evidence: The authors assessed the cancer-causing and infective potential of residual DNA from a cell-based live, attenuated influenza vaccine that is manufactured in Madin Darby Canine Kidney (MDCK) cells. They determined that 230 billion doses of vaccine would need to be administered before a potential cancer gene dosage equivalent would be reached, and 83 trillion doses would need to be administered to induce an infective event. [...] The WHO Expert Committee on Biological Standardization adopted requirements for the use of animal cells as substrates for the production of vaccines and other biologicals in 1996. In 2006, a WHO Study Group on Cell Substrates was formed to initiate revision of WHO requirements. In 2007, the Study Group agreed that data generated on the possibility of cancer or an infection caused by DNA in vaccines were important in defining potential risk for vaccine recipients. It was considered highly likely that reduction of DNA fragment size reduced the risk from DNA and increased the safety margin, as the smaller the DNA fragments, the lower the probability that intact oncogenes and other functional sequences would be present. Studies performed at the Center for Biological Evaluation and Research (CBER) suggest that DNA fragments smaller than 200 base pairs will give substantial safety margins for products that meet the 10 ng/dose limit. Therefore, the margin of safety for vaccines can be found in the high fragmentation, and therefore very small size, of DNA.