Researchers from Australia determined that pediatric liver transplant patients who received a single-dose of the H1N1 vaccine were not adequately protected against the virus compared to healthy children. This study appearing in the August issue of Liver Transplantation, a journal of the American Association for the Study of Liver Diseases, found that a second vaccination was needed to elicit an effective immune response in children 10 and older who had recently received a liver transplant.

In April 2009 a novel strain of H1N1 influenza virus (swine flu) was identified and its rapid spread led to the World Health Organization's (WHO) declaration of a pandemic in June 2009. Epidemiologists quickly determined that children and immunosuppressed patients were particularly susceptible to the virus, with studies finding higher rates of morbidity and mortality compared to seasonal flu. Following positive trial, a single dose of H1N1/09 vaccine was recommended for healthy children (10 and older) and adults. However, medical evidence has shown immunocompromised patients to have lower immune response rates to routine vaccinations.

Given the lack of data pertaining to immune response in liver transplant patients, a research team led by Dr. Wolfram Haller with the Murdoch Childrens Research Institute and Royal Children's Hospital in Melbourne, Australia, recruited 21 pediatric patients (aged 10 and older) who underwent liver transplantation at the facility. The observational study was conducted from November 2009 through October 2010. Participants were given a single dose (15μg) of the H1N1/09 vaccine, with a second dose administered to those children who did not display adequate concentrations of the vaccine to protect against the virus. Blood samples were taken before and >6 weeks following vaccination to measure antibody levels.

Results showed that 62% of subjects produced an adequate increase in antibody titres against the H1N1 virus after the first vaccine dose with the seroconversion rate increasing to 90% with a second follow-up dose. Roughly, 33% of patients had antibodies against the virus (seropositive) at baseline, that is before the vaccination. Out of these, 86% developed antibodies after the initial vaccination. Of the patients without H1N1 antibodies (seronegative) at baseline only half had an immune response after the first vaccination. Researchers determined that increasing time since transplant correlated with successful immune response.

"Our research provides the first evidence of immunogenicity of the H1N1/09 vaccine in pediatric liver transplant patients," said Dr. Haller. Study results showed that the recommended single dose of the H1N1/09 vaccine for children 10 years of age and older did not provide protection for similar aged children who underwent liver transplantation. "Immune response to the H1N1 vaccine in a non-pandemic situation should be investigated and further research is urgently needed to develop new vaccine strategies that protect high-risk patient groups from the H1N1 influenza," Haller concluded.

The Centers for Disease Control and Prevention (CDC) estimate that 43 million to 89 million Americans were infected with H1N1 between April 2009 and April 10, 2010, with approximately 14 million to 28 million of those cases in children 17 years of age and younger. On August 10, 2010 the WHO International Health Regulations Emergency Committee officially declared an end to the 2009 H1N1 pandemic.

Full citation: "Immune Response to Pandemic Influenza A (pH1N1/09) Vaccination in Pediatric Liver Transplant Recipients." Wolfram Haller, Jim Buttery, Karen Laurie, Kathe Beyerle, Winita Hardikar and George Alex. Liver Transplantation; Published Online: February 23, 2011 (DOI: 10.1002/lt.22283) Print Issue Date: August 2011.