Spanish researchers have discovered that giving two flu shots, five weeks apart, is a more effective method of vaccinating solid organ transplant recipients against the virus.

Solid organ transplant patients are more susceptible to influenza and have decreased mortality when they do acquire the virus. That's why all are encouraged to get the inactivated trivalent influenza vaccination each year.

"However, the immunological response to the influenza vaccine is heterogeneous in this population, with rates of seroprotection that range from 15 percent to 90 percent, generally lower than those of healthy individuals," said study author Dr. Elisa Cordero of the Institute of Biomedicine of Seville, Spain.

In general, transplant patients have lower serological responses to vaccines than the general population. The study aimed to assess serological markers of patients who received one standard flu shot and of those who received the vaccination followed by a booster several weeks later.

Researchers studied 500 solid organ recipients at 12 Spanish hospitals. Subjects were at least 16 years old and at least 30 days postransplant. The control group received only one flu shot, while the experimental group received two, with the booster administered five weeks later.

At 10 weeks post-vaccine, there was no noticeable difference in seroconversion rates among the control group. However, more flu antibodies developed among the group that received a booster shot. For H1N1, the seroconverstion rate was 54 percent. For H3N2 , the rate was 48 percent. For influenza B, the rate was 91 percent.

The booster group also had higher rates of seroprotection than the control group. For H1N1, the rate was 54 percent compared to 43 percent in the control group. For H2N3, it was 57 percent compared to 46 percent. For influenza B, it was 83 percent compared to 72 percent.

"This is the first randomized controlled trial that reports a vaccination strategy that improves immunological effectiveness of influenza vaccination in solid organ transplant recipients," the researchers wrote. "The safety and relevance of these results, with a number needed to treat for seroconversion and seroprotection below 10 patients, generate good-quality evidence to support the seasonal use of two doses of the influenza vaccine in solid organ transplant recipients beyond the first month after transplantation."

Patients who have received solid organ transplants have compromised immune systems and are more likely to experience complications from the flu virus. According to the World Health Organization, vaccination is the best defense against the flu for this patient population. Anti-viral medications can be effective, but dosage strategies are not well defined.

Even though vaccine coverage increased significantly in the 2010-2011 flu season, deaths and ICU hospitalizations remained high among organ transplant recipients at 7.2 percent and 16.2 percent, respectively.

"These findings will contribute to development and implementation of strategies to improve influenza vaccination response, especially early after transplantation, where the risk of severe complications due to influenza infection is higher," the authors wrote.