Included patients were from the region of Emilia-Romagna in north-east Italy suspected to be infected with CHIKV since they showed symptoms such as myalgia, severe back and joint pain, headache, and skin rash. Collaboration with the regional microbiology reference laboratory of Bologna University and the Department of Infectious and Parasitic Diseases of the National Institute of Health in Rome was initiated and identified the patients as having CHIKV. The clinical criteria was described as acute onset of fever (.38.5uC) and severe arthralgia not explained by other medical conditions. CHIKV infection was confirmed by one or more of the acute phase tests: virus isolation, reverse transcriptase-PCR (RT-PCR) positive for CHIKV nsp1 gene, seroconversion to virus-specific serum antibodies collected at least 1 to 3 weeks apart, or presence of virus-specific IgM antibodies in a single serum sample collected [15] . Acute CHIKV patient samples were determined to be in the viral stage if the sample was PCR positive for CHIKV (7 patients), in the IgM antibody initiation stage if the sample was PCR negative, IgM positive, IgG negative (6 patients) or in the seroconversion stage (22 patients) if the sample was PCR negative, IgM positive and IgG positive. The samples were considered to be high IgG if the IgG level was greater than 6400 (6 months) (31 patients high out of 50) or greater than 3200 (12 months) (20 patients out of 50). IgG levels below or equal to these thresholds were considered low IgG. CHIKV patients were considered to be non-symptomatic (15 patients out of 50 at the 6-month follow-up; 34 patients out of 50 at the 12 month follow-up), have mild symptoms (21 patients out of 50 a the 6 month follow-up; 14 patients out of 50 at the 12 month followup), or have severe symptoms (14 patients out of 50 at the 6 month follow-up; 2 patients out of 50 at the 12 month follow-up) based on their responses to a questionnaire at the time of sampling which was based on: articular pain, muscle pain, mono-arthritis, oligoarthritis, symmetric polyarthritis, asymmetric polyarthritis, tenosynovitis, arthralgia and fibromyalgia. Control samples were collected from 10 healthy volunteers screened for symptoms of viral infection.