Weighing Obesity's Impact on Flu Shots

Researchers investigated whether a weakened vaccine is behind high rates of flu-related complications and hospitalization in obese patients.

Is a yearly flu shot enough to protect obese people from influenza?

That’s the question epidemiologists are asking in the wake of the 2009 H1N1 flu outbreak, which proved especially deadly for obese patients. A pair of studies released last week offer conflicting answers: one suggests obesity weakens the vaccine’s effectiveness, while the other shows it doesn’t.

The mixed results shouldn’t stop pharmacists from encouraging everyone to get an annual flu shot, however. Experts agree that even if it is weaker for patients carrying extra pounds, some protection against influenza is better than none.

Melinda Beck, PhD, senior author of the study linking obesity with impaired immune response to the flu vaccine, told the LA Times that despite her team’s discovery, “we absolutely recommend flu shots for everyone, including [patients who are] overweight and obese.” The Marshfield Clinic’s Laura Coleman, PhD, RD, arrived at the same conclusion, but from opposite findings.

Presenting at the Infectious Disease Society of America (IDSA) 49th annual meeting in Boston, Massachusetts, Dr. Coleman said that in her study, obese patients were just as likely as nonobese individuals to respond to the vaccine’s antigens. “We do not see evidence for vaccine recommendations to be different for obese and nonobese individuals,” she told Medscape.

Dueling studies agree: annual flu shots for all

Dr. Beck’s study involved 461 patients who received the 2009-2010 flu vaccine at a University of North Carolina clinic. By examining blood samples taken at 1 month and 12 months after vaccination, she and colleagues found that although all adults showed a similar initial response to the vaccine, its effect wore off more quickly in obese patients than in healthy-weight individuals.

At the 12-month mark, antibody levels had dropped 4-fold in roughly 50% of patients who were obese, compared with less than 25% of healthy-weight participants, according to results published in the October 25 online issue of the International Journal of Obesity. The plunge in antibodies puts obese patients at greater risk for contracting influenza, the researchers noted.

In another test, they measured the functional response of patients’ CD8+ T cells at 12 months. When exposed to influenza, these white blood cells limit the spread and severity of infection by releasing the infection-fighting protein interferon-gamma (IFN-γ). Whereas 75% of healthy-weight patients’ CD8+ T cells released IFN-γ in the 12-month test, just 25% of obese patients’ cells produced it.

Without these cells functioning at full capacity, patients who come into contact with the virus face a higher risk of complications and severe illness, said coauthor Patricia Sheridan, PhD. “If antibody titers are not maintained over time in the obese individuals and memory CD8+ T cell function is impaired, they may be at greater risk of becoming ill from influenza,” she said.

Dr. Coleman’s study looked at the flu vaccine’s effectiveness in a smaller cohort of 163 obese (BMI >30) and 250 nonobese (BMI <24.9) patients. She and colleagues tested participants’ antibody levels at baseline, 21 to 28 days after vaccination, and 6 months post-vaccination. They observed no significant difference in protection between obese and healthy-weight patients.

Taken together, the 2 studies suggest that antibody levels don’t fall dramatically in obese patients until 12 months after the vaccine is administered—a promising finding, according to Andrew Pavia, MD, chief of pediatric infectious disease at the University of Utah and chair of the Pandemic Influenza Task Force of the IDSA.

In an interview with Medscape, Dr. Pavia said the results of both studies are “reassuring that the vaccine works upfront,” adding that the dropoff in antibodies Dr. Beck observed at 12 months underscores the importance of yearly immunization. “This is a clear message that, at least in the obese patient, reimmunization is needed very year,” he said.

That recommendation holds true for all patients, regardless of body mass.

“Even though you had last year’s vaccine last October and it was exactly the same as this year’s you’re probably not well protected by this October,” said Dr. Pavia. “And by January, when flu is really raging, you probably won’t be protected at all.”

For other articles in this issue, see:

• Doctors Underestimate Language Gaps
• OTC Thyroid Drugs Pose Health Risks