Vaccines containing inactivated influenza virus have been available for 50 years to prevent flu and its complications, according to background information in the article. Immune responses decrease with age, however, and currently available flu vaccines are not always effective in the elderly. Recommended vaccines contain 15 micrograms (?g) of inactivated virus per strain.
Wendy A. Keitel, M.D., Baylor College of Medicine, Houston, and colleagues studied the response to vaccines containing 15, 30 and 60 ?g of virus among 202 individuals age 65 years and older (average age 72.4) in 2002. The 60-?g level was selected as the highest dosage because manufacturers would reasonably be able to produce that type of vaccine. Participants were randomly assigned to receive one of the three vaccines or a placebo injection and then were monitored daily for one week afterward. They were also examined 30 minutes, two days and 28 days after the vaccine and were contacted six months later to inquire about serious adverse events or side effects. Blood was drawn before the immunization and one month after.
On average, individuals who received higher dosages of vaccine had higher concentrations of antibodies against the flu virus in their blood one month later. In addition, a larger percentage of those who received the higher-dose vaccines had what physicians believe is a sufficient immune response to protect them from developing the flu if they were exposed to the virus. All three vaccine dosage levels were safe and well tolerated, according to the researchers. Although discomfort, redness or swelling at the site of the injection was more common among those who received the 60 ?g dose than among those who received the 15 ?g dose, most reactions to the immunization were mild.
Although current vaccines are sufficient to protect younger adults from influenza, the elderly and others at high risk of hospitalization and death from the disease have not responded consistently, the authors write. "Improved vaccines are needed to reduce the morbidity and mortality associated with influenza in these vulnerable populations," they write. "Increasing the dose of inactivated influenza vaccine consistently and safely increases the immunogenicity [immune response] of inactivated influenza virus vaccines; moreover, vaccines that contain higher doses have conferred significantly enhanced protection against naturally occurring influenza."