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NEW YORK – The 2014-2015 flu season is “severe,” especially for those 65 and over, children under 5 years old and those with other medical conditions, warned Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention in Atlanta in a teleconferenced media briefing Friday.

Frieden also admitted the flu vaccine this season has been particularly ineffective, largely because the virus has mutated to a virulent strain of the H3N2 virus that is not included in the vaccine. About two-thirds of the H3N2 viruses analyzed this season are different from the H3N2 virus that is included in this year’s CDC-approved flu vaccine.

On Jan. 5, the CDC reported that with about half the 15-week flu season now gone, 43 states are expecting widespread high flu activity. Patient visits to the doctor for influenza-like illnesses, ILI, now are almost even with the peak of the 2012-2013 season, the last time H3N2 viruses predominated.

For the week ending Dec. 27, overall flu-related hospitalizations were 12.6 per 100,000 people, which is comparable to the 13.3 per 100,000 overall hospitalization rate seen during the same week of the 2012-2013 season but higher than the 8.9 per 100,000 rate observed during week 52 of 2013-2014, which was an H1N1-predominant season.

“H3N2 viruses continue to predominate in the United States this flu season, accounting for more than 95 percent of all influenza reported cases,” the CDC reported.

“In the past, H3N2-predominant seasons have been associated with more severe illness and higher mortality, especially in older people and young children, relative to H1N1- or B-predominant seasons,” the CDC report continued. “Between 1976 and 2007, for example, CDC estimates that an average of 28,909 people died from flu during H3N2 seasons, compared to 10,648 people during non-H3N2 predominant years.”

Frieden indicated that hospitalization rates among the over-65 population were accelerating sharply, at 92 per 100,000 in the population this week, up from 52 per 100,000 the week before. In 2012-2013, the cumulative hospitalization rate for the over-65 population reached 183 per 100,000, a cumulate rate the CDC expects to see closely repeated this year.

Despite the relative ineffectiveness of the flu vaccine this season, the CDC continued to recommend getting it, including the inoculation of children as young as 6 months old, because, as Frieden said, “the vaccine may still be of some benefit.”

The CDC reported that as of Dec. 5, an estimated 145.4 million doses of seasonal flu vaccine had been distributed. As of early November, however, only 40 percent of people in the U.S. had reported getting a flu vaccine this season.

Frieden recommended clinicians should prescribe antiviral drugs for patients with influenza-like symptoms.

There are three FDA-approved influenza antiviral drugs recommended by CDC this season to treat influenza.

The brand names for the drugs are Tamiflu (generic name oseltamivir), Relenza (generic name zanamivir) and Rapivab (generic name peramivir). Tamiflu is available as a pill or liquid, and Relenza is a powder that is inhaled. (Relenza is not for people with breathing problems like asthma or COPD, for example.) Rapivab is administered intravenously by a health care provider.

“If more people with flu-like symptoms, particularly the elderly and those with other conditions including asthma, diabetes or renal illness, got treated with antiviral drugs, we could prevent tens of thousands of hospitalizations and thousands of deaths,” Frieden said.

Flu deaths in adults are not nationally reported, so the CDC has no reliable count of the number of people who die from flu each year.

“There are several factors that make it difficult to determine accurate numbers of deaths caused by flu regardless of reporting,” the CDC website reports. “Some of the challenges in counting influenza-associated deaths include: the sheer volume of deaths to be counted; not everyone that dies with an influenza-like illness is tested for influenza; and influenza-associated deaths are often a result of complications secondary to underlying medical problems, and this may be difficult to sort out.”

In the 2014-2015 influenza season for the week ending Jan. 3, the CDC reports “the proportions of deaths to pneumonia and influenza was above the epidemic.

At the CDC press briefing Friday, the reported studies suggest that from 5 to 15 percent of the U.S. population is expected to get influenza each year, resulting in tens of millions of influenza cases, tens of millions of doctor visits, 100,000 to 400,000 influenza hospitalization and tens of thousands of deaths.

The CDC studies over the last 20 years suggest a wide variety of estimates of the number of deaths caused by influenza per year, ranging from as few as 5,000 deaths in a year to a high of 50,000 deaths in the year.

Published CDC reports analyzing the past 31 flu seasons indicate deaths resulting from influenza average 32,743 per year over the past 10 seasons, with the 31-year average at 23,607.

In some years, the published CDC studies indicate as few as 3,349 have died (in 1986-1987), but the highest annual average toll was 48,614, in 2003-2004.

The difference, the CDC suggests, is the strain of influenza predominant each year. Deaths peak in years in which particularly virulent strains of the H3N2 dominate, as is the case this year, and are lower when H1N1 dominates, a less virulent strain.

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