1 The efficacy of both vaccines
can be affected by a number of factors, including
the age and health of the vaccine recipients, and
by the extent of antigenic similarity between the
strains included in the vaccines and those that
actually circulate months later.2-6 Thus, there can
be differences in efficacy from year to year. There
are also issues related to the fact that although
two distinct type B lineages have recently been in
circulation each year, only one can be included in
the licensed vaccines.7 Similar issues may be encountered
if the novel influenza A (H1N1) virus
of swine origin continues to circulate along with
viruses of human origin.8
Beginning in the 2004–2005 influenza season,
we conducted a series of annual studies to estimate
the absolute and relative efficacies of licensed
inactivated and live attenuated vaccines in healthy
adults younger than 50 years of age.9,10 We report
here estimates of the efficacies of the two
vaccines in the 2007–2008 season, using end points
determined by viral culture and polymerasechain-
reaction (PCR) assay. Influenza-related morbidity
was high in 2007–2008, a year in which
type A (H3N2) viruses predominated; these viruses
were characterized by a slight antigenic drift
from the type A (H3N2) viral strain included in
the vaccine.11,12