By Robert Roos, Health Editor
Mar 11, 2011 (CIDRAP News) – Researchers from the University of Michigan say one measure of flu vaccine efficacy that has been used in a number of past controlled trials is not very accurate, and that this may have led to a degree of overselling of the protection the vaccines provide.
Writing in the Journal of Infectious Diseases, Joshua G. Petrie and colleagues describe the effects of using different “end points,” or measures for detecting infection, in a 4-year flu vaccine efficacy trial they conducted. They found that using serologic measures—an increase in flu antibodies—in vaccinated individuals as evidence of infection leads to an overestimate of vaccine efficacy.
They also concluded that virus isolation—growing the flu virus in cell culture—was not very accurate, as the viruses are hard to culture, resulting in missed cases. The most accurate tool, they found, was real-time polymerase chain reaction (RT-PCR).
In their trial, the use of RT-PCR showed that the protective efficacy of the vaccine was about 70%. “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,” the report says.
“Basically what we’re saying is that in some studies, depending on the design, there may have been an overestimate of vaccine efficacy based on serologic outcome,” Arnold S. Monto, MD, senior author of the study, told CIDRAP News. “It requires reevaluation of some of these studies.”
The report says that trials conducted in the US military decades ago established that inactivated flu vaccines were 70% to 90% effective in healthy adults. PCR was not available then, and the researchers typically used serology to identify infections in the vaccinated participants versus the placebo recipients.