A homologous booster shot involves the administration of the same vaccine as previously administered, while a heterologous booster shot involves the administration of a different vaccine.^{[citation needed]}

"Heterologous prime-boost immunization is administration of two different vectors or delivery systems expressing the same or overlapping antigenic inserts."^[1]

"An effective vaccine usually requires more than one time immunization in the form of prime-boost. Traditionally the same vaccines are given multiple times as homologous boosts. New findings suggested that prime-boost can be done with different types of vaccines containing the same antigens. In many cases such heterologous prime-boost can be more immunogenic than homologous prime-boost."^[2]

• "Interim statement on booster doses for COVID-19 vaccination". World Health Organization.
• Liu, Xinxue; Shaw, Robert H.; Stuart, Arabella S. V.; Greenland, Melanie; Aley, Parvinder K.; Andrews, Nick J.; Cameron, J. Claire; Charlton, Sue; Clutterbuck, Elizabeth A.; Collins, Andrea M.; Dinesh, Tanya; England, Anna; Faust, Saul N.; Ferreira, Daniela M.; Finn, Adam; Green, Christopher A.; Hallis, Bassam; Heath, Paul T.; Hill, Helen; Lambe, Teresa; Lazarus, Rajeka; Libri, Vincenzo; Long, Fei; Mujadidi, Yama F.; Plested, Emma L.; Provstgaard-Morys, Samuel; Ramasamy, Maheshi N.; Ramsay, Mary; Read, Robert C.; Robinson, Hannah; Singh, Nisha; Turner, David P. J.; Turner, Paul J.; Walker, Laura L.; White, Rachel; Nguyen-Van-Tam, Jonathan S.; Snape, Matthew D.; Munro, Alasdair P. S.; Bartholomew, Jazz; Presland, Laura; Horswill, Sarah; Warren, Sarah; Varkonyi-Clifford, Sophie; Saich, Stephen; Adams, Kirsty; Ricamara, Marivic; Turner, Nicola; Ting, Nicole Y. Yee; Whittley, Sarah; Rampling, Tommy; Desai, Amisha; Brown, Claire H.; Qureshi, Ehsaan; Gokani, Karishma; Naker, Kush; Wright, Johanna K. Kellett; Williams, Rachel L.; Riaz, Tawassal; Penciu, Florentina D.; Maso, Claudio Di; Howe, Elizabeth G.; Vichos, Iason; Farooq, Mujtaba Ghulam; Noristani, Rabiullah; Yao, Xin L.; Oldfield, Neil J.; Hammersley, Daniel; Belton, Sue; Royal, Simon; Ramos, Alberto San Francisco; Hultin, Cecilia; Galiza, Eva P.; Shiham, Farah; Solórzano, Carla; Sainsbury, Hannah; Davies, Kelly; Ambrose, Pauline; Hitchins, Lisa; Baker, Natalie; Leung, Stephanie; Fothergill, Ross; Godwin, Kerry; Buttigieg, Karen; Shaik, Imam; Brown, Phill; Knight, Chanice; Lall, Paminder; Allen, Lauren (4 September 2021). "Safety and immunogenicity of heterologous versus homologous prime-boost schedules with an adenoviral vectored and mRNA COVID-19 vaccine (Com-COV): a single-blind, randomised, non-inferiority trial". The Lancet. 398 (10303): 856–869. doi:10.1016/S0140-6736(21)01694-9. ISSN 0140-6736. PMC 8346248. PMID 34370971.
• Mukherjee, Debabrata. "Heterologous vs. Homologous Prime-Boost Schedules for COVID-19 Vaccines". American College of Cardiology. The authors concluded that the SARS-CoV-2 anti-spike IgG concentrations of both heterologous schedules were higher than that of a licensed vaccine schedule (ChAd/ChAd) with proven efficacy against coronavirus disease 2019 (COVID-19) disease and hospitalization