2014 – 2015 Flu Season: An Update

~ by James M. Wilson V, MD

In September, we advised the Sermo community of potential for an influenza vaccine mismatch this season. This observation was notable because vaccine mismatch seasons are often associated with vigorous influenza activity. On December 22nd, we provided the following take home messages [1]:

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.

News media outlets have recently highlighted the threat of influenza [2], however there is a need for context. Figure 1 displays Epidemiological Week-matched maps of influenza activity, provided by CDC’s FluView [3]. On initial glance, it would appear that the 2014-15 season is the worst on record since the 2007-8 season.

However, Figure 2 shows a more nuanced view of the data: hospitalization rates. This season is comparable to the non-vaccine mismatch season of 2012-13, with previously noted exceptions:

• 0-4 year olds: 89% of 2012-13 seasonal activity
• 5-17 year olds: 131% of 2012-13 seasonal activity
• 18-49 year olds: nearly identical as the 2012-13 season and 84% of the 2013-14 season
• 50-64 year olds: nearly identical as the 2013-14 and 2012-13 seasons.
• 65+ year olds: 134% of the 2012-13 season

Figure 3 displays pediatric mortality, which needs to be interpreted carefully since mortality data is reported late. This means back-filling data may occur. That said, currently reported pediatric mortality is 35% of that seen at Week 51 during the 2012-13 season.

Figure 4, when compared to Figure 2, reveals an important key characteristic of this season’s influenza activity: the core of the American workforce is not severely impacted. Particularly when placed in perspective with influenza activity over the prior two years.

Figure 5 displays the national level forecast for influenza. Current influenza activity has fallen within +1 standard deviation expectations.

2014 2015 Influenza Season Summary

• The current influenza season is most similar to activity observed during the non-vaccine mismatch season of 2012-13.
  o Exceptions in hospitalization rates of the 5-17 and 65+ year old age group.
  o Older adults aged 65+ years are the most severely effected.
• The bulk (75%, the 20-54 year old group) of the American workforce is not expected to be impacted more severely than that observed in the prior two seasons.

Figure 1. Displays of CDC’s maps of weekly epidemiological estimates approximated to Epidemiological Week 51 of the year. The image should be considered from top left (2007-8 Week 51), down the left column to the upper right, down the right column, with the current 2014-15 Week 51 image bottom center.  In terms of geographical spread, the 2014 – 2015 season is the worst since the 2007-2008 season for Week 51.[4]

Figure 2.  CDC Weekly data by age for the current data to Epidemiological Week 51. [5]

Figure 3.  Pediatric influenza-related mortality reported to CDC.

Figure 4.  US labor force, by age, 2013. [6]

Figure 5. Ascel Bio national level influenza forecast for all age groups in the US.  Current activity has tended to remain within +1 standard deviation of anticipated activity levels.

Biography

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.

Resources

• 1 http://blog.sermo.com/2014/12/22/coming-2014-2015-flu-season-expect/
• 2 http://www.cnn.com/2014/12/31/health/flu- epidemic/index.html?iref=allsearch
• 3, 4, 5  http://www.cdc.gov/flu/weekly/