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A marker vaccine is a vaccine which allows for the differentiation between infected and vaccinated subjects. Most vaccines are inactivated or attenuated forms of the targeted virus, therefore it is impossible to differentiate by laboratory tests which subjects are infected or only vaccinated. Marker vaccines, being a form of the virus without a particular gene coding for a non protective antigen so, do not produce a key antibody, while still protecting against the disease. Laboratory tests can test for the absence of the antibody, confirming whether the individual has vaccinated or is genuinely infected and needs treatment.