Our multiyear study of comparative vaccine efficacy was initiated in 2004–2005 to resolve some of these questions. In the first year of our trial, in which an influenza A virus with moderate antigenic drift and two lineages of influenza
B virus circulated, we found that the absolute efficacy of the inactivated vaccine in preventing symptomatic influenza was 67 to 78%, on the basis of several laboratory-confirmed end points.9 In that same year, we did not find a significant
absolute efficacy of the live attenuated vaccine with use of the same end points. Laboratory-confirmed
outcomes were too few in number to determine whether the inactivated vaccine significantly outperformed the live attenuated vaccine. Influenza activity was relatively low in the
subsequent influenza season (2005–2006). In that
year, despite an enrollment that exceeded the
target number, absolute efficacy was shown for
only the inactivated vaccine and only when serologic
end points were included.10