Jump in H3N2 flu prompts CDC warning, antiviral reminder

The US Centers for Disease Control and Prevention (CDC) today warned clinicians that flu activity has increased significantly in recent weeks, much of it from H3N2, which poses a heightened threat, especially to seniors and young children, and elevates the importance of prompt antiviral treatment.

At the global level, H3N2 is still the dominant strain, but different regions are seeing a lot of illness from influenza B and 2009 H1N1 viruses, the World Health Organization (WHO) said in its latest update.

H3N2 activity prompts reminders about speedy antiviral prescribing

In a Health Alert Network (HAN) notice, the CDC said predominant H3N2 activity has major implications for clinical treatment, given that the vaccine is generally less effective against that subtype, and that neuraminidase inhibitors, such as (oseltamivir) Tamiflu, will be a useful treatment adjunct.

Past H3N2-heavy seasons have been marked by more hospitalizations and deaths in those over age 65 and in younger children, the CDC said. It added that seasonal flu vaccines have shown less protection against H3N2 than the 2009 H1N1 and influenza B strains, and if H3N2 continues to dominate, the efficacy against H3N2 could be near the 32% seen last season.

"For this reason, in addition to influenza vaccination for prevention of influenza, the use of antiviral medications for treatment of influenza becomes even more important than usual," the CDC said.

Aside from warning clinicians to keep flu high on their list of possible diagnoses for sick patients, the CDC urged clinicians to treat all hospitalized and high-risk patients with a neuraminidase inhibitor as soon as possible. Though treatment started within 2 days of illness onset is ideal, there are still clinical benefits when given later. The drugs are often underutilized, the CDC said, but are appropriate for hospitalized patients, people with progressive disease who are not hospitalized, and those at risk for flu complications, such as seniors, young children, pregnant women, and those with underlying health conditions. Clinicians should not wait for lab-confirmation of flu before prescribing treatment for these patients, and it may be useful to provide phone triage to streamline diagnosis and treatment.

Also, clinicians can use their judgement to prescribe neuraminidase inhibitors to previously healthy people.

Flu cases rising in Northern Hemisphere

In its weekly global influenza update, the World Health Organization (WHO) said H3N2 is the dominant flu strain in North America, while influenza B is circulating more widely in Europe.

Flu is still rising across much of North America, with low levels detected in Central America, the Caribbean, and South America. Influenza B detections are above seasonal averages for most of North America, the WHO said.

Throughout much of eastern and southern Asia, flu activity is low, but China reported rising levels of influenza-like illness (ILI). Influenza B Yamagata-lineage viruses are the predominant strain in China. Western Asia also recorded elevated levels of influenza, with 2009 H1N1 detected most frequently.

Results from specimens collected in 106 countries in the first 10 days of December show that 62.4% of flu samples were typed as influenza A and 37.6% as influenza B. Of the sub-typed influenza A viruses, 30.1% were 2009 H1N1 and 69.9% were H3N2. Of the characterized B viruses, 85.2% belonged to the B-Yamagata lineage and 14.8% to the B-Victoria lineage.

See also:

Dec 27 CDC HAN notice

Dec 25 WHO update

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