Since it takes many months to create, manufacture, and distribute a new influenza vaccine, decisions on which strains to include in the next seasonal flu shot must be made about six months in advance.
Twice each year the World Health Organization gathers scientists from around the globe to try to predict which flu strains will be dominant in the upcoming flu season, so they can include them in the next flu vaccine.
Since viruses constantly evolve and change, this can prove quite a challenge.
Although it happens rarely, as we saw in 2009, a novel (even pandemic) flu strain can emerge after the seasonal strains have been selected, throwing a sizable monkey wrench into the fall vaccination campaigns.
And as I wrote last month in Three Would Make For A Crowded Viral Field, with the slow-motion emergence of a new H3N2v swine flu virus over the past 5 months, this is becoming an increasingly tricky business.
Photo Credit – CDC PHIL
The seasonal flu shot licensed each year is a trivalent vaccine – meaning it contains 3 different flu strains (2 A-strains, 1 B-strain).
In recent years there has been discussion of moving to a quadravalent vaccine, one that would include 2 A-strains and 2 B-strains, but licensing one for use in the United States would require clinical trials and FDA approval.
The two dominant influenza A strains in circulation today are the H1N1pdm09 virus - which replaced the old seasonal H1N1 - and the H3N2 seasonal virus.
But we also have a new H3N2v swine flu virus in the wings. One that - while not spreading widely yet - threatens to complicate the upcoming vaccine strain selection.
H3N2v has been detected in 5 states among a small handful of people, and is suspected to be circulating at very low levels across the United States. It has not yet been reported in any other country, but then, testing for it can be difficult.
Whether this new H3N2v strain has `legs’, and can establish itself as a viral contender for next year’s flu season, is exceedingly hard to predict.
This virus may fizzle, or lope along as a minor player.
The concern is, given the apparent lack of immunity of those under the age of 20, this strain may have the potential to spread more widely in the coming months, particularly among children and adolescents.
Complicating matters, our northern hemisphere flu season is off to a slow start, and has provided few clues as to how well this viral interloper will fare when pitted against this year’s dominant H1N1 and H3N2 strains.
While the future course of this virus is unknown, the CDC has already prepared a "seed virus" that vaccine manufacturers can use to develop a vaccine, if the need arises.
Although a separate mono-valent vaccine could be prepared specifically for this strain, as we saw in 2009, that would be both expensive and highly inconvenient.
Luckily we’ve still a month or more for experts to watch this new virus before decisions must be made regarding next fall’s vaccine. The FDA could conceivably even delay a decision for a few weeks, to give more time for data to be examined.
But the truth is, it could take many months before we really know whether this H3N2v strain has real staying power in the human population.
Adding the H3N2v strain to next fall’s trivalent vaccine would be a difficult decision, as it would entail dropping one of the three current flu strains from the shot. A move that could have serious public health ramifications by itself.
Be glad this isn’t your decision.
All of which illustrates the need for newer and faster vaccine manufacturing techniques that don’t require six months of lead time, or for the long sought after `holy grail’ of flu research, a universal flu vaccination.
Solutions that are, sadly, still years away.