In 1955, a batch of Salk's vaccine made by Cutter Laboratories in California was inadequately inactivated and, as a result, 11 children died. After this incident, production methods were changed and no further incidents were reported. Sabin became highly critical of O'Connor and the March of Dimes, who he believed were biased towards Salk's vaccine and made statements inconsistent with the scientific research. In the meantime, trials of the vaccine based on Sabin's version were carried out in the Soviet Union with important contributions made by Mikhail Chumakov.