The 2017-2018 influenza season was one of the most severe in this century, according to every indicator measured by the Centers for Disease Control and Prevention (CDC). The proportion of outpatient visits due to influenza-like illness (ILI) was elevated nationally above a baseline of 2.2% for 19 straight weeks, and for 3 weeks it was over 7%.1 High ILI activity was widespread and included all 50 states in January.
From October 2017 through April 2018, the CDC estimates that the influenza-related hospitalization rate was 106.6 per 100,000 population, with the highest rates among children 0 to 4 years (74.3/100,000), adults 50 to 64 years (115.7/100,000), and adults 65 years and older (460.9/100,000). More than 90% of adults hospitalized had a chronic condition, such as heart or lung disease, diabetes, or obesity, placing them at high risk for influenza complications.1
Influenza severity is also measured as the proportion of deaths due to pneumonia and influenza, which was above the epidemic threshold for 16 weeks in 2017-2018 and was above 10% for 4 weeks in January.1 Based on all of these indicators, the 2017-2018 influenza season was classified as high severity overall and for all age groups, the first time this has happened since the 2003-2004 season. There were 171 pediatric deaths attributed to influenza, and more than three-quarters of vaccine-eligible children who died from influenza this season had not received influenza vaccine.1
The type of influenza predominating last season was influenza A from early- through mid-season, and was influenza B later in the season (see https://stacks.cdc.gov/view/cdc/54974).1 For the entire season, 71.2% of specimens that tested positive for influenza in public health labs were Influenza A and 84.9% of these were H3N2.1
Effectiveness of influenza vaccine last season. As measured by preventing respiratory illness needing medical attention, vaccine effectiveness was 36% overall: 25% against influenza A (H3N2), 67% against influenza A (H1N1), and 42% against influenza B.1 Effectiveness varied by age, being the highest in those 8 years and younger.2 Effectiveness was questionable in those older than 65, with an estimated effectiveness of 23% but confidence intervals including 0.2
While the effectiveness of influenza vaccines remains suboptimal, the morbidity and mortality they prevent is still considerable. The CDC estimates that in 2016-2017, more than 5 million influenza illnesses, 2.6 million medical visits, and 84,700 hospitalizations were prevented.3 And effectiveness last season was similar to, or better than, what has been seen in each of the past 10 years (FIGURE).4
Three drugs were recommended for use to treat influenza in 2017-1018 (oseltamivir, peramivir, and zanamivir), and no resistance was found except in 1% of influenza A (H1N1) tested.1 No resistance was found in other A or any B viruses tested.1
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