Canada Communicable Disease Report

July 2008 Volume 34 Number 07

Monthly Report

Influenza in Canada: 2007-2008 Season Update

F Reyes, MHSc (1), S Aziz, MSc (1), B Winchester, MSc (1), Y Li, PhD (2), W Vaudry, MD (3), J Bettinger PhD, MPH (4), P Huston, MD, MPH (1), A King, MD, MHSc, FRCPC (1)

1. Centre for Immunization and Respiratory Infectious Diseases, Public Health Agency of Canada, Ottawa, Ontario
2. Influenza and Respiratory Virus Section, National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, Manitoba
3. Department of Pediatrics, University of Alberta, Edmonton, Alberta
4. Vaccine Evaluation Centre, University of British Columbia, Vancouver, British Columbia

Introduction

Canada's national influenza surveillance system, FluWatch, now in its 12th year, is coordinated through the Centre for Immunization and Respiratory Infectious Diseases (CIRID), Public Health Agency of Canada (PHAC). The program collects data and information from various sources in order to provide a national picture of influenza activity.

This report provides a summary of the results on influenza activity in Canada during the current 2007-2008 season from 26 August 2007 up to and including 17 May 2008.

Methods

FluWatch reports on five main indicators of influenza activity: 1) sentinel laboratory-based influenza and other respiratory virus detections; 2) strain identification and antiviral resistance for circulating influenza viruses; 3) sentinel influenza-like illness (ILI) primary care consultation rates; 4) regional influenza activity levels as assigned by provincial and territorial FluWatch representatives; and 5) paediatric influenza-associated hospital admissions and mortality data. In addition, the FluWatch program also conducts an assessment of international influenza activity by monitoring reports from other influenza surveillance programs worldwide. Detailed surveillance methodology has been previously described⁽¹⁾.

Results

1) Respiratory virus detectionss

Since 26 August 2007, 33 laboratories across Canada reported a total of 111,056 influenza tests, of which 12,000 (10.8%) have tested positive for influenza viruses. The provincial distribution of influenza tests conducted and the number positive are shown in Table 1.

Of the 12,000 influenza detections to date, 6,945 (57.9%) were influenza A and 5,055 (42.1%) were influenza B viruses. During the same time period, a total of 93,881 respiratory syncytial virus (RSV) tests were conducted of which 8,602 or 9.2% were positive for RSV viruses; 87,318 parainfluenza virus tests were conducted of which 1,580 or 1.8% were positive for parainfluenza viruses; and 74,880 adenovirus tests were conducted of which 1,366 or 1.8% were positive for adenoviruses.

Influenza A detections were reported across Canada, however, the majority of detections to date were from Ontario (35.3% or 2,452/6,945) and Quebec (34.2% or 2,222/6,495). Influenza B detections were also reported across Canada, the majority of which were also from Ontario (34.3% or 1,733/5,055) and Quebec (22.3% or 1,129/5,055).

The peak in influenza detections occurred in week 12 (late March) at 18.7%; influenza A detections peaked at 10.7% in week 11 (mid-March) whereas influenza B detections peaked at 9.6% in week 15 (mid-April).

Regional variations in the peak and spread of influenza A and influenza B detections were also observed. In Ontario, British Columbia and Quebec, influenza A detections predominated and peaked first (in January and February), followed by increases in influenza B detections later in the season (between late February to early April). In the Prairies and in the Atlantic Region, influenza A and B co-circulated and were detected around the same time (peaked around late-February and mid-March respectively). Dual peaks for influenza A detections were observed in Ontario; the first peak occurred in early January 2008 followed by a second peak in late March to mid-April.

To date detailed case-by-case epidemiological reports for 9,130/12,000 (76.1%) laboratory-confirmed influenza cases were received from eight provinces and three territories (Table 1). The majority of reports received were from Quebec (36.3% or 3,314/12,000), Ontario (20.9% or 1,910/12,000), and Alberta (19.2% or 1,757/12,000). For this reason, case-by-case results should be interpreted with caution as the results are not representative of the overall Canadian situation.

The majority of influenza A cases were reported in children < 5 years of age (24.1% or 1,290/5,356), followed by adults between 25 to 44 years of age (22.0% or 1,180/5,356). The majority of influenza B cases were in adults > 65 years of age (25.8% or 970/3,765) followed by those 25 to 44 years of age (19.8% or 746/3,765) (Figure 1).

Table 1. Aggregate and case-by-case influenza data by province/territory or region, Canada, 26 August 2007 - 17 May 2008

Province/ Territory or Region	Season to date: 26 August, 2007 - 17 May, 2008
	Total number of influenza tests	Aggregate data				Case-by-case data
		Number of positive tests (% of total)			% of total positive tests	Number of cases	% of total cases
		Influenza A	Influenza B	Total
N.L.	1,374	83 (38.4 %)	133 (61.6%)	216	1.8	1	0.0
P.E.I.	156	3 (10.3%)	26 (89.7%)	29	0.2
N.S.	1,046	123 (54.7%)	102 (45.3%)	225	1.9	220	2.4
N.B.	1,530	108 (35.9%)	193 (64.1%)	301	2.5
Atlantic	4,106	323 (41.9%)	454 (58.1%)	771	6.4	221	2.4
Que.	29,734	2,222 (66.3%)	1,129 (33.7%)	3,351	27.9	3,314	36.3
Ont.	34,345	2,452 (58.6%)	1,733 (41.4%)	4,185	34.9	1,910	20.9
Man.	2,818	85 (66.9%)	42 (33.1%)	127	1.1	131	1.4
Sask.	6,112	349 (54.9%)	287 (45.1%)	636	5.3	680	7.4
Alb.	30,336	931 (49.9%)	935 (50.1%)	1,866	15.6	1,757	19.2
Prairies	39,266	1,365 (51.9%)	1,264 (48.1%)	2,629	21.9	2,568	28.1
B.C.	3,605	589 (55.4%)	475 (44.6%)	1,064	8.9	996	10.9
Y.T.						4	0.0
N.W.T.						50	0.5
Nun.						65	0.7
Territories						119	1.3
Unknown						2	0.0
Total	111,056	6,945 (57.9%)	5,055 (42.1%)	12,000		9,130

Figure 1. Proportionate distributions of case-by-case data, by influenza type and by age group, Canada, 26 August 2007- 17 May 2008

2) Influenza virus strain identification

To date, 1,281 influenza viruses have been antigenically characterized by the National Microbiology Laboratory (NML): 461 (36.0%) A (H1N1), 218 (17.0%) A (H3N2) and 602 (47.0%) B viruses. Of the 461 influenza A (H1N1) viruses characterized, 439 (95.2%) were antigenically similar to A/Solomon Islands/3/2006 and 22 (4.8%) were antigenically similar to A/Brisbane/59/2007. Of the 218 influenza A (H3N2) viruses characterized, 9 (4.1%) were antigenically similar to A/Wisconsin/67/2005 and 209 (95.9%) were antigenically similar to A/Brisbane/10/2007. One of the 9 A/Wisconsin-like viruses had reduced titre to A/Wisconsin/67/2005 reference antiserum. Of the 602 influenza B isolates characterized, 16 (2.7%) were anti-genically similar to B/Malaysia/2506/2004 and 586 (97.3%) were antigenically similar to B/Florida/4/2006 (belonging to the B/Yamagata lineage) (Figure 2). The majority of influenza viruses identified early in the season were influenza A/Solomon Islands/3/2006 (H1N1)-like; however, the number of influenza B/Florida/4/2006 viruses have been increasing since early-January and since the beginning of April represent the majority of influenza strains characterized so far this season.

The 2007-2008 Canadian influenza vaccine contained an A/Solomon Islands/03/2006 (H1N1)-like virus; an A/Wisconsin/ 67/2005 (H3N2)-like virus; and a B/Malaysia/2506/2004-like virus.

Figure 2. Influenza strain characterization, Canada, 2007-2008 influenza season (n = 1,281)

Of the influenza A (H1N1) viruses characterized to date, the majority were from Ontario (39.3% or 181/461), British Columbia (26.7% or 123/461) and Quebec (15.8% or 73/461). The majority of influenza A (H3N2) viruses characterized were from Ontario (56.9% or 124/218), British Columbia (16.0% or 35/218) and Saskatchewan (10.6% or 23/218). Of the influenza B viruses characterized, 38.7% (233/602) were from Ontario, 20.9% (126/602) were from British Columbia and 13.3% (80/602) were from Alberta.

Antiviral resistance

Since the start of the season, the NML has tested 946 influenza A isolates (538 A (H1N1), 408 A (H3N2)) for amantadine resistance and found that 43.6% (412/946) were resistant. Six (1.1%) of the 538 A (H1N1) isolates tested were resistant; five (83.3%) of which were from British Columbia and one from Quebec. Of the 408 influenza A (H3N2) isolates tested, 406 (99.5%) were resistant. The proportion of A (H3N2) isolates resistant to amantadine was 100% in the majority of provinces and territories except in Ontario and British Columbia where the two H3N2 non-resistant isolates were reported.

On the basis of amantadine resistance patterns observed, PHAC continues to recommend against the use of amantadine for the treatment and prevention of influenza⁽²⁾.

The NML also tested 1,301 influenza isolates (481 A (H1N1), 221 A (H3N2) and 599 B) for oseltamivir (Tamiflu®) resistance and found that 125 (26%) of the 481 A (H1N1) isolates tested were resistant to oseltamivir. The resistant isolates were from Newfoundland and Labrador, Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, Alberta and British Columbia. Of the 125 resistant viruses, 121 were A/Solomon Islands/3/06/-like and four were A/Brisbane/59/07-like. Of the 125 oseltamivir resistant influenza strains, 124 were sensitive to the antiviral amantadine but one isolate was also found to be resistant to amantadine. Further investigation revealed that this specimen was collected when the patient had already received 5 days of oseltamivir and 2 days of amantadine.

3) ILI consultations reported by sentinel clinical practices

Weekly ILI rates have remained within or below baseline levels for the 2007-2008 season except for weeks 15, 17 and 20 where the rates exceeded baseline. To date, the highest rate observed was in week 01 (early January) with 32 consultations for ILI per 1,000 patient visits, which is below the peak observed during the previous season (50 per 1,000 in week 09). Figure 3 shows the Canadian age-standardized, census-division weighted ILI consultation rates for the current season, compared to the mean rate and 95% confidence intervals for the previous 11 years of the FluWatch program. To date, the highest ILI consultation rates were reported in children: 33/1,000 patients seen in the 0 to 4-year age group and 18/1,000 in those aged 5 to 19 years. So far this season, the median sentinel participation rate has been low compared to previous years at 60% (range from 37% to 71%) per week.

Figure 3. Census-division weighted age-standardized ILI consultation rates, by report week, Canada, 26 August 2007-17 May 2008, compared with seasons 1996-1997 to 2006-2007 (average with 95% confidence intervals)

4) Regional influenza activity levels assessed by provincial and territorial epidemiologists

Influenza activity in Canada overall was relatively mild from September to late December 2007, except in some regions in Alberta and Ontario, where localized influenza activity was reported mid- to late October 2007. Influenza activity steadilyincreased across the country from mid-January to early April and declined then after. Overall influenza activity in Canada for the 2007-2008 season remained mild to moderate and was similar to the previous two seasons.

Regional variations in the spread/timing and intensity of influenza activity were observed. Sporadic activity was first reported in British Columbia, Alberta, Ontario and Quebec at the start of the season. A few regions in Ontario and Alberta first reported localized activity in late August and mid-November respectively; followed by regions of British Columbia, Saskatchewan, Northwest Territories and Nunavut in December; Manitoba, New Brunswick and Nova Scotia in mid- to late January; and finally in Prince Edward Island in early March. Influenza activity levels remained low (only sporadic or no activity reported) in Newfoundland and Labrador and in the Yukon throughout the season.

Widespread influenza activity was reported 47 times by 10 regions in four provinces, the majority of which were reported between mid-February to mid-March 2008 and mostly in British Columbia (47% or 22/47) and in Toronto, Ontario (26% or 12/47).

In total, 464 outbreaks of influenza or ILI were reported: 253 (54.5%) in long-term care facilities (LTCF's); 19 (4.1%) in hospitals; 140 (30.2%) in schools; and 52 (11.2%) in other facilities. The majority of the LTCF outbreaks reported to date were from Quebec (21.7% or 55/253), Ontario (21.3% or 54/253) and British Columbia (20.2% or 51/253) while the majority of outbreaks of ILI in schools were from British Columbia (52.9% or 74/140) and New Brunswick (21.4% or 30/140). There were similar numbers of outbreaks reported to date this season compared to the same period in the previous two seasons (489 in 2006-2007 and 456 in 2005-2006); however there were more outbreaks in LTCF's reported this season compared with the previous two (253 versus 178 and 160 in the previous two seasons respectively).

5) Influenza hospitalizations in children

To date this season, preliminary data from the Immunization Monitoring Program Active (IMPACT) shows a total of 472 influenza-associated paediatric hospitalizations compared to 335 cases at the same time last season. Provincial and age distribution may change once the data are finalized, however to date the provincial distribution of cases is as follows: 32.8% (155/472) from Quebec; 24.6% (116/472) from Ontario; 13.8% (65/472) from Alberta; 10.6% (50/472) from British Columbia; 5.7% (27/472) from Nova Scotia; 5.5% (26/472) each from Manitoba and Saskatchewan; and 1.5% (7/472) from Newfoundland and Labrador. Influenza A was identified in 62.9% (297/472) of the cases and influenza B was identified in the remaining 37.1% (175/472).

Fifty percent (238/472) of the paediatric hospitalizations reported were hospitalized between week 7 and week 13 (10 February to 29 March, 2008), with the highest number of cases reported at 48 hospitalizations in week 9. The age distributions of the 472 cases are as follows: 25.8% (122/472) among 6 to 23 month olds; 23.1% (109/472) among 2 to 4 year olds; 21.0% (99/472) among 0 to 5 month olds; 19.9% (94/472) among 5 to 9 year olds; and 10.2% (48/472) among 10 to 16 year olds. Two influenza-associated paediatric deaths were reported in Canada: one from British Columbia and the other from Alberta. Both deaths were due to influenza B infection.

International

United States

Detailed Information on influenza activity during the 2007-2008 influenza season in the United States was reported in the Morbidity and Mortality Weekly Report from the CDC and can be accessed at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5725a5.htm .

Worldwide

Summaries of overall influenza activity in the world were reported in the Weekly Epidemiological Reports from the WHO and can be accessed at: http://www.who.int/wer/en/ .

Weekly summaries of influenza activity in Europe can be obtained from the European Influenza Surveillance Scheme electronic bulletins: http://www.eiss.org/cgi-files/bulletin_v2.cgi?season=2007 .

Acknowledgements

The authors gratefully acknowledge and thank all the FluWatch surveillance partners who participated in this year's influenza surveillance program, including the NML, laboratories reporting to the Respiratory Virus Detections Surveillance System (RVDSS), sentinel primary care practitioners, provincial ministries of health, and the Immunization Monitoring Program, ACTive (IMPACT) Network. A special thank you to Shabnam Rahimi Khameneh and Estelle Arseneault for their contributions to this report.

References

1. Reyes F, Macey JF, Aziz S et al. Influenza in Canada: 2005-2006 season. CCDR 2007;33(3):21-41.
2. Public Health Agency of Canada. Recommendation for use of Amantadine for treatment and prevention of influenza. November 2006. URL: http://www.phac-aspc. gc.ca/media/nr-rp/2006/20061101-amantadine-eng.php.