CDC FluView – A Flu Season dominated by H1N1
# 8715
The flu season just ended was notable for being the first H1N1 dominated flu season in the United States since the 2009 H1N1 pandemic virus first emerged. Since then, the (now seasonal) H1N109 virus had taken a backseat to the H3N2 virus – that is – until last winter.
H1N1 dominated flu seasons are traditionally not as severe, but are more likely to impact younger adults and children, than do H3N2 seasons. And that is the pattern we saw last year.
A look at the P&I (Pneumonia & Influenza) Mortality charts for the past four years shows that the previous year (2012-13) peaked far higher, and far earlier, than did last year.
FIGURE 4. Percentage of all deaths attributable to pneumonia and influenza (P&I), by surveillance week and year — 122 Cities Mortality Reporting System, United States, 2009–2014*
If you go back to the 2011-12 flu season, we barely saw a flu epidemic at all – in what was perhaps the lightest flu season in 30 years. Such is the variability of our yearly influenza epidemic.
Yesterday the CDC’s MMWR published a detailed review of the past flu season, with an abundance of charts and statistical information. I’ve excerpted some highlights, but follow the link to read it in its entirety.
Influenza Activity — United States, 2013–14 Season and Composition of the 2014–15 Influenza Vaccines
Weekly
June 6, 2014 / 63(22);483-490Scott Epperson, MPH1, Lenee Blanton, MPH1, Krista Kniss, MPH1, Desiree Mustaquim, MPH1, Craig Steffens, MPH1, Teresa Wallis, MS1, Rosaline Dhara, MPH1, Michelle Leon, MPH1, Alejandro Perez, MPH1, Sandra S. Chaves, MD1, Anwar Abd Elal1, Larisa Gubareva, MD1, Xiyan Xu, MD1, Julie Villanueva, PhD1, Joseph Bresee, MD1, Nancy Cox, PhD1, Lyn Finelli, DrPH1, Lynnette Brammer, MPH1 (Author affiliations at end of text)
During the 2013–14 influenza season in the United States, influenza activity* increased through November and December before peaking in late December. Influenza A (H1N1)pdm09 (pH1N1) viruses predominated overall, but influenza B viruses and, to a lesser extent, influenza A (H3N2) viruses also were reported in the United States. This influenza season was the first since the 2009 pH1N1 pandemic in which pH1N1 viruses predominated and was characterized overall by lower levels of outpatient illness and mortality than influenza A (H3N2)–predominant seasons, but higher rates of hospitalization among adults aged 50–64 years compared with recent years. This report summarizes influenza activity in the United States for the 2013–14 influenza season (September 29, 2013–May 17, 2014†) and reports recommendations for the components of the 2014–15 Northern Hemisphere influenza vaccines.
Another somewhat surprising aspect of last year’s flu season was the near-lack of novel flu detections in the United States. During the previous couple of years we saw several hundred swine variant viruses reported, mostly associated with attendance of state and county fairs where pigs were displayed (see A Variant Swine Flu Review).
One of the things we will be watching for this summer is the possible return of variant swine flu viruses. Last July, in anticipation of its return, the CDC Released Updated H3N2v Guidance.
But as the MMWR report above states:
Novel Influenza A Viruses - During the 2013–14 influenza season, one case of human infection with an H3N2v virus occurred during week 40 (the week ending October 5, 2013) in a child from Iowa with known direct exposure to swine. The child fully recovered, and no additional cases were identified in family members or other close contacts.
This year’s flu vaccine will have the same components as last year, but that doesn’t mean if you got a shot last year, you can safely skip this year. The protection of flu vaccines decreases over time (see BMC Infectious Diseases: Waning Flu Vaccine Protection In the Elderly.)
Since flu shots are generally about 60% protective most years, it makes getting the booster the smart thing to do every year.
Composition of the 2014–15 Influenza Vaccines
The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee has determined that the 2014–15 influenza vaccines used in the United States have the same antigenic composition as those used in 2013–14. The trivalent vaccines should contain an A/California/7/2009-like (2009 H1N1) virus, an A/Texas/50/2012-like (H3N2) virus, and a B/Massachusetts/2/2012-like (B/Yamagata lineage) virus. The committee also recommended that quadrivalent vaccines contain a B/Brisbane/60/2008-like (B/Victoria lineage) virus (1). These recommendations were based on global influenza virus surveillance data related to epidemiology, antigenic and genetic characteristics, serologic responses to 2013–14 seasonal vaccines, and the availability of candidate vaccine viruses and reagents.
The MMWR report sums up their findings:
What is already known on this topic?
CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. Substantial influenza activity generally begins in the fall and continues through the winter and spring months; however, the timing and severity of influenza activity varies by geographic location and season.
What is added by this report?
The 2013–14 influenza season was the first influenza A (H1N1)pdm09–predominant season since the emergence of the virus in 2009, and also had later-season influenza B activity. The highest hospitalization rates were among adults aged ≥65 years, which is consistent with previous influenza seasons; hospitalization rates among those aged 50 to 64 years were significantly higher than in all years since the 2009 pandemic. Nearly all of the influenza virus specimens sent to CDC for antigenic characterization were similar to the components of the 2013–14 Northern Hemisphere influenza vaccine. The Food and Drug Administration has recommended that the 2014–15 influenza vaccines used in the United States have the same antigenic composition as those used in 2013–14.
What are the implications for public health practice?
Influenza surveillance, including for novel influenza viruses, should continue throughout the summer months, and health-care providers should consider influenza as a cause of respiratory illness even outside the typical season. Although influenza viruses typically circulate at low levels during the summer months, timely empiric antiviral treatment is recommended for patients with severe, complicated, or progressive influenza illness and those at higher risk for influenza complications; treatment can be considered for others if it can be started within 48 hours of illness onset.
While there is no predicting what kind of flu year lies ahead, influenza has a habit of throwing us curve balls, and its potential impact should never be underestimated.
