The live attenuated rotavirus vaccine RotaShield (Wyeth-Ayerst) contained three rotavirus reassortants, with different genes encoding specific serotypes (VP4 or VP7) evoking virus-specific Ab, along with genes of Rhesus macaque-passaged rotavirus that attenuated virulence [19]. After approval, 76 cases of intussusception, in which one segment of the bowel enfolds within another segment, causing obstruction, were reported to the VAERS surveillance system. 70% of intussusception cases occurred after the first dose of vaccine. Due to this surveillance, the CDC recommended the suspension of the rotavirus vaccine until further studies could be performed. One study found one case in every 5000 to 9500 vaccinated infants, with the highest risk after the first dose. Due to the possible association with intussusception, Rotashield was withdrawn from the market in 1999.