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CDC Foresees Potentially Severe Flu Season

“Drift variants” reduce effectiveness of current vaccine

Early data suggest that the current 2014–2015 flu season could be severe, according to the Centers for Disease Control and Prevention (CDC).

So far this year, seasonal influenza A H3N2 viruses have been most common. There often are more severe flu illnesses, hospitalizations, and deaths during seasons when these viruses predominate, the agency says. For example, H3N2 viruses were predominant during the 2003–2004, 2007–2008, and 2012–2013 seasons, the three seasons with the highest mortality levels in the past decade. All of these seasons were characterized as “moderately severe.”

Increasing the risk of a severe flu season is the finding that roughly half of the H3N2 viruses analyzed are “drift variants” –– viruses with antigenic or genetic changes that make them different from that season’s vaccine virus. This means that the vaccine’s ability to protect against those viruses may be reduced, although vaccinated people may have a milder illness if they do become infected. During the 2007–2008 flu season, the predominant H3N2 virus was a drift variant, and yet the vaccine had an overall efficacy of about 40% against H3N2 viruses.

Depending on the formulation, flu vaccines protect against three or four different flu viruses. Even during a season when the vaccine is only partially protective against one flu virus, it can protect against the others, the CDC says.

“While the vaccine’s ability to protect against drifted H3N2 viruses this season may be reduced, we are still strongly recommending vaccination,” said Joseph Bresee, MD, Chief of the Influenza Epidemiology and Prevention Branch of the CDC. “Vaccination has been found to provide some protection against drifted viruses in past seasons. Also, vaccination will offer protection against other flu viruses that may become more common later in the season.”

Influenza viruses are constantly changing. The drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014–2015 Northern Hemisphere vaccine had been made in mid-February. At that time, a small number of these viruses had been found among the thousands of specimens that had been collected and tested.

A committee of experts must pick which viruses to include in the vaccine many months in advance in order for a vaccine to be produced and delivered in time for the upcoming flu season. There is always the possibility that viruses will drift during that time.

According to the CDC, the influenza antiviral drugs Tamiflu (oseltamivir, Genentech) and Relenza (zanamivir, GlaxoSmithKline) can reduce severe complications, such as hospitalization and potentially death, for people who are at high risk of serious flu complications or are very sick. Treatment of high-risk patients should begin as soon as possible after symptoms develop without waiting for laboratory tests to confirm flu infection.

Those at high risk for influenza include children younger than 5 years of age (especially those younger than 2 years of age); adults 65 years of age and older; pregnant women; and people with certain chronic health conditions, such as asthma, diabetes, or heart, lung, or kidney disease.

Source: CDC; December 4, 2014.

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