The CDC say 2014-15 flu season could be severe as this year's vaccine may not fully protect against already circulating "drift" strains of H3N2 virus that in the past have been associated with more severe illness.
This is the message the Centers for Disease Control and Prevention (CDC) is sending out this week, as they urge "immediate vaccination for anyone still unvaccinated this season," and recommend "prompt treatment with antiviral drugs for people at high risk of complications who develop flu."
The federal agency says so far this year, the most common strains of flu viruses that have been circulating are so-called "drift variants" of A H3N2 viruses that are genetically different from the strains used to make this season's vaccine.
The seasonal flu vaccine is formulated to protect against three or four different flu viruses.
The CDC note, however, that even during a season when the vaccine only partially protects against one flu virus, it can protect against the others. They also point out that it will likely offer protection against other flu viruses that may become more common later in the season.
'Drift strains' of flu virus can increase during time it takes to make vaccine
The reason that this year's flu vaccine may not cover all the strains that end up in circulation is because the time it takes to make a new season's flu vaccine is longer than the time it takes for flu viruses to mutate into new variants.
Flu vaccine scientists - like long-range weather forecasters - have to make predictions way in advance of the flu season and in the meantime, things can change. It doesn't happen every year, but sometimes tiny changes in virus genes can accumulate in a few months to cause it to drift outside the range the scientists have anticipated.
This year, the World Health Organization (WHO) made its recommendations for 2014-2015 Northern Hemisphere vaccine in mid-February. This kicked off the vaccine production cycle, which takes around 6 months to prepare the millions of doses needed.
At that time, only a very small number of drifted H3N2 viruses had been found among the thousands of samples that had been collected and tested for analysis by WHO scientists.
However, more recent surveillance has picked up that so far, seasonal influenza A H3N2 viruses have been most common. These often cause more severe illnesses, hospitalizations and deaths during seasons when they predominate, note the CDC.
The 2012-2013, the 2007-2008 and the 2003-2004 flu seasons were dominated by H3N2 viruses. All three were classed as "moderately severe" and had the highest rates of flu deaths in the past decade, they add.
Around half of H3N2 viruses analyzed so far are drift variants
Around half of the H3N2 viruses analyzed so far are drift variants, suggesting the vaccine's ability to protect against H3N2 viruses may be reduced. However, even if they do become infected by these drift strains, people are likely to experience a milder illness if they are vaccinated.
For example, in the 2007-2008 flu season, the circulating flu strains were dominated by an H3N2 drift variant, yet the vaccine was 42% effective against H3N2 viruses compared with an overall efficacy of 37%.
CDC director Tom Frieden says, "It's too early to say for sure that this will be a severe flu season, but Americans should be prepared."
Flu activity is currently low in the US as a whole but is increasing in some regions, say the CDC.
Dr. Frieden says the season is only just beginning and, "It's not too late to get your vaccine."
There are three ways to save lives in a flu season, he adds. People should get vaccinated, there should be prompt treatment for those at high risk of complications, and - to prevent spreading flu virus - people should stay home when they get sick.
Everyone over 6 months old should have flu vaccine, say CDC
The CDC recommend that everyone - with rare exceptions - over the age of 6 months should have the seasonal flu vaccine.
People at high risk of seasonal influenza include children under the age of 5, adults aged 65 and over, pregnant women, and people with certain health conditions such as asthma, diabetes, kidney, heart or lung disease.
The CDC recommend that people at high risk should seek medical advice as soon as they start with flu symptoms, which can include sore throat, runny or stuffy nose, fever, cough, body aches, headache, chills and fatigue.
There is evidence that antiviral drugs against flu are more effective when started in the first 48 hours following the start of symptoms.
According to the WHO, as of 27 November 2014, with the exception of some Pacific Islands, global influenza activity remains low.
In the northern hemisphere, annual flu epidemics occur during autumn and winter and affect up to 15% of the population.
Meanwhile, Medical News Today recently learned of a study that concludes preterm children should be prioritized for receiving seasonal flu vaccinations. The findings urge for a review of current guidelines - such as those of the US, the UK and WHO - that maintain children born prematurely are not deemed to be at high risk of flu-related complications.