Sharing a long border, the United States and Canada tend to see similar flu seasons, albeit often with slightly different timing. This winter, however, Canada is seeing far more influenza B than the U.S., and there are early indications that they are seeing less diversity among their H3N2 viruses than has the United States.
These differences aren't as pronounced as we're seeing from other places around the world (Asia is 80% Influenza B), but they are a reminder of how difficult predicting flu activity can be and just how much variety we can see across the globe in a single flu season.Some excerpts from this week's Canadian FluWatch report (in blue) follow, along with some comments showing how their flu season has differed from ours.
FluWatch report: January 21, 2018 to January 27, 2018 (week 4)
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Organization: Public Health Agency of Canada
Date published: 2018-02-02
Overall SummarySignificant differences from the U.S. Flu season (so far) include:
- Overall, influenza activity in Canada remains at peak levels but there are signs that activity is starting to slow down in parts of the country.
- The majority of influenza detections continue to be A(H3N2), although 40% of detections were influenza B in week 04.
- To date this season, the majority of lab confirmations, hospitalizations and deaths have been among adults 65 years of age and older.
- A Canadian study reported a mid-season estimate of influenza vaccine effectiveness of 42% overall. The study confirmed an anticipated low vaccine effectiveness of 17% against the A(H3N2) strain and moderate vaccine effectiveness against influenza B of 55%, which is circulating at high levels in Canada this season.
- Flu in Canada - while high - has leveled off, while it is still climbing in the U.S.
- Influenza B in Canada is running more than twice the level of that in the US
- According to reports, the vast majority H3N2 virus being seen in Canada has been clade 3C.2a, while early reports from the CDC suggest the US is seeing a more diverse set of H3N2 viruses.
How all of this will affect the expected mid-season flu vaccine effectiveness report due from the CDC is unknown, but a higher incidence of H3N2 here - combined with the low VE against that strain - could lower our overall (combined) vaccine effectiveness.
Conversely, since quadrivalent vaccines (2 A's & 2 B strains) are more commonly used in the U.S., our performance against influenza B may turn out a bit higher than the 55% reported from Canada.Moving on with this week's FluWatch report:
Laboratory-Confirmed Influenza Detections
In week 04, the overall percentage of tests positive for influenza remained similar to the previous week at 31%. Since week 02, the percentage of tests positive for influenza A has remained stable at 18%. This suggests that the peak for influenza A detections occurred in week 01 (19%). The percentage of tests positive for influenza B remained similar to the previous week.
The percentage of influenza A detections for week 04 is average for this time of year. The percentage of tests positive for influenza B in week 04 continue to be well above expected levels for this time of year. For data on other respiratory virus detections, see the Respiratory Virus Detections in Canada Report.
Provincial/Territorial Influenza Hospitalizations and Deaths
In week 04, 130 influenza-associated hospitalizations were reported by participating provinces and territories1. To date this season, 3,018 influenza-associated hospitalizations have been reported, 79% of which were associated with influenza A, and 2,078 cases (69%) were in adults 65 years of age or older. To date, 271 ICU admissions and 120 deaths have been reported.
Pediatric Influenza Hospitalizations and Deaths
In week 04, the number of laboratory-confirmed influenza-associated pediatric (≤16 years of age) hospitalizations reported by the Immunization Monitoring Program Active (IMPACT) network increased compared to the previous week. In week 04, 88 hospitaliations were reported of which influenza A and B made up almost equal proportion of cases. The number of weekly hospitalizations has been above the seven-season average since week 45.
To date this season, 446 pediatric hospitalizations have been reported by the IMPACT network, 299 (67%) of which were associated with influenza A. Children 0-23 months accounted for the largest proportion of influenza A cases (40%) and the smallest propotion of influenza B cases (22%).
Among the 147 hospitalizations due to influenza B, children 5-9 years accounted for the largest proportion of cases (29%).
Additionally, 71 ICU admissions and five deaths have been reported to date. Seventy percent of ICU cases were due to influenza A. Children aged 0-23 months and 10-16 years each accounted for 31% of ICU cases.
Genetic Characterization of A(H3N2) viruses
During the 2017-18 season, 349 A(H3N2) viruses did not grow to sufficient titers for antigenic characterization by HI assay. Therefore, genetic characterization was performed to determine to which genetic group they belong. Sequence analysis showed that 295 A(H3N2) viruses belonged to genetic group 3C.2a, 53 viruses belonged to subclade 3C.2a1 and one virus belonged to the clade 3C.3a.
Additionally, of the 118 influenza A(H3N2) viruses that were characterized antigenically as similar to A/Hong Kong/4801/2014, 75 belonged to genetic group 3C.2a and 12 viruses belonged to subclade 3C.2a1. The four viruses that showed reduced titer belonged to genetic clade 3C.3a. Sequencing is pending for the remaining 27 virus isolates.
A/Hong Kong/4801/2014-like virus belongs to genetic group 3C.2a and is the influenza A/H3N2 component of the 2017-18 Northern Hemisphere influenza vaccine.
Genetic Characterization of Influenza B viruses
Among the viruses characterized antigenically as having reduced titer to ferret antisera produced against cell-propagated B/Brisbane/60/2008, sequence analysis showed that all 10 viruses had a two amino acid deletion in the HA gene.
With a population a little over 10% of the United States., the totals from Canada are understandably a lot lower than we're seeing in the U.S., but they are nonetheless enduring a heavy influenza season.
With influenza B rising to 40% this week in Canada - and not being a close match to their trivalent vaccine - we'll be watching to see how much of a factor it turns out to be this spring.As always, these numbers are all subject to revision, and like all surveillance reports should be viewed as indicative of trends, and not as a full accounting of disease activity during any given time period.