OBJECTIVETo characterize the prevalence and antigenic drift of influenza A viruses isolated during the period from 2001 to 2005 in infants and young children in Beijing.

METHODSMDCK cell culture, indirect immunofluorescence assay (IFA) and hemagglutination inhibition (HI) assay were used to isolate and identify type A influenza viruses (H1N1 and H3N2) from clinical samples collected from outpatients and inpatients who visited the Affiliated Children's Hospital because of acute respiratory infections from Oct. 2001 to Aug. 2005. The HA1 regions of hemagglutinin gene of H3N2 isolates were amplified by using RT-PCR followed by sequencing.

RESULTSOut of 7338 clinical samples collected during this surveillance period, 347 (4.7%) were positive for influenza A viruses, including 48 (13.8%) of H1N1, 273 (78.7%) of H3N2 and 26 (7.5%) of subtype-unidentified influenza A viruses. Although there was a prevalence season of influenza A from October each year to April of next year during the 2001-2004 period, it was worth noting that a consecutive influenza A activity was detected from Aug. 2004 to Aug. 2005, when some influenza A viruses were detected even in summer. The positive rate of H3N2 was 14.2% in August, 2005, which was equal to that of the peak season of 2003-2004. H3N2 were predominant in most of the influenza seasons during the surveillance period, and H1N1 was detected only in the influenza seasons of the 2001-2002 and 2004-2005 along with H3N2. The positive rates for both H3N2 and H1N1 were higher in specimens from outpatients than those from inpatients. A total of 46.6% (110/236) of the H3N2 were detected from children younger than 2 years of age, and 14.0% (33/236) were from children older than 5 years, whereas, more H1N1 was found in children older than 5 years (48.0%, 12/31) than in those younger than 2 years (6.5%, 2/31) during a period from Nov. 2003 to Aug. 2005. Sequence analysis of the HA1 regions of hemagglutinin of H3N2 isolated in a series of years revealed amino acid changes in the HA1 domain of H3N2 isolates in the antigenic sites (A-E) each year.

CONCLUSIONH3N2 and H1N1 prevailed in each influenza season during the surveillance period in Beijing, and H3N2 strains were predominant. The data from all-year around surveillance of influenza in Beijing indicate that continuous surveillance throughout a year and use of both antigenic and molecular analysis will be more helpful for early identification of any antigenic variants as well as prevention and control of influenza by promoting development of vaccines.