2018-2019 season retrospective
Last year’s influenza season was longer than usual. Infections, as measured by the percentage of outpatient visits due to influenza-like illness, increased in early November 2018, peaked in early February to mid-March of 2019, and remained above baseline levels through mid-May.1,2 Ninety six percent of influenza-positive samples were influenza A,1 and 57% of those were H1N1.2 In the second half of the season, H3N2 became the predominant circulating virus and there was a genetic shift in this strain that caused a decrease in the effectiveness of influenza vaccines (FIGURE).1 The influenza-confirmed hospitalization rate was 65.3/100,000, with the highest rate (221.7/100,000) occurring among those 65 years of age and older.2 Of those hospitalized with influenza, 93% of adults and 55% of children had an underlying medical condition and 29% of women of childbearing age were pregnant.2
Morbidity and mortality from influenza during the 2018-2019 influenza season were moderate compared with previous years. Pneumonia and influenza mortality reached close to 8% of all deaths during the peak of the season (considered a modest peak), but stayed above the epidemic threshold for 10 weeks.2 There were 119 pediatric deaths.1 Overall, in the United States, there were an estimated 37 to 43 million influenza-related illnesses, 17 to 20 million flu-related medical visits, 531,000 to 647,000 flu-related hospitalizations, and 36,400 to 61,200 deaths.1
Influenza viral resistance to oseltamivir remained very low throughout the season for both A and B viruses.2
Vaccine effectiveness was subpar
The effectiveness of influenza vaccine last season was disappointing. When assessed using laboratory-confirmed medically attended influenza, the vaccine was 29% effective; when assessed by age group, the confidence intervals included 0 in ages 9 to 17 years and 50 years and older.3 In the age group 6 months to 8 years, the vaccine was 49% effective.3 The vaccine was not effective against the predominant H3N2 strain circulating. It was 25% effective in preventing hospitalization, with a lack of benefit seen in individuals ages 18 to 49 years and those 65 and older.3
Vaccination was associated with increased rates of hospitalizations from infections cause by H3N2. It is not known if this finding was due to chance, unstable results from small numbers, an unknown bias, or some biological cause not yet understood. This is a topic of ongoing research.
Effectiveness in preventing pediatric hospitalizations was estimated at 31%, again with no effectiveness against H3N2.3 The estimate of vaccine effectiveness in the United States was similar to that in Canada.2
While these results are much lower than desired, influenza vaccine did prevent an estimated 40,000 to 90,000 hospitalizations and decreased influenza-like illnesses by 44%.3
Continue to: A look at vaccine safety