AT THIS POINT in time, a discussion of the problems encountered with inactivated poliomyelitis vaccine, Salk vaccine, is relatively simple. Once the difficulties which were encountered in 1955 had been surmounted, there were no further problems with safety in the field. The revised safety testing procedures introduced in 1955 have been consistently effective in providing a safe product. Problems do occur on occasion in the course of manufacture which emphasize the validity of the revised test procedures. Some relatively minor problems have arisen from time to time, such as the presence of penicillin in the vaccine as originally produced, but it is now possible readily to obtain vaccine prepared with other antibiotics.
The introduction of more stringent manufacturing and testing procedures in 1955 was associated with a fall in average potency. Although remaining within the required potency limits, this was nevertheless undesirable in a vaccine which is not 100% effective
Murray R. Standardization, Licensing, and Availability of Live Poliovirus Vaccine. JAMA. 1961;175(10):843-846. doi:10.1001/jama.1961.03040100007003