Shafer is pregnant with twins, according to the orchestra, and has been advised not to perform for health reasons. She was scheduled to sing Berg's Suite From Lulu on April 19 at Carnegie Hall, the New Jersey Performing Arts Center on April 21, and the Kennedy Center on April 22.
In place of the Berg work, the SFS will perform Ravel's Piano Concerto in D major for the Left Hand, with Jean-Yves Thibaudet as soloist.
The tour will also include works by Debussy, Mahler, Wagner, Webern, and Ives. Soprano Lorraine Hunt Lieberson, who will record Mahler's R‹ckert Lieder with the orchestra in San Francisco before the tour, will perform the work at a second Carnegie Hall concert on April 20.
Music director Michael Tilson Thomas is the conductor.