Social Isolation and Aging health in Schizophrenia spectrum disorders (SIAS): study protocol for a multinational longitudinal study - Koster, Merel; van der Pluijm, Marieke; Baldwin, Helen; Greenburgh, Anna; King, Diana; Arnold, Cole; Lopez-Morinigo, Javier-David; Underwood, Slayton; Larrauri, Carlos A; Racher, Matthew; Glahn, David C; Harvey, Philip D; et al. (BMC Psychiatry, 2026)