High-dose Influenza Vaccine vs Standard-dose for Seniors

The highest morbidity and mortality consequences of influenza occur in senior citizens. The efficacy of “standard” flu vaccine varies depending on the outcome that is examined. For instance, as has been best demonstrated in nursing home trials, even when standard-dose flu vaccine fails to prevent clinical disease, it mitigates disease severity enough to reduce mortality. Evolving vaccines, such as the high-dose influenza vaccine, are trying to improve on the already impressive results of earlier versions.

High-dose influenza vaccine (HDVax) contains four times the amount of hemagglutinin as standard-dose vaccine (SDVax). It has already been established that higher antibody titers are achieved with HDVax than SDVax, but whether that translates into improved protection from influenza has not been fully clarified.

This very large randomized, double- blind, active-controlled trial (n = 31,989) compared HDVax vs SDVax through two influenza seasons in North America. Both vaccines were highly effective, since < 2% of recipients of either vaccine contracted clinical influenza. HDVax was found to be superior to SDVax for the primary trial endpoint (acquisition of documented clinical influenza), showing a 24% lower frequency than SDVax. As previously demonstrated, higher antibody levels were also achieved with HDVax.

Before we celebrate the 24% risk reduction with HDVax, recall that this is a relativerisk reduction. The absolute risk reduction was very small (0.5%), translating into a number needed to treat of 200. Whether using HDVax instead of SDVax is a worthwhile investment — since this trial indicates that 200 persons must be treated with HDVax instead of SDVax to prevent one case of clinical influenza — will require further consideration, especially since there was no difference in mortality between the groups.