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.