Map of overall influenza/ILI activity level by province and territory, Canada, Week 51
Due to the Holidays our regularly scheduled Friday CDC FluView report will be released on Monday, but all signs are pointing to another hefty increase in flu activity across the country. Influenza surveillance and reporting during the second half of December is always problematic, and the data often lags behind by an extra week or so getting to the CDC.
We do have Canada’s latest FluWatch report – which reflects their flu activity as of December 20th.
Not surprisingly, they too are seeing an increasingly tough flu season, with H3N2 far and away the predominant flu strain, and with most of the H3 viruses examined a poor match for this year’s flu vaccine (see HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus).
Between it being an H3N2 year – a subtype which often produces more severe seasons than H1N1 – and reduced vaccine effectiveness, there are concerns that the next couple of months may be particularly rough (see Early Data Suggests Potentially Severe Flu Season).
Some excepts from a much longer, data rich report:
- In week 51, laboratory detections of influenza increased sharply for the fifth consecutive week. The majority of laboratory detections continued to be reported in AB, ON and QC; but with increasing activity in SK and NL.
- A(H3N2) continues to be the most common type of influenza affecting Canadians. In both laboratory detections and hospitalizations, the majority of cases have been among seniors ≥65 years of age.
- Similar to the previous week, there were a large number of newly-reported laboratory-confirmed outbreaks of influenza: 125 influenza outbreaks in 7 provinces, of which 94 were in long-term care facilities (LTCF).
- The rate of antiviral prescriptions more than doubled from the previous week, increasing especially among seniors.
Influenza and Other Respiratory Virus Detections
In week 51, the number of positive influenza tests increased sharply to 2,833 influenza detections (29.1% of tests), predominantly due to influenza A (Figure 2). To date, 97% of influenza detections have been influenza A, and 99.8% of those subtyped have been A(H3) (Table 1). The timing of the season and predominant A(H3N2) subtype is similar to the pattern observed during the 2012-13 influenza season when percent positive for influenza peaked in week 52 (35%). To date, among the cases of influenza with reported age, the largest proportion was in adults ≥65 years of age (56%) (Table 2).
Figure 2. Number of positive influenza tests and percentage of tests positive, by type, subtype and report week, Canada, 2014-15
Influenza Strain Characterizations
During the 2014-2015 influenza season, the National Microbiology Laboratory (NML) has characterized 59 influenza viruses [37 A(H3N2), 2 A(H1N1) and 20 influenza B]. Influenza A (H3N2): When tested by hemagglutination inhibition (HI) assay, 31 of the 37 A(H3N2) viruses characterized were antigenically similar to A/Switzerland/9715293/2013, which is the influenza A(H3N2) component recommended for the 2015 Southern Hemisphere influenza vaccine. One was antigenically similar to A/Texas/50/2012, which is the influenza A(H3N2) component recommended for the 2014-15 influenza vaccine. The remaining five A(H3N2)viruses showed reduced titer to A/Texas/50/2012.
Additionally, 112 A(H3N2)viruses were unable to be tested by HI assay; however, sequence analysis showed that 111 belonged to a genetic group that typically shows reduced titers to A/Texas/50/2012. Influenza A(H1N1): Two A(H1N1) viruses characterized were antigenically similar to A/California/7/2009, which is the influenza A(H1N1) component recommended for the 2014-15 influenza vaccine. Influenza B: Of the 20 influenza B viruses characterized, 17 viruses were antigenically similar to the vaccine strain B/Massachusetts/2/2012. Three viruses showed reduced titers with antiserum produced against strains recommended for the seasonal influenza vaccine (Figure 4).
Figure 4. Influenza strain characterizations, Canada, 2014-2015, N = 59
The NML receives a proportion of the number of influenza positive specimens from provincial laboratories for strain characterization and antiviral resistance testing. Characterization data reflect the results of haemagglutination inhibition (HAI) testing compared to the reference influenza strains recommended by WHO.
The recommended components for the 2014-2015 northern hemisphere trivalent influenza vaccine include: an A/California/7/2009(H1N1)pdm09-like virus, an A/Texas/50/2012 (H3N2)-like virus, and a B/Massachusetts/2/2012-like virus (Yamagata lineage). For quadrivalent vaccines, the addition of a B/Brisbane/60/2008-like virus is recommended.
Despite its reduced effectiveness, the CDC continues to recommend that people get the flu shot – partially because it may provide some modicum of protection against this drifted flu strain, but mostly because we often see a wave of Influenza B late in the flu season, and the shot can help protect against that virus.
Beyond that, practicing good flu hygiene; Staying home when sick, washing your hands, covering your coughs, and disposing of your tissues properly are all important habits to maintain during this flu season.