Influenza activity shot up across the United States last week, according to the latest FluView weekly report today from the Centers for Disease Control and Prevention (CDC). Geographic indicators and increasing hospitalizations all suggest high flu activity.
Now 46 states are experiencing widespread flu activity, with only Hawaii, Maine, New Hampshire, and New Jersey reporting regional levels of influenza. New York City and 26 states reported high influenza-like illness (ILI) activity. Nine states and Puerto Rico reported moderate activity, and 15 states and the District of Colombia reported low or minimal activity.
In the week before, 21 states had reported high ILI activity.
H3N2 dominates lab samples
Last week, with 36 states reporting widespread activity, the CDC issued a warning stating that H3N2, an influenza A subtype that's notoriously hard on children and the elderly, was dominating this season's flu landscape.
Further evidence of H3N2's prevalence was shown in this week's laboratory surveillance data. Influenza A was the dominant strain, accounting for 84.9% of all laboratory-confirmed flu samples. Of those influenza A specimens, 87.0% were H3N2 and 7.7% were H1N1. Only 15.1% of all samples tested were influenza B, with the majority (58.3%) subtyped as Yamagata.
The proportion of laboratory samples testing positive for flu went up from 22.4% last week to 25.5% this week.
For the sixth week in a row, the percentage of clinic visits for flu was above the national baseline of 2.2%. Last week, that percentage was 5.0%; this week it climbed to 5.8%.
Hospitalizations due to ILI also rose this week, from 8.7 hospitalizations per 100,000 population to 13.7. Adults over 65 were most likely to be hospitalized, with 56.6 hospitalizations per 100,000 population, followed by adults aged 50 to 64 (15.4 per 100,000 population) and children aged 0 to 4 years (9.9 per 100,000 population). The vast majority (90.1%) of hospitalizations were associated with influenza A.
"Antiviral treatment as early as possible is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for serious influenza-related complications," the CDC said, echoing last week's Health Alert Network reminder that clinicians should use antivirals in certain high-risk patients when influenza is suspected.
One additional pediatric death was reported last week, bringing the total number of pediatric deaths for the 2017-18 season to 13. The child suffered from an influenza A infection, but subtyping was not performed, the CDC said.
Flu in Europe
The European Centre for Disease Prevention and Control (ECDC) said in its weekly flu report that activity has also increase in Northern, Southern, and Western Europe. Influenza A and B are co-circulating across the continent.
Of individuals presenting with ILI or respiratory illness to sentinel primary healthcare sites, 44% tested positive for influenza viruses, an increase from 38% in the previous week.
Jan 5 CDC FluView
Dec 27 CDC Health Alert Network post
Jan 5 Flu News Europe