This year’s influenza season is the longest in a decade, and although influenza activity is subsiding, it remained elevated for the 21st week, according to a report and report summary from the US Centers for Disease Control and Prevention (CDC).
In February, the predominant influenza strain flipped from the milder A(H1N1)pdm09 virus to the A(H3N2) virus, which is often more severe, and experts believe this change in the predominant strain is responsible for the longer influenza season.
Influenza–like illness activity was widespread in 11 states during the week ending April 13 (week 15), down from 20 states the week before. Activity was regional in Puerto Rico and 20 states, with local activity in 17 states and sporadic activity in the US Virgin Islands and two states (Indiana and Texas). The CDC expects sporadic activity to continue for a number of weeks.
Influenza activity was high in Rhode Island; moderate in five states (Arizona, Hawaii, Kentucky, Louisiana, and Missouri); low in New York City, Puerto Rico, and 14 states; and minimal in the District of Columbia and 30 states.
The Influenza Hospitalization Surveillance Network (FluSurv-NET) has received reports of 17,979 laboratory-confirmed influenza-associated hospitalizations since October 1, 2018, translating to a cumulative overall rate of 62.3 hospitalizations per 100,000 population in the United States.
The hospitalization rate was highest among adults aged 65 years and older (206.5 per 100,000), followed by adults aged 50-64 years (77.8 per 100,000), and children younger than 5 years (71.0 per 100,000).
Five more pediatric deaths were reported to the CDC during week 15, bringing the total this season to 91. Three of the deaths were linked to an influenza A(H3) virus and occurred during the weeks ending February 23, March 23, and April 13, 2019. Two deaths were related to an influenza A virus that did not undergo subtyping; they occurred during the weeks ending March 23 and March 30, 2019.
The proportion of deaths from pneumonia and influenza was 6.6% during week 14 — below the epidemic threshold of 7.0% for week 14. That proportion has been at or above the epidemic threshold for 9 weeks during the 2018-2019 season (weeks 1-3, weeks 7-9, and weeks 11-13).
Nationally, the percentage of respiratory specimens that tested positive for influenza in clinical laboratories dropped from 15.0% the previous week to 11.8%, after peaking at 26.2% during the week ending February 23, 2019 (week 8).
Although nationally influenza A(H3) viruses were reported more often than influenza A(H1N1)pdm09 viruses and predominated in all 10 regions, influenza A(H1N1)pdm09 viruses are still predominant nationally overall.
Most influenza A(H1N1)pdm09, B/Victoria, and B/Yamagata influenza viruses collected in the United States from September 30, 2018, to April 6, 2019, continue to be described as being antigenically and genetically similar to their cell-grown reference viruses representing the 2018-2019 Northern Hemisphere influenza vaccine viruses.
“However, the majority of influenza A(H3) viruses are antigenically distinguishable from A/Singapore/INFIMH-16-0019/2016 (3C.2a1), a cell-propagated reference virus representing the A(H3N2) component of 2018-2019 Northern Hemisphere influenza vaccines,” the CDC writes in its report summary.
“The vast majority (> 99%)” of influenza viruses tested were susceptible to oseltamivir, zanamivir, and peramivir. To date this season, three (0.2%) influenza A(H1N1)pdm09 viruses showed “highly reduced inhibition by oseltamivir and peramivir.” Two (0.1%) further influenza A(H1N1)pdm09 viruses demonstrated reduced inhibition by oseltamivir. All influenza viruses tested were susceptible to zanamivir.
“While CDC continues to recommend influenza vaccination as long as influenza viruses are circulating, influenza antiviral drugs are an important second line of defense that can be used to treat flu illness,” the CDC writes in the report.