Volume 27-11
1 June 2001

[Table of Contents]

RESPIRATORY VIRUS SURVEILLANCE

FluWatch Project, 2000-2001
End of Season Update

This report provides a summary of influenza surveillance in Canada during the 2000-2001 season, up to the final week of the season, ending 21 April 2001.

Synopsis

The 2000-2001 season was a relatively mild season worldwide. In Canada, lower than usual activity was reported for all national indicators of influenza activity, including the rate of influenza-like illness (ILI), the percentage of laboratory-confirmed cases of influenza and provincial/territorial influenza activity levels. However, there were a number of interesting characteristics of this year's influenza season. In contrast to the predominance of influenza A, and in particular the A/Sydney/5/97 (H3N2)-like virus over the past 3 years, influenza B predominated overall this season. Influenza A (H3N2) accounted for < 1% of all characterized isolates (H1N1 accounted for 49% of isolates). Increased laboratory-confirmed influenza activity began in the West (Yukon, prairie provinces and British Columbia) in mid-December, followed by the Atlantic provinces in mid- to late January and Ontario and Quebec in mid-February and March.

2000-2001 Influenza Season Update

Although there was no prominent peak in ILI reporting rates during the 2000-2001 season, the highest weekly rate, 31.4 cases of ILI per 1,000 patient visits, occurred during week seven (week ending 17 February 2001) (Figure 1). This was considerably lower, and later, than last season's peak of 149.1 cases per 1,000 during week 52. During week seven, 2001 the age groups with the highest ILI rates were 5 to 19 years of age (27/1,000) and <= 5 years of age (25/1,000), compared to those > 65 years who had the lowest ILI rate (3/1,000). The peak of laboratory-confirmed influenza occurred around the same time as the peak in ILI. Of all ILI tests done 16% were positive for influenza during weeks seven through 10. The peak was due to simultaneous influenza B and influenza A activity during this 3-week period. The ratio of positive tests for influenza A was highest in week seven, while the number of positive tests for influenza B was highest in week nine (Figure 2). The peak in provincial/territorial reported influenza activity levels occurred slightly earlier, with the greatest number of regions reporting localized and widespread activity during week five. This year was the first year the implementation of national collection of outbreak data, by type of facility. Since week 40 (ending 7 October 2001), 219 outbreaks of ILI were reported across Canada, 49 of these were in long-term care facilities, six were in hospitals and 164 were in other sites.

Figure 1: Census Division weighted age-standardised influenza-like illness (ILI) rates*, Canada, by report week for the 2000-2001 Influenza season compared to 1996/97 through 1999/2000 seasons (mean rate with 95% confidence intervals)

A total of 46,336 reports for influenza laboratory tests were received by the Respiratory Virus Detection system between 27 August, 2000 and 21 April, 2001. 4,154 (9%) tests were confirmed positive for influenza; 1,324 (32%) were influenza A and 2,830 (68%) were influenza B. The provincial distribution of positive influenza A tests is as follows: Newfoundland (28), Nova Scotia (33), Prince Edward Island (two), New Brunswick (44), Quebec (246), Ontario (400), Manitoba (44), Saskatchewan (161), Alberta (248), British Columbia (118). In comparison, the provincial distribution for positive influenza B tests is Newfoundland (102), Nova Scotia (150), Prince Edward Island (18), New Brunswick (95), Quebec (710), Ontario (452), Manitoba (110), Saskatchewan (445), Alberta (543), British Columbia (205).

Twelve percent, (506/4,154) positive influenza isolates, were referred to the National Microbiology Laboratory (NML) for strain characterization. Of the influenza A isolates identified, 19% (248/1,324) were antigenically characterized: 99% (246) were H1N1 and 1% (two) were H3N2. Of the influenza A (H1N1) isolates, 98% (241/246) were similar to A/New Caledonia/20/99 and 2% (5/246) were similar to A/Johannesburg/82/96. Although, A/Johannesburg/82/96 (H1N1)-like viruses are antigenically distinct from A/New Caledonia/20/99 (H1N1)-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titres of antibody that cross-react with A/Johannesburg/82/96 (H1N1)-like viruses⁽¹⁾. The two influenza A (H3N2) viruses that have been characterized were antigenically similar to the vaccine strain A/Panama/2007/99. Of the influenza B isolates, 9% (258/2,830) were antigenically characterized as B/Yamanashi/166/98. However, it has been noted that 89% of the influenza B isolates characterized by the Centers for Disease Control and Prevention (CDC) were more closely related antigenically to B/Sichuan/379/99, a reference strain which has been found to cross react with B/Yamanashi/166/98*. Preliminary results of genetic analyses completed at the NML showed that this antigenic change was found in a percentage of the Canadian influenza B isolates⁽²⁾. It should also be noted that B/Sichuan/379/99 virus exhibits cross-reactivity with the 2000-2001 vaccine strain. The vaccine strains in the 2000-2001 influenza vaccine either matched or produced high titres of antibody that cross-reacted with all of the A and B strains characterized by the NML during the 2000-2001 season.

In the USA, influenza activity peaked in late January and early February, 2001. During weeks three to five, the overall percentage of patient visits for ILI peaked at 4%, compared to a peak range of 5% to 6% in the previous 3 years. Likewise, the peak in the percentage of laboratory tests positive for influenza was 24% in week 4, compared to a peak range of 28% to 33% in the previous 3 years. The proportion of all deaths due to pneumonia and influenza has been below the epidemic threshold throughout the season⁽¹⁾. The overall percentage of positive laboratory tests for influenza A was 55%. Thirty eight percent of influenza A isolates were characterized, of which 95% were A/New Caledonia/20/99 (H1N1)-like and 5% were A/Bayern/07/95 (H3N2)-like. Six percent of influenza B isolates were characterized, of which 88% were B/Sichuan/379/99-like and 12% were B/Beijing/184/93-like.

Internationally, influenza activity in the Northern hemisphere was initially reported during the third week of November 2000 and peaked in early to mid-February 2001. Influenza activity was mainly due to influenza A (H1N1) which co-circulated with influenza B. Influenza A (H3N2) viruses were isolated sporadically. Many countries reported outbreaks of both influenza A (H1N1) and influenza B. However, no countries reported any outbreaks of influenza A (H3N2). In most countries, children and young adults were most greatly affected⁽²⁾.

Since, 24 September, 2000, 22,568 influenza virus isolates have been reported to FluNet. Of these, 49% (11,132) were influenza B and 51% (11,436) were influenza A. Of the influenza A viruses, 8% (1,733) were influenza A (H1N1), 4% (805) were influenza A (H3N2) and 39% (8,898) were unsubtyped influenza A⁽²⁾.

The World Health Organisation recommends that the vaccine for the 2001-2002 season (Northern hemisphere, winter) should contain the following components:

• an A/New Caledonia/20/99(H1N1)-like virus
• an A/Moscow/10/99(H3N2)-like virus†
• a B/Sichuan/379/99-like virus‡

*    The CDC reports that although A/Bayern-like viruses are antigenically distinct from the A/New Caledonia-like viruses, the A/New Caledonia/20/99-like vaccine strain produces high titres of antibody that cross-react with A/Bayern/07/95-like viruses⁽¹⁾. Likewise, the B/Beijing/184/93-like vaccine strain (and the Canadian equivalent, B/Yamanashi/166/98-like vaccine strain) produce high titres of antibody that cross-react with B/Sichuan/379/99-like virus⁽¹⁾.

†    The widely used vaccine strain A/Panama/2007/99 (H1N1) is an A/Moscow/10/99(H1N1)-like virus.

‡    B/Johannesburg/5/99 and B/Victoria/504/2000 are B/Sichuan/379/99-like viruses, which have been used for vaccine production⁽²⁾.

FluWatch reports are published weekly (October through April) and can be accessed through Health Canada's FluWatch website: <http://www.phac-aspc.gc.ca/fluwatch/index.html>.

References

1. CDC. Influenza summary update, Week ending April 21, 2001-Week 16. URL: <http://www.cdc.gov/ncidod/diseases/flu/weeklychoice.htm>. Date accessed: May 3, 2001.

2. Recommended composition of influenza virus vaccines for use in the 2001-2002 season. Weekly Epidemiologic Record. 2001;76:58.

3. WHO, FluNet. Isolates/activity, ILI activity, World. URL: <http://www.who.int/GlobalAtlas/home.asp>. Date accessed: May 3, 2001.

Source: JF Macey, MA, MSc, Field Epidemiology Training Program, Centre for Surveillance Coordination and Division of Respiratory Diseases, Bureau of Infectious Diseases; B Winchester, BSc, MSc, SG Squires, MSc, T Tam, MD, Division of Respiratory Diseases; P Zabchuk, Division of Disease Surveillance, Bureau of Infectious Diseases, Centre for Infectious Disease Prevention and Control, Ottawa; Y Li, PhD, National Microbiology Laboratory, Winnipeg, Manitoba.

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