Invited commentary: Use of selective viral cultures to adjust nonvirologic endpoints in studies of influenza vaccine efficacy.

Influenza vaccine was first developed during the Second World War to protect the US military and to maintain readiness of combat troops (1). The virus was grown in fertile eggs, and the antigen was concentrated and inactivated before administration. Vaccines currently licensed for use in the United States are produced by using similar techniques. They are now more highly standardized, and the virus has been disrupted so that viral components considered unnecessary for protection can be removed; in general, however, they are little changed from those developed approximately 60 years ago (2). Most estimates of protective efficacy are also old. Nearly every year through the 1960s, randomized controlled trials comparing vaccine with placebo were conducted in the US military. The endpoint for comparison was infection as determined not by virus isolation but by rise in antibody titer, based on the hemagglutination-inhibition test, between the blood specimens collected before and after the influenza season. Protective efficacy, so determined, ranged from 70 percent to 90 percent except in years when the circulating virus was antigenically different from the strains included in the vaccine (3). While these evaluations were confirmed by more recent studies using similar endpoints and remain the “gold standard,” they were criticized even contemporaneously for not using virus isolation as an endpoint (4). The reason, partially theoretical, was based on the fact that vaccine would produce a rise in titer in the vaccinees only and that it might be more difficult to detect a further rise in titer among this group, given infection, than in the unvaccinated. A concept of being near the “antibody ceiling” was even introduced as part of this concern, without much biologic justification. The major reason that virus isolation was not used, except to define the period of virus circulation, involved the logistics of specimen collection from all participants at the time of illness. The problem in using virus isolation as an endpoint in large studies is that the procedure is more labor intensive and less comprehensive than using serology. Blood specimens for serologic studies can be easily collected from all participants before and after the season, while collecting specimens for virus isolation involves first recognizing an illness meeting the case definition and then contacting the case and carrying out the clinical procedure. There should not be a potential for selection bias in choosing who should be cultured in a blinded study, such as those carried out in the military, or, for that matter, in civilian populations when it may be more difficult to collect specimens in a planned fashion. Those who questioned using serology as the only endpoint proposed simply using differences in risk of all clinical respiratory illness meeting a case definition by vaccine status as a way of defining efficacy. Given the occurrence of acute respiratory illnesses of noninfluenzal etiology even in the influenza season, these analyses would always result in lower clinical efficacy estimates, biased toward no effect, although the exact degree of bias would vary. Over the years, influenza vaccination became accepted and even recommended for certain segments of the civilian population. However, estimates of the efficacy of inactivated influenza vaccine again became a policy-related issue in the early 1990s when it was decided that only if there could be some demonstration of cost-effectiveness of the vaccine for the US Medicaid system would it be approved for coverage. The problem here was further complicated by the inability in the United States to conduct randomized, placebo-controlled trials in the target population, those 65 years of age or older, since, by then, influenza vaccination was an absolute recommendation for this age group. As a result, only observational studies could be conducted. These observational studies concentrated on hospitalization as an endpoint, since hospitalization was a great expense to the health care system and the investigation had an economic underpinning. The outcomes examined in those studies that used a case-control design were those discharge diagnoses corresponding to the categories classically defined as “pneumonia and influenza”