Although influenza activity sometimes get off to an early start, most years we don’t see a big uptick until after the Thanksgiving or Christmas holidays. Thus far the big seasonal influenza story hasn’t been about the numbers (which remain low), but focuses instead on the antigenic diversity of flu viruses in circulation this year.
Two weeks ago, in A `Drift’ In A Sea Of Influenza Viruses, I wrote about the emergence over the spring and summer of an antigenically `drifted’ H3N2 virus – one that differs from the A/Texas/50/2012 (H3N2)-like virus contained in this year’s flu vaccine.
The strains to be included in each season’s flu shot must be selected six months in advance to give manufacturers time to produce and distribute the vaccine. It is always a bit of a gamble to try to guess what the flu landscape will look like six to twelve months in the future, and this year is no different.
While the A/Texas/50/2012 (H3N2)-like virus remains in circulation, coming up rapidly is a divergent A/Switzerland/9715293/2013 (H3N2)-like virus, against which this year’s flu shot is less effective.
Right now, among handful of H3N2 viruses that have been antigenically tested in the United States, about half are `covered’ by the vaccine strain. In Canada (in a much smaller sampling), that number is about 25%. The H3N2 strain for next year’s Southern Hemisphere flu shot has already been changed to meet the challenge of this emerging strain.
Another concern was raised earlier this month when we learned that last year’s (and presumably this year’s) H1N1 component of the LAIV (FluMist nasal spray vaccine) vaccine was not effective in children aged 2-8. Given the dearth of H1N1 virus detections this fall, and decent prospects of seeing that trend continue, the impact of this vaccine failure will hopefully be small.
As I’ve written here often , flu vaccines are considered very safe – and most years provide a moderate level of protection against influenza – and can do so even years when they aren’t a perfect match (see CDC’s What if there is a mismatch between circulating viruses and the vaccine viruses? ).
I get my flu shot every year, and encourage others to do so as well.
All data are preliminary and may change as more reports are received.
During week 45 (November 2 - 8, 2014), influenza activity was low in the United States.
- Viral Surveillance: Of 9,138 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 45, 678 (7.4%) were positive for influenza.
- Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
- Influenza-associated Pediatric Deaths: No influenza-associated pediatric deaths were reported.
- Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 1.6%, which is below the national baseline of 2.0%. All 10 regions reported ILI below region-specific baseline levels. Puerto Rico experienced moderate ILI activity; two states experienced low ILI activity; New York City and 48 states experienced minimal ILI activity; and the District of Columbia had insufficient data.
- Geographic Spread of Influenza: The geographic spread of influenza in Guam was reported as widespread; three states reported regional activity; Puerto Rico, the U.S. Virgin Islands, and 13 states reported local activity; the District of Columbia and 31 states reported sporadic activity; and three states reported no influenza activity.
CDC has antigenically characterized 19 influenza viruses (one 2009 H1N1 virus, 13 influenza A (H3N2) viruses, and five influenza B viruses) collected by U.S. laboratories since October 1, 2014 by hemagglutination inhibition (HI).
2009 H1N1 :
- The 2009 H1N1 virus tested was characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2014-2015 Northern Hemisphere influenza vaccine.
Influenza A (H3N2) :
- Seven (54%) of the 13 influenza A (H3N2) viruses tested have been characterized as A/Texas/50/2012-like, the influenza A (H3N2) component of the 2014-2015 Northern Hemisphere influenza vaccine. Six (46%) of the 13 viruses tested showed reduced titers with antiserum produced against A/Texas/50/2012. Among viruses that showed reduced titers with antiserum raised against A/Texas/50/2012, the majority were antigenically similar to A/Switzerland/9715293/2013, the H3N2 virus selected for the 2015 Southern Hemisphere influenza vaccine. A/Switzerland/9715293/2013 is related to, but antigenically and genetically distinguishable, from the A/Texas/50/2012 vaccine virus. A/Switzerland-like H3N2 viruses were first detected in the United States in small numbers in March of 2014 and began to circulate in greater numbers over the spring and summer.
Influenza B : Both B/Victoria and B/Yamagata-lineage viruses are currently circulating in the United States. However to date, results of antigenic characterization are available only for B/Victoria lineage viruses.
- Victoria Lineage : All five B/Victoria-lineage viruses were characterized as B/Brisbane/60/2008-like, which is included as an influenza B component of the 2014-2015 Northern Hemisphere quadrivalent influenza vaccine.
- Yamagata Lineage: No antigenic characterization data is available for influenza B/Yamagata-lineage viruses collected after October 1, 2014.
From this week’s FluWatch report:
- As expected, overall influenza activity in week 45 increased from the previous week. Yukon reported activity for the first time this season but only at sporadic levels.
- A(H3N2) continues to be the most common type of influenza affecting Canadians.
- To date, 40-55% of influenza laboratory detections and hospitalizations have been in seniors ≥65 years of age.
Influenza Strain Characterizations
During the 2014-2015 influenza season, the National Microbiology Laboratory (NML) has characterized 12 influenza viruses [four A(H3N2) and eight influenza B]. One influenza A was antigenically similar to A/Texas/50/2012, and four influenza B viruses were antigenically similar to the B/Massachusetts/2/2012 (Yamagata lineage) recommended by the WHO for the 2014-15 seasonal influenza vaccine. Three influenza A(H3N2) viruses and one influenza B virus showed reduced titers to antisera produced against strains recommended for the seasonal influenza vaccine (Figure 4).
Figure 4. Influenza strain characterizations, Canada, 2014-2015, N = 12
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.