In recent years, there has been substantial controversy about how well influenza vaccines protect groups such as the elderly population; much of this debate has involved observational studies without virologic end points [22
]. However, there has also been a longer-standing controversy about the precise efficacy of inactivated vaccine in healthy adults. That debate was mainly centered on the end points that were used in the trials conducted in the US military, studies that established that the inactivated vaccine was 70%–90% effective [10
]. These studies were performed before rtPCR assays were available, and outcomes typically involved serologic assessments (ie, using an increase in antibody titer to identify infections in the vaccinated group versus the placebo group). Some have suggested that this outcome might overestimate the efficacy of vaccines because of the concept of an “antibody ceiling”—that is, that once antibody titers were increased in response to the vaccine, they could go no higher in response to infection [14
]. Despite this concern, the serologic end point has continued to be used in efficacy studies [11
], in part because it can be challenging to have subjects report illnesses and present for specimen collection for virus identification shortly after illness onset.
Serologic results from our recent comparative efficacy trials have demonstrated that, using only a serologic outcome, estimates will be biased in favor of overestimating inactivated vaccine efficacy [4
]. In analyses presented here, only 23% of inactivated vaccine failures demonstrated serologic confirmation of rtPCR-confirmed A (H3N2) infection. Similar results were not found for cases of influenza B. The small numbers might be a reason; circulation of different type B lineages involved might also be an issue [4
]. The situation with estimating live attenuated vaccine efficacy using a serologic outcome is somewhat different. In the pivotal trials of live attenuated vaccine leading to licensure, a serologic end point was not used to determine efficacy [2
]. Data from our studies have paradoxically shown that using a serologic end point in the trials in children might have been successful, in part because the live attenuated vaccine does not produce major serologic antibody responses, and an increase in antibody produced by subsequent infection is easily demonstrated.
Use of rtPCR for detecting influenza infection has now become a validated standard. It was used, along with isolation in cell culture and serologic testing, to identify infection in our comparative vaccine efficacy study. The test is highly sensitive, so much so that concern has been raised that it might be detecting infections that are not clinically relevant. However, studies following influenza illness over time have shown that detectable virus does not persist, particularly in adults [24
]. In our trials, rtPCR positivity was also associated with an increase in antibody titer in the live attenuated vaccine and placebo groups, even at high Ct values—a surrogate for low viral load. This and the fact that the illnesses met a symptomatic case definition indicate their clinical relevance. Low viral load, as estimated by Ct values, was also associated with failure to isolate the virus in cell culture. This applied only to A (H3N2) virus, which is known to be more difficult to isolate [18
], and was particularly seen in live attenuated vaccine failures. These findings would suggest that rtPCR with appropriately designed primers and probes should be the primary end point used in future efficacy studies. Isolation in cell culture must still be used in those rtPCR positive to further characterize the viruses, but use of it alone as an end point could result in missed cases and biased results.
Studying the characteristics of vaccine failures is difficult when evaluating efficacious vaccines. Through use of an unbalanced randomization design, 83% of subjects in our efficacy trials received one or the other vaccine, but still there were relatively few influenza cases in the vaccinated group, particularly among those receiving the inactivated vaccine. That may be the reason why it was difficult to demonstrate evidence of milder illness in the vaccinated, with the exception of reduced health care use. Characterization of severity has always been a challenge in influenza illness studies, even in those involving antiviral treatment, for which it was the primary outcome [25
]. An interesting finding, which was most apparent in the more numerous live attenuated vaccine failures, was reduced viral shedding among vaccinated subjects, as estimated by Ct values; this finding was unrelated to timing of specimen collection relative to illness onset. If this can be confirmed in children, it might partially explain the mechanism of live attenuated vaccine in producing indirect protection in school children [27
There are clear problems with using serologic end points in studies involving inactivated vaccine, as well as with using isolation in cell culture without also performing rtPCR assays in studies involving both live attenuated and inactivated vaccines. How protective then are our currently licensed vaccines since one or the other of serology and isolation have been used in past efficacy studies? That is more easily answered for the inactivated vaccine, at least in terms of efficacy among young adults. In both years in which there was sufficient virus transmission in our studies, demonstrated protective efficacy, using rtPCR alone, was ~70% [4
]. That may suggest that we should lower the usual description of vaccine efficacy from 70%–90% in healthy adults to closer to 70%; however, further confirmation by other studies is desirable. Given 70% efficacy in a population with 50% vaccine coverage, approximately one-quarter of influenza cases may occur among vaccinated persons, regardless of attack rate in a given year. This should be kept in mind when considering treatment strategies with influenza antivirals. In any event, these findings reinforce the need for improved influenza vaccines, perhaps even for young adults, the group in which the vaccine is thought to work best.