Inactivated Flu Vaccine vs Live, Attenuated Flu Vaccine – Which Gives Better Protection?
A recent study published in the New England Journal of Medicine (NEJM, 9/24/09) comparing the efficacy of live vs inactivated influenza vaccines given to adults during the 2007-2008 flu season, showed that the inactivated virus provided better protection against influenza A in healthy adults.
The study involved 1952 healthy adults between 18 and 49 years of age who were randomly assigned to receive either the inactivated vaccine administered by injection or the live, attenuated (weakened) influenza vaccine administered intranasally.
The study was designed primarily to evaluate the absolute efficacy (the effectivness of each vaccine compared to placebo) and secondarily to examine the relative efficacy (comparing the effectiveness of the inactivated to the live vaccine). Subjects were then asked to report 2 or more respiratory or systemic symptoms and throat cultures were then obtained in order to confirm the presence of the virus.
What is interesting is why, in adults, the inactivated vaccine proved to be 50% more effective than the live, attenutated vaccine which incidentally, is more efficacious in children. The authors speculate that the live, attenuated virus may be too similar to previous viral strains that infected adults and thus, adults were not able to become infected.
I presume this occurs because of the limited viral load present in the live, attenuated vaccine. The adult is able to suppress the infection from occurring when exposed to the viral load present in the vaccine but when the adult comes in contact with a higher viral load, as would likely be the case when exposed to someone who is sick, the immune system quickly becomes overwhelmed by the sheer number of viruses, causing the person to develop influenza.
The fact remains that scientists are not clear as to why there are age-specific differences. In the meantime, giving children the intranasal influenza vaccine and adults the inactivated vaccine seems the best way to confer protection.
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