution in the HA1 site of H3N2 was the starting point of two evolutionary branches. No change was observed in H1N1 isolates. B-type virus was mainly represented by Victoria-lineage strains, though Yamagata-lineage viruses were also identified. The fluctuating circulation of these two clades has characterised B virus epidemics in recent years.

Conclusions: The assessment of the H3N2 molecular change in this area was in line with whole results used for establishing the vaccine composition for the incoming season. The particular epidemiological features of two B virus clades, namely Yamagata- and Victoria-like, may be taken into consideration to be introduced into the influenza vaccine.