The molecular epidemiology of influenza viruses: a lesson from a highly epidemic season.
OBJECTIVE
To analyse the epidemiological and molecular features of a long-lasting epidemic (12 weeks) of influenza in north-eastern Italy during the 2004-05 season.


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


RESULTS
The incidence peak (16.4 per thousand) was the highest recorded over the last six years in north-eastern Italy. The epidemic was sustained by two subsequent waves of circulating viruses: an H3N2 variant and two type B variants, respectively. In addition, scattered isolation of an H1N1 variant occurred. Antigenic and molecular characterisation showed the emergence of an H3N2 virus drifted with respect to vaccine strain, which also had a substantial impact on morbidity in vaccinated subjects. 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 results used for establishing the vaccine composition for the incoming season. The particular epidemiological features of two B virus clades, namely Yamagata-like and Victoria-like, may be considered for introduction into the influenza vaccine.
D'Agaro
P
P
Department of Public Medicine Sciences, U.C.O. Hygiene and Preventive Medicine, CIRI-IV, University of Trieste and IRCCS Burlo Garofolo, Trieste, Italy.
Rossi
T
T
Burgnich
P
P
Molin
G Dal
GD
Coppola
N
N
Rocco
G
G
Campello
C
C
D000293:Adolescent; D000328:Adult; D000940:Antigenic Variation; D002648:Child; D002675:Child, Preschool; D019049:Developed Countries; D004196:Disease Outbreaks; D014943:Global Health; D006385:Hemagglutination Inhibition Tests; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D006801:Humans; D015994:Incidence; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D053122:Influenza A Virus, H3N2 Subtype / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology; D007558:Italy; D008875:Middle Aged; D017720:Molecular Epidemiology; D009017:Morbidity; D009975:Orthomyxoviridae / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification*; D010802:Phylogeny; D017422:Sequence Analysis, DNA
D019267:Hemagglutinin Glycoproteins, Influenza Virus; C493831:hemagglutinin HA1, influenza B virus