WASHINGTON — Vaccine site has little impact on the vaccine's effect, but using a 25-mm needle instead of a 16-mm needle may be more effective in administering flu vaccine to older patients, based on results of a study conducted at the Mayo Clinic in Rochester, Minn.
The Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices currently recommends influenza vaccination for all adults older than 50 years, but studies have shown that vaccine efficacy may be reduced in older adults, said Dr. Prikish Tosh, an infectious disease fellow at the Mayo Clinic.
“We wondered whether vaccine site had any effect,” said Dr. Tosh, who presented the study results at the joint annual meeting of the Interscience Conference on Antimicrobial Agents and Chemotherapy and the Infectious Diseases Society of America.
He and his colleagues also examined whether a longer needle would increase penetration into the muscle and affect immunogenicity and reactogenicity in older patients. Flu vaccine manufacturers recommend 25-mm needles, but some single-dose vials are currently packaged with 16-mm needles, Dr. Tosh said.
In this study, Dr. Tosh and colleagues randomized 133 adults aged 50–88 years to receive the trivalent inactivated influenza vaccine in either the deltoid muscle or the deltoid fat pad. The groups were similar in terms of age, gender, weight, and other demographic characteristics. Patients who were immunocompromised or had previously received the vaccine were excluded.
Antibody titers for each of the three strains of influenza in the vaccine were measured at baseline and at 4–6 weeks after vaccination.
The researchers found no significant differences in antibody response rates between the two groups. “The results … were surprising,” Dr. Tosh said. “At baseline, the antibody levels in the two groups were the same. However, after vaccination, we were expecting to see a substantial difference between the two groups. But we didn't see any difference for any of the three vaccine components.”
Similarly, the researchers found no significant differences in immunogenicity between those who seroconverted and those who did not. “Injecting in the fat pad did increase reactogenicity,” Dr. Tosh noted. The patients who received deltoid fat pad injections reported significantly more redness and swelling, compared with those who had intramuscular injections (34% and 5%, respectively).
In a subset analysis, 66 patients underwent ultrasound before vaccination to assess fat pad thickness. “Based on the ultrasound, a 25-mm needle would have worked for 97% of the subjects,” Dr. Tosh noted. A 25-mm needle would have penetrated the muscle in all of the men and all but 3% of the women, but a 16-mm needle would have failed to penetrate the muscle in 26% of men and 51% of women, he said.
Based on these findings, the use of 25-mm needles, compared with 16-mm needles, increases the likelihood of injection into the muscle, which reduces the reactogenicity of flu shots in the elderly, Dr. Tosh said. Dr. Tosh stated that he had no financial conflicts to report.