Although hardly a surprise, the news is nonetheless disappointing that the percentage of `antigenically drifted’ H3N2 specimens tested by the CDC – those that are less likely to be covered by this year’s flu vaccine – continues to rise.
Six weeks ago (see A `Drift’ In A Sea Of Influenza Viruses) 70% of the H3N2 viruses characterized since the start of this year’s flu season (Oct. 1st) appeared to be a good match to the vaccine.
Since then, that percentage has been cut by more than half, with now just 32.5% of the samples to date matching the vaccine strain.
This decline becomes even more dramatic when you look at the results of antigenic testing from the last survey week, where just 16 of 83 H3N2 (19.2%) viruses tested appeared to a good match to the vaccine. Complicating matters, unlike last year – which primarily saw the H1N1 virus – this year is being dominated by H3N2.
Despite this vaccine mismatch, it should be noted that the flu vaccine protects against more than just the H3N2 strain, and the CDC believes it may even provide some limited benefit against the `drifted’ strain.
A little over a week ago we saw the CDC issue a HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus, along with a warning that Early Data Suggests Potentially Severe Flu Season. Since H3 dominant years tend to produce more severe flu seasons, and the vaccine may be of limited benefit against this drifted strain, doctors are being urged to prescribe antiviral medications early for `at risk’ patients (see below).
Those at high risk from influenza include children younger than 5 years (especially those younger than 2 years); adults 65 years and older; pregnant women; and people with certain chronic health conditions such as asthma, diabetes, heart or lung disease, and kidney disease.
CDC recommends that people at high risk check with their doctor or other health care professional promptly if they get flu symptoms. Studies show that flu antiviral drugs work best for treatment when they are started in the first 48 hours after symptoms appear. Flu symptoms can include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue.
The good news is, while there are signs of a potentially bad flu season ahead, so far we’re not seeing a huge amount of flu activity around the country. It is increasing, however, and often doesn’t really accelerate until after the holidays.
Some excerpts from today’s FluView report include:
All data are preliminary and may change as more reports are received.
During week 49 (November 30-December 6, 2014), influenza activity continued to increase in the United States.
- Viral Surveillance:Of 16,093 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 49, 3,415 (21.2%) 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: Two influenza-associated pediatric deaths were reported.
- Influenza-associated Hospitalizations: A cumulative rate for the season of 3.8 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.
- Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 2.5%, above the national baseline of 2.0%. Eight of 10 regions reported ILI at or above region-specific baseline levels. Puerto Rico and six states experienced high ILI activity; two states experienced moderate ILI activity; seven states experienced low ILI activity; New York City and 35 states experienced minimal ILI activity; and the District of Columbia had insufficient data.
- Geographic Spread of Influenza: The geographic spread of influenza in 14 states was reported as widespread; Guam, Puerto Rico and 25 states reported regional activity; the U.S. Virgin Islands and seven states reported local activity; and the District of Columbia and four states reported sporadic activity.
Influenza Virus Characterization*:
CDC has characterized 236 influenza viruses [10 A(H1N1)pdm09, 197 A(H3N2), and 29 influenza B viruses] collected by U.S. laboratories since October 1, 2014.
Influenza A Virus 
- A(H1N1)pdm09 : All 10 H1N1 viruses tested were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2014-2015 Northern Hemisphere influenza vaccine.
- A (H3N2) : Sixty-four (32.5%) of the 197 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. One hundred thirty-three (67.5%) of the 197 viruses tested showed either reduced titers with antiserum produced against A/Texas/50/2012 or belonged to a genetic group that typically shows reduced titers to A/Texas/50/2012. Among viruses that showed reduced titers with antiserum raised against A/Texas/50/2012, most 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 increase through the spring and summer.
Influenza B Virus 
Twenty (69%) of the influenza B viruses tested belong to B/Yamagata/16/88 lineage and the remaining nine (31%) influenza B viruses tested belong to B/Victoria/02/87 lineage.
- Yamagata Lineage : All 20 B/Yamagata-lineage viruses were characterized as B/Massachusetts/2/2012-like, which is included as an influenza B component of the 2014-2015 Northern Hemisphere trivalent and quadrivalent influenza vaccines.
- Victoria Lineage : Seven (78%) of the nine B/Victoria-lineage viruses were characterized as B/Brisbane/60/2008-like, the virus that is included as an influenza B component of the 2014-2015 Northern Hemisphere quadrivalent influenza vaccine. Two (22%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
*CDC routinely uses hemagglutination inhibition (HI) assays to antigenically characterize influenza viruses year-round to compare how similar currently circulating influenza viruses are to those included in the influenza vaccine, and to monitor for changes in circulating influenza viruses. However, a portion of recent influenza A(H3N2) viruses do not grow to sufficient hemagglutination titers for antigenic characterization by HI. For many of these viruses, CDC is also performing genetic characterization to infer antigenic properties.
While even in a year with perfect antigenic matches the flu vaccine can’t promise 100% protection, it – along with practicing good flu hygiene (washing hands, covering coughs, & staying home if sick) – remain your best strategy for avoiding the flu (and other viruses) this winter.