"Finally, foetuses are thought to be the “cleanest” possible source of cells, since they are less likely to have picked up any viruses from the outside world which might contaminate vaccines or confound the results of experiments. Back in 2017, Hayflick asked Olshansky to quantify exactly how many lives the cells had spared until that point. By comparing the global prevalence of certain infectious diseases in the 1960s, when the cell line was discovered, with the prevalence of infectious diseases then, he calculated that vaccines made with WI-38 may have prevented around 4.5 billion infections. In total, the cells are likely to have spared 10.3 million lives."