For more than a century, the idea of a vaccine against cancer has occupied an uncertain space between scientific ambition and medical fantasy. Cancer is not an invading virus that can simply be identified and neutralised. It arises from the body’s own cells, exploiting the biological mechanisms designed to sustain life. Teaching the immune system to distinguish malignant cells from healthy ones has therefore been one of medicine’s most difficult challenges. That challenge may now be entering a new phase.

Recent clinical results involving personalised mRNA vaccines against melanoma offer the strongest indication yet that the immune system can be trained to recognise the unique genetic signature of a tumour and prevent the disease from returning. The significance lies less in the prospect of a single vaccine that eradicates cancer than in a fundamental shift in how the disease may be fought. For decades, cancer treatment has relied on standard categories: surgery, chemotherapy, radiation and, more recently, immunotherapy and targeted drugs. Even the most sophisticated treatments have generally been designed for categories of patients sharing a diagnosis or a........

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