Following warning from the Centers for Disease Control and Prevention in December that this flu season is set to be a particularly harsh one, the organization’s latest influenza surveillance report states that this year’s flu season has now crossed the epidemic threshold.

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Half of the H3N2 viruses analyzed in this season are what are known as “drift variants” – viruses with antigenic or genetic changes that may make the virus less responsive to vaccination.

The Centers for Disease Control and Prevention (CDC) released a statement on December 4th urging anyone still unvaccinated to seek immediate vaccination. The CDC also recommended that people at high risk of complications who develop flu should receive prompt treatment with antiviral drugs.

The basis for the CDC’s concern was the predominance of the more severe strains of flu this season – seasonal influenza A H3N2 viruses – which are usually responsible for the flu-related illnesses that cause hospitalizations and deaths.

“For example,” reads the CDC statement, “H3N2 viruses were predominant during the 2012-2013, 2007-2008 and 2003-2004 seasons, the three seasons with the highest mortality levels in the past decade. All were characterized as ‘moderately severe.'”

In addition, approximately half of the H3N2 viruses analyzed in this season are what are known as “drift variants” – viruses with antigenic or genetic changes that may make the virus less responsive to vaccination.

In the 2007-2008 flu season, a drift variant of H3N2 was the predominant strain, which resulted in a vaccine efficacy of 37% against that strain and 42% against H3N2 viruses overall.

However, in the organization’s latest influenza surveillance report, it is noted that 15 children have died across the country from flu, and there has been an increase in the number of states reporting a “high” level of flu activity – from 13 to 22 in just 1 week.

The report also states that during October-December 2014, there have been 2,643 laboratory-confirmed flu-related hospitalizations – an overall rate of 9.7 hospitalizations per 100,000 population.

Seniors have been most hard hit by the current flu season, with 38.3 hospitalizations per 100,000 population of the over-65s.

The majority of hospitalized adults – 96% – had at least one reported underlying medical condition, the report says. Most commonly, these underlying conditions were metabolic disorders, cardiovascular disease and obesity.

Although the word “epidemic” sounds alarming, it is not uncommon for a harsh flu season to be described as such. The CDC consider a flu season to reach epidemic level when a threshold of deaths attributable to pneumonia and influenza has passed. The surveillance report confirms that the current proportion of deaths is right on that threshold, which is 6.8%.

Speaking to The Washington Post, Erin Burns, a health communications specialist within the CDC’s Influenza division, said of the report:

It is a bit early to make any kind of characterization about pediatric deaths this season, but from looking at the curve going back to 2011-2012, it doesn’t seem like anything unusual is happening.”

However, Burns acknowledged that pediatric death data “typically lag behind” other measures used to calculate the severity of a flu season.

Each year, a committee of experts pick which viruses should be included in the flu vaccine many months in advance, in order to maximize the vaccine’s effectiveness. However, there always remains the possibility that the viruses will drift during that time.

Some critics have commented that this drift is the most concerning aspect of the current epidemic.

For instance, the head of the Johns Hopkins Medicine Office of Epidemiology and Infection Prevention, Trish Perl, is quoted by The Post as saying “the most concerning thing about the flu season this year is there is a mismatch between the predominant strain that is circulating and what was put in the vaccine.”

The CDC’s position is that vaccination remains vital, and that it is not too late to get vaccinated.

“While the vaccine’s ability to protect against drifted H3N2 viruses this season may be reduced, we are still strongly recommending vaccination,” says Dr. Joseph Brezee, chief of the Influenza Epidemiology and Prevention Branch at the CDC, in a statement.

“Vaccination has been found to provide some protection against drifted viruses in past seasons,” Dr. Brezee reassures. “Also, vaccination will offer protection against other flu viruses that may become more common later in the season.”