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How can a modified vaccinia virus carry genes from another pathogen?

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Infographic showing estimated lives saved by childhood vaccinations worldwide from 1974 to 2024, with syringe-shaped bars for each disease. Measles tops the list at 93.71 million lives saved, followed by tetanus, whooping cough, and others, for a total of 153.84 million lives saved. — vaccine Vaccines and the estimates of lives saved over the period 1974–2024.

A modified vaccinia virus can carry genes from another pathogen through the use of recombinant DNA technology. In this process, researchers identify the specific genes of a target pathogen that encode for antigens—the proteins that stimulate an immune response.1

This genetic material is then inserted into the attenuated form of the large vaccinia virus. The vaccinia virus acts as a carrier, allowing the foreign genes to "piggyback" within its structure. When this altered virus is injected into an individual, it does not cause the original disease but instead prompts the body to produce antibodies against the foreign proteins, thereby conferring immunity. This approach is particularly valuable for developing vaccines against viruses that are inherently dangerous or difficult to grow in a laboratory setting.1