Inclusion Criteria:  Subject participated in the VLA1553-301 clinical study; Subject has an understanding of the study and its procedures, agrees to its provisions, and voluntarily gives written informed consent prior to any study-related procedures; Subject had immunogenicity blood samples taken at baseline (Visit 1) and either Day 29 (Visit 3), Day 85 (Visit 4) or Day 180 (Visit 5) in Study VLA1553-301 and was negative for neutralizing antibodies at baseline  Exclusion Criteria:  Subject presents with clinical conditions representing a contraindication to blood draws; Subject has donated blood or use of blood products prior 30 days of immunogenicity sampling; Subject has received an active drug with potential immunosuppressive action or investigational drug or device within a period of 30 days prior a study visit; Subject has a known or suspected problem with alcohol or drug abuse as determined by the Investigator; Subject has any condition that, in the opinion of the Investigator, may compromise the subject's well-being, might interfere with evaluation of study endpoints, or would limit the subject's ability to complete the study; Subject is committed to an institution (by virtue of an order issued either by the judicial or the administrative authorities); Subject is a member of the team conducting the study or in a dependent relationship with one of the study team members. Dependent relationships include close relatives (i.e., children, partner/spouse, siblings, parents) as well as employees of the Investigator or site personnel conducting the study.