The 2015-2016 flu season was less severe than the last three seasons, with a lower hospitalization rate and fewer pediatric deaths.
Cases of influenza appeared later in the season that typically seen, and activity didn’t peak until March, Stacy L. Davlin, Ph.D., wrote in the June 10 issue of the Morbidity and Mortality report (MMWR 2016; 22:567-75)
“During the most recent 18 influenza seasons, only two other seasons have peaked in March (2011-2012 and 2005-2006),” wrote Dr. Davlin, an epidemiologist at the Centers for Disease Control and Prevention, Atlanta. This serves as a reminder that influenza can and does occur outside the traditionally expected season, and that clinicians shouldn’t discount the possibility of flu when a patient presents with typical symptoms.
Cynthia Goldsmith/CDC photo #10073
“Although summer influenza activity in the United States typically is low, influenza cases and outbreaks have occurred during summer months, and clinicians should remain vigilant in considering influenza in the differential diagnosis of summer respiratory illnesses,” Dr. Davlin said.
The most common influenza virus of the last season was A(H1N1), which accounted for about half of cases in those aged 5-24 years, and about 70% of cases in those younger than 5 years and those 65 years and older.
Three novel viruses were seen as well: variants of A(H1N1), A(H1N2), and A(H3N2). The A(H1N1) variant occurred in a Minnesota resident who lived and worked in an area of swine farming, but who denied direct contact with pigs. The A(H3N2) variant occurred in a New Jersey resident who reported visiting a farm shortly before symptom onset. There was no evidence of human-to-human transmission. Both recovered fully without hospitalization. The A(H1N2) variant occurred in a Minnesota resident who was hospitalized but who recovered. This person was not interviewed so no possible source of infection was identified.
The CDC tested 2,408 viral specimens for susceptibility to antiviral medications. Among the 2,193 A(H1N1) specimens, less than 1% were resistant to oseltamivir and peramivir. All were susceptible to zanamivir. However, the testing found persistent high levels of resistant to amantadine and rimantadine in the A viruses. Amantadine is not effective on the B strains at all. Therefore, CDC does not recommend the use of amantadine as an anti-influenza medication.
Reports of influenza first exceeded the 2.1% baseline level in the week ending Dec. 26, 2015, according to the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet). They remained elevated for the next 17 weeks, with a peak of 3.6% of all outpatient visits in the week ending March 12. From October 2015-April 2016, the overall hospitalization rate for influenza-like illness was 31 per 100,000. This was highest in those aged 65 years and older (85/100,000), and lowest in those aged 5-17 years (10/100,000). About 92% of adults hospitalized for flu-like illness had at least one underlying medical comorbidity, including obesity (42%), cardiovascular disease (40%), and metabolic disorders (38%). Almost half of children (48%) also had medical comorbidities, including asthma or other reactive airway disease (22%) and neurologic disorders (18%).
CDC’s National Center for Health Statistics Mortality Surveillance System found that the percentage of deaths attributed to pneumonia and influenza peaked at 8% during the week ending March 19. This is slightly lower than the death rate seen in the last 5 years, which ranged from 9% in 2011-2012 to 11% in 2012-2013.
Of this season’s deaths, 74 occurred in children. The mean and median ages of these patients were 7 years and 6 years, respectively; the range was 2 months-16 years. This total was lower than that recorded in any of the past three influenza seasons: 171 pediatric deaths in 2012-2013, 111 in 2013-2014, and 148 in 2014-2015.
Dr. Davlin also announced the Food and Drug Administration’s recommendations for composition of the 2016-2017 influenza vaccine.
Trivalent vaccines should contain an A/California/7/2009 (H1N1) pdm09-like virus, an A/Hong Kong/4801/2014 (H3N2)-like virus, and a B/Brisbane/60/2008-like virus (B/Victoria lineage). Quadrivalent vaccines, which have two influenza B viruses, should include the viruses recommended for the trivalent vaccines, as well as a B/Phuket/3073/2013-like virus (B/Yamagata lineage).
“The vaccine viruses recommended for inclusion in the 2016-2017 Northern Hemisphere influenza vaccines are the same vaccine viruses that were chosen for inclusion in 2016 Southern Hemisphere seasonal influenza vaccines,” Dr. Davlin noted. “These vaccine recommendations were based on a number of factors, including global influenza virologic and epidemiologic surveillance, genetic and antigenic characterization, antiviral susceptibility, and the availability of candidate vaccine viruses for production.”
As a CDC employee, Dr. Davlin had no financial disclosures.
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