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One can understand from the passage that flu vaccines currently on the market ----.
A new vaccine that may offer lifetime protection against
the flu has shown promise in human trials. It works on the
deadly type A strain, responsible for pandemics. Current
flu vaccines work by giving immunity against two proteins,
called haemagglutinin and neurominidase, found on the
surface of flu viruses. However, as these proteins
continually mutate, vaccines have to be reformulated every
year to keep on working. The new vaccine, known as
ACAM-FLU-A, gets around this problem by homing in on a
protein called M2, found on all type A strains, that does not
mutate so readily. The vaccine could be quickly produced
in response to a flu outbreak. The doses can be mass
produced and used at any time, because there is no need
to identify the most prevalent strains. In theory, a single
injection could offer lifetime protection. Whether this will
work in practice is a matter for future trials.