Earlier this month, a report from the Centers for Disease Control and Prevention revealed that the 2014-15 flu season had already crossed the threshold for epidemic status, with 15 child deaths from the virus so far. Now, a new report from the organization estimates this season's flu vaccine is only 23% effective across all age groups.

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The CDC say this season's flu vaccine has low effectiveness and people at high risk of flu-related complications should seek additional prevention and treatment measures.

As a result, the Centers for Disease Control and Prevention (CDC) urge all individuals at high risk of flu-related complications to seek treatment as early as possible if any symptoms of influenza are identified.

"Physicians should be aware that all hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient's vaccination status and without waiting for confirmatory testing," says Dr. Joseph Bresee, branch chief of the Influenza Division at the CDC.

"Health care providers should advise patients at high risk to call promptly if they get symptoms of influenza," he adds.

The CDC reached their estimate, published in this week's Morbidity and Mortality Weekly Report (MMWR), by analyzing 2,321 children and adults with acute respiratory illness.

Of these, 950 (41%) had the influenza virus, with 916 (96%) testing positive for influenza A - all of which were H3N2 viruses - and 35 (4%) testing positive for influenza B. The 2014-15 flu vaccine was found to have been administered to 49% of patients with influenza and 56% of patients without the virus.

From this, the CDC estimate that the flu vaccine has reduced an individual's risk of visiting a doctor due to flu by 23%. This result remained after accounting for patients' age, sex, race/ethnicity, self-reported health and the number of days between illness onset and study enrollment.

Vaccine effectiveness against H3N2 viruses was estimated to be highest among children and adolescents aged 6 months to 17 years, at 26%. Vaccine effectiveness was lower among adults aged 18-49 and 50 and over, at 12% and 14%, respectively.

The report authors say their findings show the effectiveness of this season's vaccine is relatively low, compared with some seasons where vaccine effectiveness has reached 50-60%.

70% of this season's H3N2 viruses are 'drift variants,' making flu vaccine less effective

The CDC say their estimate supports previous findings that the 2014-15 flu vaccine has low effectiveness against circulating influenza A H3N2 viruses, which, as the report indicates, have been the most predominant viruses this season.

H3N2 viruses are the most severe, causing the highest number of hospitalizations and deaths. During the three seasons with the highest mortality rates over the past 10 years - 2012-13, 2007-08 and 2003-04 - H3N2 viruses were the most prevalent.

The CDC note that around 70% of this season's H3N2 viruses have been identified as "drift variants" - viruses that possess antigenic or genetic changes that make them different from the virus included in this season's flu vaccine, meaning the vaccine's effectiveness is reduced.

Vaccine effectiveness, the CDC say, is also dependent on the age and health of the individual receiving the vaccine; it tends to be most effective in young, healthy people and less effective among older people - as demonstrated in the report's estimates.

Findings should not deter from vaccination

Despite the low effectiveness of the 2014-15 flu vaccine, the CDC continue to recommend that all people aged 6 months and older receive the vaccine, as it may still prevent infections from some circulating influenza A H3N2 viruses and reduce severe flu-related complications.

"Also, vaccine might protect against other influenza viruses that can circulate later," the report authors add. "As of early November, 2014, fewer than half of US residents had reported receiving influenza vaccine this season. Influenza vaccination, even when effectiveness is reduced, can prevent thousands of hospitalizations."

In addition, the CDC say their estimate stresses the importance of additional prevention and treatment measures against influenza - particularly for individuals aged 65 and older, young children and others at high risk of flu-related complications.

The report authors write:

"Influenza antiviral medications should be used as recommended for treatment in patients, regardless of their vaccination status. Antiviral treatment can reduce the duration of illness and reduce complications associated with influenza.

Antiviral treatment should be used for any patient with suspected or confirmed influenza who is hospitalized, has severe or progressive illness, or is at high risk for complications from influenza, even if the illness seems mild."

They add that antiviral treatment should be administered within 48 hours of flu symptoms presenting, but it can still be effective among some patients even if initiated after this time.

The need for more effective influenza vaccines

The report authors say that while flu vaccines are the best protection we have against influenza at present, there is a need for more effective ones, and this is something that may soon be on the cards according to another study reported by MNT today.

Researchers from McMaster University and the Icahn School of Medicine at Mount Sinai in New York, NY, reveal they have discovered a new class of antibodies that could lead to a universal flu vaccine becoming available within the next 5 years.

"Unlike seasonal vaccines, which must be given annually, this type of vaccine would only be given once, and would have the ability to protect against all strains of flu, even when the virus mutates," explains senior author Matthew Miller, of McMaster University. "This would prevent the occurrence of flu pandemics and poor vaccine efficiency in the case of mismatches, which actually occurred this year."

While effective influenza vaccines are crucial for protecting against the virus, we should not underestimate the importance of good personal hygiene and other practices.

The CDC say avoiding close contact with sick individuals, staying at home when you are sick, covering your mouth and nose when coughing or sneezing, washing your hands often and avoiding touching your eyes, nose and mouth are all factors that can help prevent flu.