V451 was a COVID-19 vaccine candidate developed by the University of Queensland and the Australian pharmaceutical company CSL Limited. The vaccine candidate used the University of Queensland's molecular clamp technology^[1] and the MF59 adjuvant.^[2]

V451 is a protein subunit vaccine. As part of the vaccine's design, researchers added "a fragment of one protein found on the HIV virus"^[3] as a "ground-breaking molecular clamp technology".^[4]

The development of the vaccine was cancelled on 11 December 2020 during its Phase I trial, after a number of trial participants were found to give false positive test results for HIV antibodies when they did not in fact have HIV.^[5][6] This was due to the HIV virus fragment used as a molecular clamp leading to "a partial antibody response" to HIV. This is an undesirable outcome as it will interfere with future HIV screening tests for affected participants.^[7]

Nine days prior to the termination, on 2 December, the first emergency use authorisation had been granted to a COVID-19 vaccine; the Pfizer–BioNTech COVID-19 vaccine in the United Kingdom.^[8] Following the termination of V451, vaccine production capacity by CSL Limited was diverted to the Oxford–AstraZeneca COVID-19 vaccine.^[9]