 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
|
|
||||||||||||||||
| |
|
||||||||||||||||
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Volume 25-02 |
1997-1998 INFLUENZA SEASON: CANADIAN LABORATORY DIAGNOSES AND STRAIN CHARACTERIZATIONIntroduction In collaboration with the World Health Organization (WHO) international collaborating laboratories, provincial laboratories, and other Canadian hospital and university-based laboratories, the Laboratory Centre for Disease Control (LCDC) conducts national surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. Canadian influenza surveillance information and actual representative strains are then shared with the WHO's collaborating centres for influenza to contribute to global influenza monitoring. This report summarizes influenza surveillance in Canada for the 1997-1998 influenza season. Influenza Activity The 1997-1998 influenza season in Canada began in early November 1997 and continued until May 1998. During this period, there were 4,470 cases of laboratory-confirmed influenza infections reported to LCDC by laboratories that contribute to the Canadian Virus Reporting (CVR) program, a surveillance program covering all laboratory-diagnosed viral infections. Figure 1 shows the numbers of and the months of laboratory-confirmed influenza virus isolations, detections, and serodiagnoses. Of the 4,470 reports, 4,441 (99.4%) were influenza type A, with the peak (2,139) occurring in February. There were only 29 (0.6%) cases of influenza type B (not shown in Figure 1). These data indicated that influenza A viruses predominated during the 1997-1998 season, with peak activity between January and March (Figure 1). This is in contrast to the 1996-1997 influenza season, in which both influenza type A and B viruses (1,953 and 1,173 reports, respectively) prevailed in Canada(1). Furthermore, the level of activity of type A viruses in the 1997-1998 season appeared to be relatively higher than those of the previous 5 years(1). Figure 1 Laboratory-confirmed influenza virus infections in Canada during the 1997-1998 season
Strain Characterization During the 1997-1998 influenza season, 441 influenza virus isolates (440 of influenza A and one of influenza B) were submitted to LCDC for strain typing and subtyping analysis. Of the 440 subtyped influenza A isolates, 432 (98%) were H3N2 and eight (2%) were H1N1 (Figure 2 and Table 1). Among the 432 H3N2 isolates, 71 (16%) were similar to A/Wuhan/359/95, the strain included in the 1997-1998 influenza vaccine(2), and 361 (82%) were similar to A/Sydney/05/97, a related but antigenically distinguishable variant of A/Wuhan/359/95 (Table 1). The proportion of influenza strains that were A/Sydney/05/97-like rapidly increased from January through April: 72% (41 of 57) in January, 80% (107 of 133) in February, 87% (139 of 160) in March, and 92% (70 of 76) in April (Figure 1). Only one type B isolate was submitted to LCDC, and was similar to the 1997-1998 vaccine component strain B/Beijing/184/93. All of the eight H1N1 isolates were characterized as A/Texas/36/91-like, which is antigenically different from the vaccine component strains A/Johannesburg/82/96 and A/Bayern/07/95. Table 1indicates the provincial source and identity of submitted isolates. Figure 2 Antigenic characterization completed on influenza virus isolates in the 1997-1998 season, by month of submission
Table 1 Strain characterization completed on influenza isolates in Canada, submitted from November 1997 to 6 May 1998
Consistent with antigenic analysis, genetic characterization of 20 Canadian A/Sydney/05/97-like isolates, which were collected from across Canada during the first half of the 1997-1998 influenza season, confirmed the antigenic difference between Canadian A/Sydney/05/97-like isolates and the 1997-1998 H3N2 vaccine component strain A/Wuhan/359/95(3). Discussion The 1997-1998 season was dominated by influenza A viruses (99.4%) and, during this season, the highest number of laboratory-confirmed cases (4,470) was recorded by LCDC for any winter in the period 1978-1997(4). This increase in cases was not due to an increase in reporting laboratories; 40 laboratories participated in CVR in this season compared to 42 in the previous one(1). Strain characterization showed that two antigenetically-related but distinguishable strains of influenza A(H3N2) viruses, A/Wuhan/359/95-like and A/Sydney/05/97-like, co-circulated in Canada during the 1997-1998 influenza season; the A/Sydney/05/97-like strain was predominant. Whereas, in the previous season, all of the characterized isolates of influenza A and B were antigenically similar to the vaccine component strains(1). The A/Sydney/05/97-like strain also caused considerable morbidity and mortality in Canada and the United States(2,5). Identification of the new strain of influenza A(H3N2) virus, A/Sydney/05/97-like, in Canada further emphasizes the importance of timely submission of influenza isolates for antigenic characterization. Similar trends were observed worldwide; influenza A(H3N2) viruses were mostly identified as A/Sydney/05/97-like in many countries, suggesting that the 1997-1998 vaccine component, A/Wuhan/359/95, provided only limited protection to vaccinated individuals(2). Influenza A(H1N1) and influenza B viruses were isolated only sporadically during the 1997-1998 influenza season in many countries, including Canada. Most influenza B isolates were similar to B/Beijing/184/93 and the 1997-1998 vaccine component strain B/Harbin/07/94. Although influenza A(H1N1) viruses were isolated in a number of countries during the 1997-1998 influenza season, an increasing number of antigenically characterized isolates showed variation from the vaccine component strain A/Bayern/07/95 but were close to A/Beijing/262/95 that was identified recently in several countries in other continents. Current vaccines containing B/Harbin/07/94 induced antibodies with similar frequency and titre to the vaccine viruses and to recently isolated B/Beijing/184/93-like strains. However, vaccines containing A/Wuhan/359/95-like(H3N2) and A/Bayern/07/95-like(H1N1) viruses induced good antibody responses to the vaccine strains, but less frequent and reduced antibody responses to recent isolates such as A/Sydney/05/97 and A/Beijing/262/95(2,6). Therefore, WHO recommended the following strains as vaccine components for the 1998-1999 season(6):
Acknowledgments The collaboration of laboratories in the CVR program and of provincial and hospital laboratories that forwarded early and representative isolates of influenza virus is a vital part of influenza surveillance in Canada. Influenza virus isolates were submitted from the following centres:
Carol Stansfield of LCDC conducted the influenza strain typing. References
Source: Y Li, PhD, Respiratory Viruses Section, Bureau of Microbiology, LCDC, Federal Laboratories, Winnipeg MB.
[Previous]
[Table of Contents] [Next] |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|||
|
Last Updated: 2002-11-08 |  |
|
