There is limited prospective data on influenza infections in transplant recipients. However, influenza can be a significant cause of morbidity and mortality in some organ transplant populations. Reported attack rates have varied considerably and are likely due to differences in transplant populations, immunosuppression protocols, exposures, and type and virulence of circulating influenza viruses. Complications of influenza infection appear to be common in solid organ transplant (SOT) populations. There appears to be a relatively high rate of progression to viral pneumonia in some reports especially in lung transplant recipients. In one study of organ transplant recipients over a 10-year period, the rate of influenza infection ranged from 2.8 cases/1000 person years (liver transplant) to 41.8 cases/1000 person years (lung transplant). Complications including secondary bacterial pneumonia (17%) as well as extrapulmonary complications such as myocarditis, and myositis were observed. Influenza (and other respiratory viral infections) may lead to important immunological sequelae resulting in graft rejection and/or graft dysfunction. This may be secondary to activation of immunological mechanisms, including the upregulation of pro-inflammatory cytokines such as TNF-α, IL-6 and IL-8. Some studies of kidney and liver recipients have reported a high incidence of acute rejection following infection with influenza. However, while associations between influenza infection and rejection have been reported, a causal relationship has yet to be established.

Influenza vaccination has become the standard of care for several population groups. The Centers for Disease Control and Prevention (CDC) and the Canadian National Advisory Committee on Immunization (NACI) currently recommends this vaccine for children \> 6 months, healthy adults, the elderly and all immunocompromised patients such as those with organ transplants (www.cdc.gov). The currently available influenza vaccine is a subunit vaccine containing the purified surface glycoproteins of the virus, hemagglutinin and neuraminidase. The vaccine contains 15g antigen from each of 2 circulating subtypes of influenza A and 15 g of an influenza B subtype (www.who.int). The vaccine is administered by the intramuscular route, generally in the deltoid muscle in adults. The standard dose is 0.5 mL.

Annual influenza vaccination is currently considered the main strategy to prevent influenza infection for all organ transplant recipients. However, a large number of studies have shown that the immunogenic response to this vaccine is suboptimal and ranges from 15-70%. This number varies depending on the organ transplanted and the immunosuppressive regimen used. Indeed, some studies in kidney transplant recipients, especially in the pediatric population, have shown similar responses to the influenza vaccine than in healthy controls.

Monitoring of immune response to influenza vaccine. Laboratory monitoring of response to influenza vaccine can be done by hemagglutination inhibition assay. Although laboratory serology is a surrogate marker for protection, it is widely used. This is primarily due to the large numbers of patients that would be needed to show vaccine efficacy if documented influenza infection were used as the outcome. The HIA is also used by the World Health Organization to validate annual vaccines. However, it is not known if the criteria used for the general population may be applied in immunocompromised patients. In fact, in the elderly it has been clearly proved that the standard cutoff for antibody levels of 40 units did not protect against influenza. In a study with 397 elderly vaccinated against influenza, 60% of persons who developed influenza had an antibody level of 40 units. To our knowledge, the absolute titer of antibody required to confer significant protection from illness is unknown in transplant patients.

Cell-mediated immunity after influenza vaccination. Recently, specific cellular responses have also been implicated in protection against influenza, independently of the humoral response. Cellular mediated immunity has been evaluated measuring IL-2 and IFN-(for T-helper type 1 response), IL-10 (for T-helper type 2 response), and granzyme B expression (which correlates with lytic activation). The cell-mediated immune response has been mainly used to evaluate the response to influenza vaccine in the elderly. In a study with 90 persons \>60 year-old, the IFN-/IL-10 ratio and granzyme B levels were significantly higher in persons who did not develop influenza during the follow-up, compared to persons who developed influenza. There was no correlation between the antibody levels and the risk of developing influenza infection. In a previous study, granzyme B levels were found to be higher in young people than in old people. Taken together, these data suggest that cell-mediated immune response may correlate better with the risk of influenza after vaccination.

The cell-mediated immune response after influenza vaccine has been only assessed in one study involving solid organ recipients. Mazzone et al. evaluated in 43 lung transplant recipients and in 21 healthy controls. IL-2, IL-10, IFN-, and granzyme B levels did not increase from pre- to post-vaccination in the lung transplant group. Both pre- and post-cytokine levels were lower in the transplant group compared to the control group.

The study we propose is a prospective cohort trial designed to assess the immunogenicity after a standard influenza vaccination measuring the cellular immunity in a cohort of kidney transplant recipients, and correlate it with the standard humoral immunity.