More Severe Strain of Flu Now Dominant as Pediatric Deaths Rise

Troy Brown, RN

March 04, 2019

Influenza activity continues to be elevated in the United States amid 15 more reported pediatric deaths during the week ending February 23 (influenza season week 8), according to a weekly report from the Centers for Disease Control and Prevention (CDC).

In addition, the predominant strain of influenza has flipped from influenza A(H1N1)pdm09 to a more dangerous strain, influenza A(H3N2).

The proportion of outpatient healthcare visits for influenza-like illness (ILI) fell slightly but was still elevated at 5%, above the national baseline of 2.2%. Influenza-like illness is defined as fever (temperature of 100°F [37.8°C] or greater) with cough and/or sore throat.

Influenza activity was at or above region-specific baseline levels in all 10 regions. New York City and 33 states reported high influenza-like illness activity; the District of Columbia and eight states reported moderate activity; Puerto Rico and eight states reported low ILI activity; one state (New Hampshire) reported minimal ILI activity; and the US Virgin Islands provided insufficient data to calculate an activity level.

Geographically, influenza was widespread in Puerto Rico and 49 states; the District of Columbia and Hawaii experienced local activity; the US Virgin Islands experienced sporadic activity; and Guam did not report influenza activity.

More Dangerous Strain

The percentage of respiratory specimens that tested positive for influenza fell from 26.7% during week 7 to 26.4%. Last season at this point, the percentage was 21.6%.

Influenza A(H1N1)pdm09, influenza A(H3N2)and influenza B viruses are co-circulating. Nationally, the predominant strain for the last 3 weeks flipped from influenza A(H1N1)pdm09 to influenza A(H3N2) after spreading throughout the Southeast (Department of Health & Human Services Region 4) most of the season. During the most current 3 weeks, influenza A(H3N2) — which tends to be more deadly — was reported more often than influenza A(H1N1)pdm09 in HHS regions 2, 4, 6, and 7.

Influenza A(H3N2) predominated in the New England states early in the season.

Percentage of Flu Deaths Near Epidemic Threshold

The percentage of deaths that were caused by pneumonia and influenza was 7.1% — just below the National Center for Health Statistics Mortality Surveillance System's epidemic threshold of 7.3% for week 7.

The 15 pediatric deaths reported during week 8 bring the total this season to 56. Eight were linked to an influenza A(H1N1)pdm09 virus and occurred during weeks 5, 6, 7, and 8. One child's death was related to an influenza A(H3) virus and occurred during the week ending February 9 (week 6). The remaining six deaths were linked to an influenza A virus for which no subtyping was done and occurred during weeks 5, 6, 7, and 8. Additional information on influenza-associated pediatric deaths is available on the CDC website.

Hospitalizations Rose

The overall hospitalization rate rose to 32.1 per 100,000 population, from 27.4 the week before. The hospitalization rate was almost three times higher among adults ages 65 years and older, at 91.5 per 100,000 population, followed by children ages 0 to 4 years (45.5) and adults aged 50 to 64 years (43.2).

Of 9274 hospitalizations, 95% (8810 hospitalizations) were related to an influenza A virus, 386 (4.2%) to influenza B virus, 31 (0.3%) to influenza A and influenza B virus co-infection, and 47 (0.5%) to influenza virus for which no type was determined. Among those with influenza A subtype information, 1460 (69.7%) were associated with A(H1N1)pdm09 virus and 634 (30.3%) with A(H3N2).

More than 99% of influenza viruses tested demonstrated susceptibility to oseltamivir (Tamiflu, Genentech) , zanamivir (Relenza, GlaxoSmithKline) and peramivir (Rapivab, BioCryst) according to an accompanying report summary. No new viruses with decreased susceptibility were reported during week 8. This season, two (0.2%) influenza A(H1N1)pdm09 viruses showed reduced susceptibility to oseltamivir and peramivir and an additional two (0.2%) had reduced susceptibility to oseltamivir. All viruses tested were susceptible to zanamivir.

The CDC will begin reporting susceptibility testing for baloxavir (Xofluza, Genentech) later in the season.

Most (91.2%) of the 1403 adults who were hospitalized for influenza had one or more comorbidity, most common among them cardiovascular disease, metabolic disorder, and obesity. Underlying conditions were reported in 43.8% of the 265 hospitalized children who had information on underlying conditions; the most frequently reported condition was asthma. Forty-six (20.7%) of the 223 hospitalized women of childbearing age (15 - 44 years) with pregnancy status information were pregnant.

FluView week ending February 23, 2019 (week 8)

CDC Influenza Situation Update — week ending February 23, 2019

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