The Coming 2014 – 2015 Flu Season: What to expect

2014 - 2015 flu season

~ by James M. Wilson V, MD

Each year, our team at Ascel Bio monitors a variety of reporting from foreign countries, the World Health Organization, and the U.S. Centers for Disease Control and Prevention (CDC) in an effort to determine whether we may be entering a season at-risk for vaccine mismatch.

The key type of influenza virus we focus on is type A due to the greater level of mortality and medical infrastructure disruption associated, versus types B or C. Within the type A influenza viruses, we mainly focus on which virus will dominate: type A/H3N2 or A/H1N1.

All seasons considered and excluding the first wave of the 2009 A/H1N1 pandemic, type A/H3N2 is the main concern. The reason is A/H3N2, which is an echo of the 1968 influenza pandemic, drifts quickly and tends to cause more medical infrastructure disruption than A/H1N1. It dominates influenza seasons in the US at a baseline frequency of 80% since the 1999-2000 season. About half of these A/H3N2-dominant seasons are associated with vaccine mismatch. This leaves the US with a current baseline probability of about 1/3 chance of seeing a vaccine “issue” on any given season.

Does vaccine mismatch automatically equate to “a bad season?”

The reasons for vaccine mismatch with A/H3N2 relate to several factors including; availability of isolates for consideration by the World Health Organization’s international body that decides the seasonal vaccine composition, whether or not the choice made by the decision group was the correct one, whether the virus drifts again after vaccine production, and so on. Most importantly, A/H3N2 has proven more difficult to grow in culture over time.

Right now, the CDC reports 1) dominance of type A/H3N2, and 2) that 70% of the A/H3N2 isolates do not match the strain chosen for the vaccine. So now everyone is, understandably, concerned about the prospects for this season. So the key question, again, is, “Does vaccine mismatch equate to a ‘bad season of influenza?’” Let’s take a look at the current season in context with prior seasons of influenza:

CDC flu chart

credit: CDC | click to enlarge

Credit: US Centers for Disease Control and Prevention.[1]

The above graph for influenza activity from the perspective of outpatient clinics shows a current picture of activity that follows the activity seen in 2012-13 fairly closely. Time will tell if this season is ultimately better than, similar to, or worse than 2012-13. But the real question with this figure is which of these seasons was the vaccine mismatch year (excluding the pandemic season of 2009-10)?

The answer is 2011-12, not 2012-13.

In the 2011-12 season, A/H3N2 was dominant. Of those viruses, 22% were associated with mismatch.[2] In the 2012-13 season, A/H3N2 was again dominant, however matched to the vaccine strain.[3]

On the other hand, 2007-8 was another vaccine mismatch season where A/H3N2 was dominant, but associated with a high percentage (73%) of the isolates mismatched to the vaccine. There was also a minority percentage of A/H1N1 mismatched to the vaccine that year as well. This was a moderately severe season of influenza.[4]

During the 2003-04 season, we observed another A/H3N2 dominant season with 89% mismatch with the vaccine strain. This was a severe season considered worse than the 2007-8 season.[5]

For the 2003-4, 2007-8, and pandemic 2009-2010 seasons, the early warning indicator was early seasonal influenza activity in the pediatric population[6] [7]:

Flu Chart CDC 0 - 4 years

credit CDC | click to enlarge

Pediatric hospitalization rates per CDC for the 0-4 year-old age group, 2003-4 to 2007-8 seasons.[8]

Now let’s take another look at the current season to help put things in perspective, focusing on the key indicator for influenza season severity: pediatric disease.

pediatric hospitalizations for flu from CDC

credit: CDC | click to enlarge

Pediatric hospitalization rates per 100,000 population for the 0-4 year-old age group, estimated, by CDC as of 12 Dec 2014 (Week 50).[9]

CDC pediatric hospitalizations for flu season, ages 5 to 17

credit: CDC | click to enlarge

Pediatric hospitalization rates per 100,000 population for the 5-17 year-old age group, estimated, by CDC as of 12 Dec 2014 (Week 50).[10]

Hospitalization rates for the 0-4 year-old age group are approximately 90% of that seen in 2012-13 for Week 50. The hospitalization rates for the 5-17 year-old group, however, are higher than that seen in 2012-13: currently at 3.7 compared to 2.7 per 100,000. The hospitalization rates for the 18-49 year-old age group are currently reported to be equivalent to the 2012-2013 season.

To put all of these observations in perspective, the 2009-2010 pandemic season was associated with a 0-4 year-old pediatric hospitalization rate of 65.1 at Week 50. In other words, seven times what we are currently seeing.[11]

Early season activity with high rates of pediatric hospitalizations is an early warning indicator the country may see a season of high activity. Currently, we are seeing it is approximately proportional to that seen in the 2012-13 season when the vaccine was well matched.

There is an important caveat: influenza seasons affect different age groups in different ways. In the 65+ year-old age group, we are seeing hospitalization rates that exceed the estimates during the 2009-2010 pandemic season: 23.5 per 100,000, compared to 21.9 per 100,000. The pandemic was known to effect the pediatric patient population worse than the older adult population.[12]

In short, in pediatrics this season is so far shaping up to a roughly equivalent severity to the 2012-13 season. For our older adults, this season is looking severe.

The take home messages

  1. The phrase “vaccine mismatch” or “vaccine failure,” by itself, does not guarantee a severe season of influenza. That said we are seeing a high percentage (70+%) of mismatch this season associated with prior seasons of at least moderate severity.
  2. Different influenza seasons affect different age groups differently. So far this season, the older adult population is being hit the hardest. Activity in the pediatric population is similar to that seen during the 2012-13 season.
  3. Both the World Health Organization and the CDC recommend influenza vaccination.


Dr. James Wilson, a pediatrician and infectious disease expert with AscelBio, he is a frequent contributor to the SERMO blog.

Season flu and it’s movements in the US are discussed frequently inside SERMO.  If you’re an MD or DO please join us for the latest news shared by your peers.


[1] CDC FluView.

[2] CDC FluView.

[3] CDC FluView.

[4] CDC FluView.

[5] CDC FluView.

[6] CDC FluView.

[7] CDC FluView.

[8] CDC FluView.

[9] CDC FluView.

[10] CDC FluView.

[11] CDC FluView.

[12] CDC FluView.

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