Influenza in Canada - 1999-2000 Season

Volume 27-01
1 January 2001

[Table of Contents]

INFLUENZA IN CANADA - 1999-2000 SEASON

Introduction

The Centre for Infectious Diseases Prevention and Control (CIDPC), formerly part of the Laboratory Centre for Disease Control, maintains a national influenza surveillance program, FluWatch. The objective of FluWatch is to provide a national picture of influenza activity across Canada during the influenza season. FluWatch has four main data components: 1) laboratory-based influenza virus identification in Canada; 2) influenza-like illness (ILI) surveillance in Canada; 3) provincial and territorial influenza activity levels, as assigned by the provincial and territorial FluWatch representatives, and; 4) international influenza activity reports from the United States Centers for Disease Control and Prevention (CDC) and other countries' government agencies, and the World Health Organization (WHO).

The FluWatch program disseminates information on influenza activity to public-health professionals and the public using a variety of mechanisms including the CIDPC FAXlink (dial 613-941-3900 from a telephone-equipped Fax machine), Fax, E-mail, and Health Canada's Division of Respiratory Diseases' Website <http://www.phac-aspc.gc.ca/fluwatch/index.html>. FluWatch reports are made available, for the most part, on a weekly basis during the influenza season and summaries of laboratory surveillance data are made available weekly throughout the year. Summaries of influenza activity worldwide are included periodically in the weekly Infectious Diseases News Brief which is also available on the Division of Disease Surveillance Website <http://www.phac-aspc.gc.ca/bid-bmi/dsd-dsm/nb-ab/index.html>. Surveillance reports on influenza virus activity are published periodically in the Canada Communicable Disease Report.

This report summarizes case-by-case data on laboratory-confirmed influenza infection, reports of ILI, and other influenza activity indicators for the1999-2000 influenza season. Comparisons are made with previous seasons^(1-3).

Methods

Laboratory-confirmed influenza: Laboratories participating in the case-by-case surveillance program were asked to report the epidemiologic and laboratory information for each isolation and identification made by viral culture, direct antigen detection, and seroconversion (i.e. >= fourfold rise in antibody titre by any method) to the Division of Disease Surveillance, CIDPC. Laboratory-confirmed case-by-case data were presented by the province from which the specimen originated (some laboratories received out-of-province samples), and were analyzed by week of onset of illness, the age of the case, influenza type and subtype.

Influenza-like illness reported by sentinel physicians: The College of Family Physicians of Canada (CFPC) National Research System (NaReS) was responsible for recruiting sentinel physicians across Canada, except in British Columbia, Alberta, and Saskatchewan, where provincial sentinel physician surveillance programs were already in place. FluWatch was able to tap into these existing surveillance systems. Ontario and Quebec had two sentinel physician systems each: FluWatch and a provincial system. The objective was to recruit at least one physician from each of the census divisions across Canada or for census divisions with large populations, to recruit one sentinel physician per 250,000 population. For one clinic day each week, between 10 October1999 and 22 April 2000, sentinels were asked to complete a report form with the number of patients seen (denominator) and the number of patients meeting a standard case definition for ILI (numerator). The case definition for ILI was "acute onset of respiratory illness with fever and cough and with one or more of the following: sore throat, arthralgia, myalgia, or prostration which could be due to influenza virus. (Presentations could vary in pediatric or geriatric populations.)" Age group information was collected for all patients (numerator and denominator) seen through sentinel physicians recruited by NaReS, and for patients seen through provincial surveillance systems in British Columbia, Saskatchewan, Ontario. In Alberta, age group information was collected only on numerator data; age group for denominator data was generated by applying the Canadian population distribution. In Quebec, the provincial surveillance system did not collect age group information. Sentinel report forms were either returned by Fax, or the information was conveyed via E-mail or telephone to the CIDPC on a weekly basis. The CIDPC then collated and analyzed the data and prepared a report which was distributed, for the most part, on a weekly basis.

Influenza activity assessed by the provincial and territorial epidemiologists: On a weekly basis, provincial and territorial epidemiologists or influenza surveillance representatives assessed the influenza activity level in their respective jurisdictions using a variety of sources of information which may have included: laboratory confirmation of influenza, sentinel physician ILI surveillance, reports of outbreaks, school and work-site absenteeism, and emergency and department and hospital admission data. Most provinces and territories were subdivided into influenza surveillance regions as defined by the provincial or territorial epidemiologist. For the 1999-2000 influenza season, there were a total of 53 surveillance regions: British Columbia (four), Alberta (three), Saskatchewan (three), Manitoba (12), Ontario (five), Quebec (one), New Brunswick (seven), Nova Scotia (four), Prince Edward Island (one), Newfoundland (10), Yukon (one), Northwest Territories (one), and Nunavut (one). Influenza activity levels were defined as: no activity reported, sporadic, localized, and widespread activity.*

Results

Laboratory-confirmed influenza: During the 1999-2000 laboratory surveillance period (4 September 1999 to 31 August 2000), a total of 5,907 case-by-case records were reported to CIDPC by 16 laboratories in 10 provinces (Table 1). This compared with 4,203 cases reported by 16 laboratories in nine provinces for the previous season (1998-1999). The variation in numbers of confirmed cases and distribution of virus type and subtype among provinces should be interpreted with caution; these numbers are likely to reflect differences in population size and distribution, testing and reporting practices and criteria, and the availability of diagnostic services.

Table 1 Laboratory-confirmed cases of influenza reported to CIDPC, by laboratory, Canada, 1999-2000
Province	Laboratory	No. of Cases
Newfoundland	Newfoundland Public Health Laboratory, St. John's	115
Prince Edward Island	Queen Elizabeth Hospital Inc., Charlottetown	14
Nova Scotia	Queen Elizabeth II Health Science Centre - Victoria General Site, Halifax	195
New Brunswick	Hôpital G.L. Dumont, Moncton	107
Quebec	Laboratoire de santé publique du Québec, Sainte-Anne-de-Bellevue	1,604
Ontario	Kingston Public Health Laboratory Central Public Health Laboratory, Toronto Hospital for Sick Children, Toronto Toronto Medical Laboratory Thunder Bay Public Health Laboratory	138 1,264 179 23 39
Manitoba	Cadham Provincial Laboratory,Winnipeg	305
Saskatchewan	Department of Health, Regina Saskatoon Public Health Laboratory	386 64
Alberta	Provincial Laboratory of Public Health for Northern Alberta, Edmonton Provincial Laboratory of Public Health for Southern Alberta, Calgary	770 330
British Columbia	Division of Laboratories, Health Branch, Vancouver	374
Total		5,907

Table 2 shows laboratory-confirmed case-by-case data, by province and influenza type and subtype for cases reported in the 1999-2000 season. The largest number and proportion of cases were recorded in Ontario, 1,643 cases (27.8%); Quebec, 1,602 cases (27.1%); Alberta, 1,098 cases (18.6%); and Saskatchewan, 450 cases (7.6%). The majority of isolates, 5,820 (98.5%), were of type A virus; 87 (1.5%), were of type B. This represents a decrease in the reporting of influenza B virus infections when compared with the previous season⁽¹⁾. Alberta and Saskatchewan reported the greatest number and proportion of influenza B isolates in Canada, with 36 (41.4%) and 24 (27.6%) respectively. Of the 5,820 influenza A virus identifications, 772 were subtyped: 722 (93.5%) were of the H3N2 subtype and 50 (6.5%) were of the H1N1 subtype.

Figure 1 shows laboratory-confirmed case-by-case data, by type and week of onset, for Canada and the six regions: Atlantic (Newfoundland, Prince Edward Island, Nova Scotia, New Brunswick), Quebec, Ontario, the Prairies (Manitoba, Saskatchewan, Alberta), British Columbia, and the Territories. Although confirmed cases were consistently reported earlier in the Prairies, 66% of all cases in Canada were reported in December and January, with 25% of all cases reported in the 2-week period (week 52 of 1999 and week 1 of 2000). Marked peaks in influenza A laboratory-confirmed cases were evident in all the regions except the Territories.

Figure 1
Laboratory-confirmed cases of influenza by region, type, and week of onset, Canada, 1999-2000
Figure 1 Fluwatch 1999-2000

Figure 2 shows the proportionate distribution of laboratory-confirmed case-by-case infections, by age group. During the 1999-2000 season, 42% of laboratory-confirmed cases were recorded in persons >= 65 years of age.

Figure 2
Proportionate distribution of laboratory-confirmed cases of influenza by age group, Canada, 1999-2000

Figure 2 Fluwatch 1999-2000

Table 2 Laboratory-confirmed cases of influenza, by province and by influenza type and subtype, Canada, 1999-2000
Influenza Type	Nfld.	P.E.I.	N.S.	N.B.	Que.	Ont.	Man.	Sask.	Alta.	B.C.	Yukon	NWT	Nunavut	Total
Type A
Not subtyped	104	14	195	101	1,534	1,280	267	384	785	322	23	1	38	5,048
H1N1	0	0	0	0	17	26	0	1	6	0	0	0	0	50
H3N2	11	0	0	6	35	335	0	41	271	23	0	0	0	722
Total type A	115	14	195	107	1,586	1,641	267	426	1,062	345	23	1	38	5,820
Type B	0	0	0	0	16	2	2	24	36	6	0	1	0	87
Total	115	14	195	107	1,602	1,643	269	450	1,098	351	23	2	38	5,907

Laboratory confirmations: Virus isolation (78% of cases) and direct antigen detection (21% of cases) were the most commonly recorded methods for laboratory confirmation of case-by-case influenza infection. This represented an increase of confirmation by virus isolation compared to the previous season. The remaining cases (1%) for which information was available were confirmed by serology.

Types of influenza virus circulating during the 1999-2000 season: Figure 3 compares the seasonal distribution of laboratory-confirmed case-by-case influenza infections for the 1999-2000 season with the previous four seasons. Only one period of peak activity was observed for the most recent influenza season and, as already mentioned, the majority of isolates were of type A.

Strain characterization by the Respiratory Viruses Section of the National Microbiology Laboratory, was completed on 622 isolates: 579 influenza A isolates and 43 influenza B isolates. Of 579 influenza A isolates, 480 (83%) were identified as A/Sydney/5/97-like(H3N2) and 99 (17%) were identified as A/New Caledonia/20/99-like(H1N1). All of 43 influenza B isolates were characterized as B/Beijing/184/93-like. Table 3 shows the provincial distribution of identified strains for the 1999-2000 season. A/Sydney/5/97-like(H3N2) isolates were identified in all provinces. A/New Caledonia/20/99-like(H1N1) isolates were identified in western and central Canada and Nunavut. B/Beijing/184/93-like isolates were identified in western and central Canada⁽⁴⁾.

Figure 3
Seasonal distribution of laboratory-confirmed influenza infections, Canada, 1995-2000
Figure 3 Fluwatch 1999-2000

Influenza-like illness reported by sentinel physicians: Three hundred and thirty-two sentinel physicians were recruited in 236 (82%) of the 288 census divisions across Canada; most of the well-populated urban and rural divisions were represented, with the exception of those in Quebec. The physician response rate in 1999-2000 was good. Each week between late October and mid-April, CIDPC received ILI data from an average of 68% of FluWatch sentinels. During this same period of time, 239 sentinels reported ILI data for at least 50% of the reporting weeks and 39 sentinels reported ILI data for at least 90% of the reporting weeks.

Figure 4 shows the standardized rates of ILI across Canada by reporting week, from 10 October 1999 (week 41) to 22 April 2000 (week 16). The curve obtained was smoothed using the technique of Hamming and Tukey⁽⁵⁾. The ILI rates increased in mid-December (reporting week 49, 1999), and remained elevated for 6 weeks. The peak occurred during the last week of December (reporting week 52, 1999). Rates decreased in late January (reporting week 4, 2000) and remained low for the rest of the season. Over the ILI surveillance period, a total of 8,970 cases of ILI were diagnosed from 219,790 patients seen — an ILI rate of 41 per 1,000 patients seen compared to a rate of 33 per 1,000 patients seen in 1998-1999. The highest rates of ILI were in children: 78 cases of ILI per 1,000 patients seen in the 1- to 4-year-old age group and 61 per 1,000 patients seen in the 5- to 9-year-old age group.

Figure 4
Canadian ILI rate for the 1999-2000 season compared with 1996-1997 to 1998-1999 seasons
Figure 4 Fluwatch 1999-2000

Influenza activity level assessment: Figure 5 shows the number of influenza surveillance regions reporting widespread or localized influenza activity by week from 10 October 1999 (week 41) through 22 April 2000 (week 16). Alberta was the first province to report localized influenza activity in the week ending 16 October 1999 (week 41). Widespread activity was first reported in the week ending 11 December 1999 (week 49). Influenza activity was reported as widespread or localized in 30% or more of the 53 influenza surveillance regions between the week ending 18 December 1999 (week 50) and 29 January 2000 (week 4).

Figure 5 Number of influenza surveillance regions reporting widespread or localized influenza activity, Canada, by week and year - 10 October 1999 through 22 April 2000
Figure 5 Fluwatch 1999-2000

Discussion

In the 1999-2000 influenza season, the Prairie provinces were the first to report increased laboratory isolates and reports of regional influenza activity. Nationally, there was a peak in the reporting of laboratory-confirmed cases from the beginning of December to the end of January, and increased rates of ILI were spread over the same period. In the peak of the epidemic, ILI rates were higher in the 1999-2000 season compared to the previous three seasons. As well, the epidemic appeared as a well defined, single peak (i.e. more concentrated activity in a shorter period of time). In previous seasons, epidemic curves have differed: some seasons have had a second peak while other seasons have had a smaller peak, with a wider curve (i.e. the epidemic appeared over a longer period of time).

During the 1999-2000 influenza season the numbers of laboratory-confirmed case-by-case influenza infections reported to CIDPC were higher than for any influenza seasons in the past decade^(1-4,6-11). This increase in cases may be partly explained by the increase in influenza surveillance activities, small increases in the number of reporting laboratories, but likely also represented an increase in influenza activity. The age distribution among cases in 1999-2000 is similar to that of the1998-1999 season⁽¹⁾.

Table 3 Distribution of influenza strains characterized by the Respiratory Viruses Section of the National Microbiology Laboratory for the 1999-2000, by province and territory
Influenza	Nfld.	P.E.I.	N.S.	N.B.	Que.	Ont.	Man.	Sask.	Alta.	B.C.	Yukon	NWT	Nunavut	Total
Type A (H1N1) A/New Caledonia/20/99-like					28	38	4	1	8	2		18		99
Type A (H3N2) A/Sydney/5/97-like	13	4	21	7	49	278	10	37	32	29				480
Total A	13	4	21	7	77	316	14	38	40	31		18		579
Type B B/Beijing/184/93-like					9	3	1	17	9	4				43
Total B					9	3	1	17	9	4				43
Total	13	4	21	7	86	319	15	55	49	35		18		622

For the 1999-2000 season, 98.5% of the isolates were influenza A. As in the previous two seasons, A/Sydney/5/97-like(H3N2) predominated and represented 77.1% of all influenza strains characterized by the National Microbiology Laboratory. A/New Caledonia/20/99-like(H1N1) virus was isolated in Canada for the first time and represented 15.9% of influenza strains. The trends observed in Canada were somewhat similar to those in the United States, however, the United States identified two additional H1N1 strains (A/Bayern/07/95 and A/Beijing/262/95)⁽¹²⁾.

Two of the strains included in the 1999-2000 trivalent influenza vaccine matched the identified circulating strains in Canada: A/Sydney/05/99(H3N2) and B/Beijing/184/93 (B/Yamanashi/166/98 was the most common vaccine strain used to represent the B/Beijing/184/93 virus). A/New Caledonia/20/99(H1N1) was not included in the 1999-2000 vaccine, but is an antigenic variant of the H1N1 vaccine strain A/Beijing/262/95⁽⁴⁾.

In 1999-2000, children had the highest ILI rates compared to adults and the elderly. However, senior citizens (³ 65 years of age) represented 42% of laboratory confirmed cases of influenza. This apparent incongruity may be because of an age-related bias in terms of why people consult their physicians (hence children have higher ILI rates) and who physicians test for influenza (probably seniors because of the higher probably of influenza-related complications). Therefore caution should be used when interpreting age-specific data.

The FluWatch program likely provides a good overall picture of influenza activity in Canada. While each component of the program has its limitations, they appear to complement each other well. The main limitations were the following: 1) specimen collection and submission to the national laboratory were subject to the individual practices of the attending physicians and the availability of the test within and between provinces and territories, 2) the distribution of the sentinel physicians did not correlate with the population distribution in Quebec (although in the rest of Canada, it did correlate much better than in previous seasons), 3) the instability of comparing this season's ILI rate with baseline data which is based only on the three previous seasons (the baseline will become more stable over time) and, 4) the activity level provided by the provincial and territorial epidemiologists, although based on many indicators, is somewhat subjective.

Laboratories wishing to participate in the FluWatch surveillance program should contact Mr. Peter Zabchuk, Division of Disease Surveillance, Bureau of Infectious Diseases, CIDPC, at 613-952-9729.

Acknowledgements

We would like to thank the staff of the laboratories who participated in the respiratory virus surveillance program during the 1999-2000 season, and Dr. Yan Li and Ms. Carol Stansfield of the Respiratory Viruses Section, National Microbiology Laboratory, for information regarding influenza virus strain characterization. We also wish to thank all the physicians and nurse practitioners who contributed to the ILI surveillance program in association with the CFPC, NaReS, and the sentinel influenza surveillance programs in British Columbia, Alberta and Saskatchewan. Finally, we wish to express our thanks to the provincial and territorial epidemiologists and FluWatch representatives for providing information about influenza activity levels in their jurisdictions.

* For the 1999-2000 FluWatch program, activity levels were defined as:

1 =	No activity reported.
2 =	Sporadic: sporadically occurring ILI and confirmed influenza^† with no outbreaks detected within the surveillance region.^‡
3 =	Localized: sporadically occurring ILI and confirmed influenza^† and outbreaks of ILI in < 50% of the surveillance region(s).^‡
4 =	Widespread: sporadically occurring ILI and confirmed influenza^† and outbreaks of ILI in >= 50% of the surveillance region(s).^‡
^†	Confirmation of influenza within the surveillance region at any time within the prior 4 weeks.
^‡	Sub-regions within the province/territory as defined by the provincial/territorial epidemiologist.

References

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Source: SG Squires, MSc, L Pelletier, MD, MPH, FRCPC, P Zabchuk, B Winchester, MSc, T Tam, MD, FRCPC, Division of Respiratory Diseases and Division of Disease Surveillance, Bureau of Infectious Diseases, Centre for Infectious Disease Prevention and Control

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