Volume 23-18
September 15, 1997

[Table of Contents]

1996-1997 INFLUENZA SEASON: CANADIAN LABORATORY DIAGNOSES AND STRAIN CHARACTERIZATION

Introduction

In collaboration with the World Health Organization (WHO) international collaborating laboratories, provincial laboratories, and other Canadian hospital and university-based virus laboratories, the Laboratory Centre for Disease Control (LCDC) conducts national surveillance on human influenza viruses. This surveillance monitors influenza activity, detects and describes antigenic changes in the circulating strains of influenza virus in Canada, and estimates, through periodic serosurveys, susceptibility to currently circulating and emerging strains. 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.

Influenza Activity

Figure 1 shows the number and month of laboratory-confirmed influenza virus isolations, detections, and serodiagnoses reported from laboratories that contribute to the Canadian Virus Reporting (CVR) program, a surveillance program covering all laboratory-diagnosed viral infections. Since 1989, the number of laboratories participating in CVR has increased gradually (30, 30, 32, 33, 37, 37, 39, 42, respectively), which should be taken into consideration when comparisons are made over different years. The 1996- -1997 influenza season in Canada began in late November 1996 and continued to May 1997. During this period, there were 1,173 reports of influenza B viruses, with the largest number (492) occurring in March. There were 1,953 laboratory reports of influenza A and influenza A subtypes in the same period; the peak was reached in January.

During influenza seasons, influenza A and influenza B viruses have usually predominated alternatively during a particular season. However, in the 1996-1997 season, both A and B types prevailed in Canada, although the type A viruses peaked earlier whereas the type B viruses occurred late during the season. Furthermore, the activity level of both type A and type B viruses in the 1996-1997 season was relatively higher than those of the previous 4 years, which could not be explained solely by the increasing number of reporting laboratories.

Strain Characterization

During the 1996-1997 influenza season, isolates submitted from provinces to LCDC for strain typing turned out to be highly homogenous. All type A isolates except one were A/Wuhan/359/ 95-like(H3N2) (223) and all of the 62 type B isolates were B/Beijing/184/93-like ( Figure 2 and Table 1 ). No subtype H1N1 isolates were submitted to LCDC. A large number of influenza A isolates were still arriving at LCDC in March and most of the type B isolates were received in February and March. Table 1 indicates the provincial source and identity of submitted isolates.

A quick genetic approach has been developed at LCDC to facilitate the screening for influenza virus variants. Preliminary testing of the approach on influenza isolates received during the 1996-1997 season indicated that some strains showed variation from the vaccine strains, although the standard hemagglutination inhibition assay did not identify significant antigenic changes. Work is under way at LCDC to correlate identified genetic variations with antigenic and genetic characterization of influenza virus variants.

Discussion

The past 1996-1997 influenza season in Canada was more severe than the previous ones; both type A and type B infections reached high levels in the season according to laboratory reports from provinces. However, all of the characterized isolates were homogenous and antigenically similar to the vaccine component strains.

Figure 1 Laboratory evidence of human influenza virus infections in Canada

Figure 2 LCDC antigenic characterization completed on influenza virus isolates in the 1996-1997 season, by month of submission

Globally, influenza A(H3N2), A(H1N1), and B viruses continued to circulate worldwide⁽¹⁾. Most antigenically characterized influenza A(H3N2) viruses were similar to the reference strain A/Wuhan/359/95 and the antigenically equivalent vaccine strain A/Nanchang/933/95. Although influenza A(H1N1) viruses were isolated only sporadically during the 1996-1997 influenza season, an increasing number of antigenically characterized isolates showed variation from the vaccine strain A/Texas/36/91 but were close to A/Bayern/07/95. Most influenza B viruses were similar to the reference strains B/Beijing/184/93 and B/Harbin/07/94. A small number of type B viruses that were related to the antigenically distinct B/Victoria/02/87 were isolated in Asia⁽¹⁾.

Vaccines containing A/Nanchang/933/95 and B/Harbin/07/94 induced antibodies with similar frequency and titre to the vaccine viruses and to recently isolated H3N2 and B strains. However, the H1N1 component A/Texas/36/91 induced a good antibody response to the vaccine strain but less frequent and reduced antibody responses to recent H1N1 isolates such as A/Bayern/ 07/95⁽¹⁾. Therefore, WHO recommended the following strains as vaccine components for the 1997-1998 season⁽²⁾: A/Wuhan/359/95-like(H3N2), A/Bayern/07/95-like(H1N1), and B/Beijing/184/93-like. Basically, the H1N1 component is changed and the antigenically equivalent strain that will be used by American vaccine manufacturers is A/Johannesburg/82/96⁽¹⁾.

Acknowledgements

The collaboration of laboratories in the CVR program and of provincial and hospital laboratories who 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:

British Columbia Centre for Disease Control, Virology Services, Vancouver, BC;

Virology and Reference Laboratory, U.B.C., Vancouver, BC;

Provincial Laboratory of Public Health for Southern Alberta, Calgary, AB;

Provincial Laboratory of Public Health for Northern Alberta, Edmonton, AB;

Saskatchewan Public Health Laboratory, Laboratory and Disease Control Services Branch, Regina, SK;

Cadham Provincial Laboratory, Winnipeg, MB;

Regional Public Health Laboratory, Laboratory Services Branch, Virus Laboratory, Toronto, ON;

Regional Public Health Laboratory, Peterborough, ON;

Regional Public Health Laboratory, Kingston, ON;

Regional Public Health Laboratory, Ottawa, ON;

Children's Hospital of Eastern Ontario, Ottawa, ON;

Regional Public Health Laboratory, Timmins, ON;

Regional Public Health Laboratory, Thunder Bay, ON;

Laboratoire de santé publique du Québec, Sainte-Anne-de-Bellevue, QC;

Centre hospitalier St-Joseph, Trois-Rivières, QC;

Hôpital G.L. Dumont, Moncton, NB;

Victoria General Hospital, Halifax, NS.

Carol Stansfield of LCDC conducted the influenza strain typing.

References

1. CDC. Update: Influenza activity - United States and Worldwide, 1996-97 season, and composition of the 1997-98 influenza vaccine. MMWR 1997;46:325-30.

2. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1997-1998 season. WHO Wkly Epidemiol Rec 1997;72:57-61.

Source: S Zou, PhD, Surveillance, Influenza and Viral Exanthemata, National Laboratory for Special Pathogens, Bureau of Microbiology, LCDC, Ottawa, ON.

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