Viral vector vaccine explained

A viral vector vaccine is a vaccine that uses a viral vector to deliver genetic material (DNA) that can be transcribed by the recipient's host cells as mRNA coding for a desired protein, or antigen, to elicit an immune response.[1], six viral vector vaccines, four COVID-19 vaccines and two Ebola vaccines, have been authorized for use in humans.[2]

Understanding viral vectors

History

The first viral vector was introduced in 1972 through genetic engineering of the SV40 virus.[3] [4] A recombinant viral vector was first used when a hepatitis B surface antigen gene was inserted into a vaccinia virus.[5] [6] Subsequently, other viruses including adenovirus, adeno-associated virus, retrovirus, cytomegalovirus, sendai virus, and lentiviruses have been designed into vaccine vectors.[7] Vaccinia virus and adenovirus are the most commonly used viral vectors because of robust immune response it induces.[8]

The incorporation of several viruses in vaccination schemes has been investigated since the vaccinia virus was created in 1984 as a vaccine vector.[9] Human clinical trials were conducted for viral vector vaccines against several infectious diseases including Zika virus, influenza viruses, respiratory syncytial virus, HIV, and malaria, before the vaccines that target SARS-CoV-2, which causes COVID-19.

Two Ebola vaccines that used viral vector technology were used to combat Ebola outbreaks in West Africa (2013–2016), and in the Democratic Republic of the Congo (2018–2020). The rVSV-ZEBOV vaccine was approved for medical use in the European Union in November 2019,[10] and in December 2019 for the United States.[11] [12] Zabdeno/Mvabea was approved for medical use in the European Union in July 2020.[13] [14]

Technology

Viral vector vaccines enable antigen expression within cells and induce a robust cytotoxic T cell response, unlike subunit vaccines which only confer humoral immunity.[15] In order to transfer a nucleic acid coding for a specific protein to a cell, the vaccines employ a variant of a virus as its vector. This process helps to create immunity against the disease, which helps to protect people from contracting the infection. Viral vector vaccines do not cause infection with either the virus used as the vector or the source of the antigen.[16] The genetic material it delivers does not integrate into a person's genome.[17]

The majority of viral vectors lack the required genes, making them unable to replicate.[7] In order to be widely accepted and approved for medical use, the development of viral vector vaccines requires a high biological safety level. Consequently, non or low-pathogenic viruses are often selected.[18]

Advantages

Viral vector vaccines have benefits over other forms of vaccinations depending on the virus which they produced thanks to their qualities of immunogenicity, immunogenic stability, and safety. Specific immunogenicity properties include highly efficient gene transduction, highly specific delivery of genes to target cells, and the ability to induce potent immune responses. The immunogenicity is further enhanced through intrinsic vector motifs that stimulate the innate immunity pathways,[19] [20] [21] so the use of an adjuvant is unnecessary. Replicating vectors imitate natural infection, which stimulates the release of cytokines and co-stimulatory molecules that produce a strong adjuvant effect.[22] The induction of innate immunity pathways is crucial to stimulating downstream pathways and adaptive immunity responses.

Additionally, viral vectors can be produced in high quantities at relatively low costs, which enables use in low-income countries.[23]

Viral vectors

Adenovirus

Adenovirus vectors have the advantage of high transduction efficiency, transgene expression, and broad viral tropism, and can infect both dividing and non-dividing cells. A disadvantage is that many people have preexisting immunity to adenoviruses from previous exposure.[24] [25] [26] The seroprevalence against Ad5 in the US population is as high as 40%–45%.[27] Most Adenovirus vectors are replication-defective because of the deletion of the E1A and E1B viral gene region. Currently, overcoming the effects of adenovirus-specific neutralizing antibodies is being explored by vaccinologists.[28] These studies include numerous strategies such as designing alternative Adenovirus serotypes, diversifying routes of immunization, and using prime-boost procedures.[29] Human adenovirus serotype 5 is often used because it can be easily produced in high titers.[7]

As of April 2021, four adenovirus vector vaccines for COVID-19 have been authorized in at least one country:

Zabdeno, the first dose of the Zabdeno/Mvabea Ebola vaccine, is derived from human adenovirus serotype 26, expressing the glycoprotein of the Ebola virus Mayinga variant. Both doses are non-replicating vectors and carry the genetic code of several Ebola virus proteins.[35]

Safety

With the increasing prevalence of adenoviral vaccines, two vaccines, Ad26.COV2.S and ChadOx1-nCoV-19, have been linked to the rare clotting disorder, thrombosis with thrombocytopenia syndrome (TTS).

Vaccinia virus

The vaccinia virus is part of the poxvirus family. It is a large, complex, and enveloped virus that was previously used for the smallpox vaccine. The vaccinia virus's large size allows for a high potential for foreign gene insertion. Several vaccinia virus strains have been developed including replication-competent and replication-deficient strains.

Modified vaccinia Ankara

Modified vaccinia ankara (MVA) is a replication-deficient strain that has been safely used for a smallpox vaccine. The Ebola vaccine regimen approved by the European Commission was developed by Janssen Pharmaceutials and Bavarian Nordic, and utilizes MVA technology in its second vaccine dose of Mvabea (MVA-BN-Filo).[36]

Vesicular stomatitis virus

Vesicular stomatitis virus (VSV) was introduced as a vaccine vector in the late 1990s.[37] In most VSV vaccine vectors, attenuation provides safety against its virulence.[38] VSV is an RNA virus and is part of the Rhabdoviridae family. The VSV genome encodes for nucleocapsid, phosphoprotein, matrix, glycoprotein, and an RNA-dependent RNA polymerase proteins.

The rVSV-ZEBOV vaccine, known as Ervebo, was approved as a prophylactic Ebola vaccine for medical use by the FDA in 2019.[39] The vaccine is a recombinant, replication-competent vaccine[40] consisting of genetically engineered vesicular stomatitis virus.[41] The gene for the natural VSV envelope glycoprotein is replaced with that from the Kikwit 1995 Zaire strain Ebola virus.[42] [43] [44]

Routes of administration

Intramuscular injection is the commonly used route for vaccine administration.[4] The introduction of alternate routes for immunization of viral vector vaccines can induce mucosal immunology at the site of administration, thereby limiting respiratory or gastrointestinal infections.[45] [46] Also, studies are being done on how these diverse routes can be used to overcome the effects of specific neutralizing antibodies limiting the use of these vaccines. These routes include intranasal,[47] [48] oral, intradermal, and aerosol vaccination.[49] [50]

Further reading

Notes and References

  1. Sasso E, D'Alise AM, Zambrano N, Scarselli E, Folgori A, Nicosia A . New viral vectors for infectious diseases and cancer . Seminars in Immunology . 50 . 101430 . August 2020 . 33262065 . 10.1016/j.smim.2020.101430 . 227251541 . free .
  2. Wang F, Qin Z, Lu H, He S, Luo J, Jin C, Song X . Clinical translation of gene medicine . The Journal of Gene Medicine . 21 . 7 . e3108 . July 2019 . 31246328 . 10.1002/jgm.3108 . 195695440 . free .
  3. Jackson DA, Symons RH, Berg P . October 1972 . Biochemical method for inserting new genetic information into DNA of Simian Virus 40: circular SV40 DNA molecules containing lambda phage genes and the galactose operon of Escherichia coli . Proceedings of the National Academy of Sciences of the United States of America . 69 . 10 . 2904–2909 . 1972PNAS...69.2904J . 10.1073/pnas.69.10.2904 . 389671 . 4342968 . free.
  4. Travieso T, Li J, Mahesh S, Mello JD, Blasi M . July 2022 . The use of viral vectors in vaccine development . npj Vaccines . 7 . 1 . 75 . 10.1038/s41541-022-00503-y . 9253346 . 35787629.
  5. McCann . Naina . O'Connor . Daniel . Lambe . Teresa . Pollard . Andrew J . 2022-08-01 . Viral vector vaccines . Current Opinion in Immunology . en . 77 . 102210 . 10.1016/j.coi.2022.102210 . 35643023 . 0952-7915 . 9612401 .
  6. Smith . Geoffrey L. . Mackett . Michael . Moss . Bernard . 1983. Infectious vaccinia virus recombinants that express hepatitis B virus surface antigen . Nature . en . 302 . 5908 . 490–495 . 10.1038/302490a0 . 6835382 . 1983Natur.302..490S . 4266888 . 1476-4687 . 2023-02-16 . 2023-02-16 . https://web.archive.org/web/20230216145628/https://www.nature.com/articles/302490a0 . live .
  7. Ura T, Okuda K, Shimada M . July 2014 . Developments in Viral Vector-Based Vaccines . Vaccines . 2 . 3 . 624–641 . 10.3390/vaccines2030624 . 4494222 . 26344749 . free.
  8. Mackett M, Smith GL, Moss B . December 1982 . Vaccinia virus: a selectable eukaryotic cloning and expression vector . Proceedings of the National Academy of Sciences of the United States of America . 79 . 23 . 7415–7419 . 1982PNAS...79.7415M . 10.1073/pnas.79.23.7415 . 347350 . 6296831 . free.
  9. Humphreys IR, Sebastian S . January 2018 . Novel viral vectors in infectious diseases . Immunology . 153 . 1 . 1–9 . 10.1111/imm.12829 . 5721250 . 28869761.
  10. Web site: 12 December 2019 . Ervebo EPAR . 1 July 2020 . European Medicines Agency (EMA) . 8 March 2021 . https://web.archive.org/web/20210308035916/https://www.ema.europa.eu/en/medicines/human/EPAR/ervebo . live . Text was copied from this source which is copyright European Medicines Agency. Reproduction is authorized provided the source is acknowledged.
  11. Web site: 19 December 2019 . First FDA-approved vaccine for the prevention of Ebola virus disease, marking a critical milestone in public health preparedness and response . live . https://web.archive.org/web/20191220052152/https://www.fda.gov/news-events/press-announcements/first-fda-approved-vaccine-prevention-ebola-virus-disease-marking-critical-milestone-public-health . 20 December 2019 . 19 December 2019 . U.S. Food and Drug Administration (FDA).
  12. Web site: 19 December 2019 . Ervebo . 1 July 2020 . U.S. Food and Drug Administration (FDA) . 14 February 2021 . https://web.archive.org/web/20210214105332/https://www.fda.gov/vaccines-blood-biologics/ervebo . live .
  13. Web site: 26 May 2020 . Zabdeno EPAR . 23 July 2020 . European Medicines Agency (EMA) . 23 July 2020 . https://web.archive.org/web/20200723174605/https://www.ema.europa.eu/en/medicines/human/EPAR/zabdeno . live .
  14. Web site: 26 May 2020 . Mvabea EPAR . 23 July 2020 . European Medicines Agency (EMA) . 23 July 2020 . https://web.archive.org/web/20200723174621/https://www.ema.europa.eu/en/medicines/human/EPAR/mvabea . live .
  15. 6 . Li JX, Hou LH, Meng FY, Wu SP, Hu YM, Liang Q, Chu K, Zhang Z, Xu JJ, Tang R, Wang WJ, Liu P, Hu JL, Luo L, Jiang R, Zhu FC, Chen W . March 2017 . Immunity duration of a recombinant adenovirus type-5 vector-based Ebola vaccine and a homologous prime-boost immunisation in healthy adults in China: final report of a randomised, double-blind, placebo-controlled, phase 1 trial . The Lancet. Global Health . 5 . 3 . e324–e334 . 10.1016/S2214-109X(16)30367-9 . 28017642. free .
  16. Deng . Shaofeng . Liang . Hui . Chen . Pin . Li . Yuwan . Li . Zhaoyao . Fan . Shuangqi . Wu . Keke . Li . Xiaowen . Chen . Wenxian . Qin . Yuwei . Yi . Lin . Chen . Jinding . 2022-07-18 . Viral Vector Vaccine Development and Application during the COVID-19 Pandemic . Microorganisms . en . 10 . 7 . 1450 . 10.3390/microorganisms10071450 . 35889169 . 9317404 . 2076-2607. free .
  17. Web site: 25 February 2021. Understanding and Explaining Viral Vector COVID-19 Vaccines. 2 April 2021. U.S. Centers for Disease Control and Prevention. 2 February 2021. https://web.archive.org/web/20210202160930/https://www.cdc.gov/vaccines/covid-19/hcp/viral-vector-vaccine-basics.html. live.
  18. Ura . Takehiro . Okuda . Kenji . Shimada . Masaru . 2014-07-29 . Developments in Viral Vector-Based Vaccines . Vaccines . en . 2 . 3 . 624–641 . 10.3390/vaccines2030624 . 26344749 . 4494222 . 2076-393X. free .
  19. Dempsey . Alan . Bowie . Andrew G. . May 2015 . Innate immune recognition of DNA: A recent history . Virology . en . 479-480 . 146–152 . 10.1016/j.virol.2015.03.013. 25816762 . 4424081 .
  20. Kell . Alison M. . Gale . Michael . May 2015 . RIG-I in RNA virus recognition . Virology . en . 479-480 . 110–121 . 10.1016/j.virol.2015.02.017. 25749629 . 4424084 .
  21. Akira . Shizuo . Uematsu . Satoshi . Takeuchi . Osamu . February 2006 . Pathogen Recognition and Innate Immunity . Cell . en . 124 . 4 . 783–801 . 10.1016/j.cell.2006.02.015. 16497588 . 14357403 . free .
  22. Robert-Guroff . Marjorie . December 2007 . Replicating and non-replicating viral vectors for vaccine development . Current Opinion in Biotechnology . en . 18 . 6 . 546–556 . 10.1016/j.copbio.2007.10.010 . 2245896 . 18063357.
  23. Schrauf . Sabrina . Tschismarov . Roland . Tauber . Erich . Ramsauer . Katrin . Current Efforts in the Development of Vaccines for the Prevention of Zika and Chikungunya Virus Infections . Frontiers in Immunology . 2020 . 11 . 592 . 10.3389/fimmu.2020.00592 . 32373111 . 7179680 . 1664-3224. free .
  24. Fausther-Bovendo H, Kobinger GP . Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what's important? . Human Vaccines & Immunotherapeutics . 10 . 10 . 2875–2884 . 2014-10-03 . 25483662 . 10.4161/hv.29594 . 5443060 .
  25. Barouch DH, Kik SV, Weverling GJ, Dilan R, King SL, Maxfield LF, Clark S, Ng'ang'a D, Brandariz KL, Abbink P, Sinangil F, de Bruyn G, Gray GE, Roux S, Bekker LG, Dilraj A, Kibuuka H, Robb ML, Michael NL, Anzala O, Amornkul PN, Gilmour J, Hural J, Buchbinder SP, Seaman MS, Dolin R, Baden LR, Carville A, Mansfield KG, Pau MG, Goudsmit J . 6 . International seroepidemiology of adenovirus serotypes 5, 26, 35, and 48 in pediatric and adult populations . Vaccine . 29 . 32 . 5203–5209 . July 2011 . 21619905 . 10.1016/j.vaccine.2011.05.025 . 3138857 .
  26. 2017-08-08 . Virally vectored vaccine delivery: medical needs, mechanisms, advantages and challenges . Swiss Medical Weekly . en . 147 . 3132 . 10.4414/smw.2017.14465 . 28804866 . 1424-7860 . Pinschewer . D. D. . w14465 . 2023-01-05 . 2023-01-05 . https://web.archive.org/web/20230105102222/https://smw.ch/index.php/smw/article/view/2341 . live . free .
  27. Pichla-Gollon . Susan L. . Lin . Shih-Wen . Hensley . Scott E. . Lasaro . Marcio O. . Herkenhoff-Haut . Larissa . Drinker . Mark . Tatsis . Nia . Gao . Guang-Ping . Wilson . James M. . Ertl . Hildegund C. J. . Bergelson . Jeffrey M. . June 2009 . Effect of Preexisting Immunity on an Adenovirus Vaccine Vector: In Vitro Neutralization Assays Fail To Predict Inhibition by Antiviral Antibody In Vivo . Journal of Virology . en . 83 . 11 . 5567–5573 . 10.1128/JVI.00405-09 . 19279092 . 2681979 . 0022-538X.
  28. Tatsis N, Ertl HC . Adenoviruses as vaccine vectors . Molecular Therapy . 10 . 4 . 616–629 . October 2004 . 15451446 . 10.1016/j.ymthe.2004.07.013 . 7106330 .
  29. May 2007 . 149. Nasal Delivery of Adenovirus-Based Vaccine Bypasses Pre-Existing Immunity to the Vaccine Carrier and Improves the Quality of the Immune Response . Molecular Therapy . 15 . S58 . 10.1016/s1525-0016(16)44355-8 . 1525-0016 . free .
  30. Web site: 21 April 2020. A Phase 2/3 study to determine the efficacy, safety and immunogenicity of the candidate Coronavirus Disease (COVID-19) vaccine ChAdOx1 nCoV-19. live. https://web.archive.org/web/20201005201654/https://www.clinicaltrialsregister.eu/ctr-search/trial/2020-001228-32/GB. 5 October 2020. 3 August 2020. EU Clinical Trials Register. European Union. EudraCT 2020-001228-32.
  31. Chauhan . Anil . Agarwal . Amit . Jaiswal . Nishant . Singh . Meenu . November 2020 . ChAdOx1 nCoV-19 vaccine for SARS-CoV-2 . The Lancet . en . 396 . 10261 . 1485–1486 . 10.1016/S0140-6736(20)32271-6. 33160563 . 7832915 .
  32. News: 8 January 2021. How Gamaleya's Vaccine Works. The New York Times. 27 January 2021. Corum J, Carl Z. 20 April 2021. https://web.archive.org/web/20210420022117/https://www.nytimes.com/interactive/2021/health/gamaleya-covid-19-vaccine.html. live.
  33. FDA Briefing Document Janssen Ad26.COV2.S Vaccine for the Prevention of COVID-19. U.S. Food and Drug Administration (FDA). PDF. 2021-04-02. 2021-04-29. https://web.archive.org/web/20210429162601/https://www.fda.gov/media/146217/download. live.
  34. Zhu FC, Guan XH, Li YH, Huang JY, Jiang T, Hou LH, Li JX, Yang BF, Wang L, Wang WJ, Wu SP, Wang Z, Wu XH, Xu JJ, Zhang Z, Jia SY, Wang BS, Hu Y, Liu JJ, Zhang J, Qian XA, Li Q, Pan HX, Jiang HD, Deng P, Gou JB, Wang XW, Wang XH, Chen W . 6 . Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial . Lancet . 396 . 10249 . 479–488 . August 2020 . 32702299 . 7836858 . 10.1016/S0140-6736(20)31605-6 . free .
  35. Johnson & Johnson Announces European Commission Approval for Janssen's Preventive Ebola Vaccine. 1 July 2020. Johnson & Johnson. 16 July 2020. 22 May 2022. https://web.archive.org/web/20220522181844/https://www.jnj.com/johnson-johnson-announces-european-commission-approval-for-janssens-preventive-ebola-vaccine. live.
  36. Web site: Ebola Vaccine Regimen Zabdeno (Ad26.ZEBOV) and Mvabea (MVA-BN-Filo) . 2023-02-16 . www.precisionvaccinations.com . en-US . 2023-02-16 . https://web.archive.org/web/20230216160847/https://www.precisionvaccinations.com/vaccines/ebola-vaccine-regimen-zabdeno-ad26zebov-and-mvabea-mva-bn-filo . live .
  37. Roberts A, Kretzschmar E, Perkins AS, Forman J, Price R, Buonocore L, Kawaoka Y, Rose JK . 6 . Vaccination with a recombinant vesicular stomatitis virus expressing an influenza virus hemagglutinin provides complete protection from influenza virus challenge . Journal of Virology . 72 . 6 . 4704–4711 . June 1998 . 9573234 . 10.1128/JVI.72.6.4704-4711.1998 . 109996 .
  38. Humphreys . Ian R. . Sebastian . Sarah . January 2018 . Novel viral vectors in infectious diseases . Immunology . en . 153 . 1 . 1–9 . 10.1111/imm.12829 . 5721250 . 28869761.
  39. Woolsey C, Geisbert TW . Current state of Ebola virus vaccines: A snapshot . PLOS Pathogens . 17 . 12 . e1010078 . December 2021 . 34882741 . 10.1371/journal.ppat.1010078 . 8659338 . Dutch RE . free .
  40. Marzi A, Ebihara H, Callison J, Groseth A, Williams KJ, Geisbert TW, Feldmann H . Vesicular stomatitis virus-based Ebola vaccines with improved cross-protective efficacy . The Journal of Infectious Diseases . 204 . Suppl 3 . S1066–S1074 . November 2011 . 21987743 . 3203393 . 10.1093/infdis/jir348 . doi . free .
  41. Web site: Ervebo (Ebola Zaire Vaccine, Live) Suspension for intramuscular injection. Merck Sharp & Dohme. PDF. 2021-04-02. 2020-03-29. https://web.archive.org/web/20200329050507/https://www.fda.gov/media/133748/download. live.
  42. Martínez-Romero C, García-Sastre A . Against the clock towards new Ebola virus therapies . Virus Research . 209 . 4–10 . November 2015 . 26057711 . 10.1016/j.virusres.2015.05.025 .
  43. Choi WY, Hong KJ, Hong JE, Lee WJ . Progress of vaccine and drug development for Ebola preparedness . Clinical and Experimental Vaccine Research . 4 . 1 . 11–16 . January 2015 . 25648233 . 4313103 . 10.7774/cevr.2015.4.1.11 .
  44. Regules JA, Beigel JH, Paolino KM, Voell J, Castellano AR, Hu Z, Muñoz P, Moon JE, Ruck RC, Bennett JW, Twomey PS, Gutiérrez RL, Remich SA, Hack HR, Wisniewski ML, Josleyn MD, Kwilas SA, Van Deusen N, Mbaya OT, Zhou Y, Stanley DA, Jing W, Smith KS, Shi M, Ledgerwood JE, Graham BS, Sullivan NJ, Jagodzinski LL, Peel SA, Alimonti JB, Hooper JW, Silvera PM, Martin BK, Monath TP, Ramsey WJ, Link CJ, Lane HC, Michael NL, Davey RT, Thomas SJ . 6 . A Recombinant Vesicular Stomatitis Virus Ebola Vaccine . The New England Journal of Medicine . 376 . 4 . 330–341 . January 2017 . 25830322 . 5408576 . 10.1056/NEJMoa1414216 .
  45. Hassan AO, Shrihari S, Gorman MJ, Ying B, Yuan D, Raju S, Chen RE, Dmitriev IP, Kashentseva E, Adams LJ, Mann C, Davis-Gardner ME, Suthar MS, Shi PY, Saphire EO, Fremont DH, Curiel DT, Alter G, Diamond MS . 6 . An intranasal vaccine durably protects against SARS-CoV-2 variants in mice . Cell Reports . 36 . 4 . 109452 . July 2021 . 34289385 . 10.1016/j.celrep.2021.109452 . 8270739 .
  46. Xu F, Wu S, Yi L, Peng S, Wang F, Si W, Hou L, Zhu T . 6 . Safety, mucosal and systemic immunopotency of an aerosolized adenovirus-vectored vaccine against SARS-CoV-2 in rhesus macaques . Emerging Microbes & Infections . 11 . 1 . 438–441 . December 2022 . 35094672 . 10.1080/22221751.2022.2030199 . 8803102 .
  47. Chavda . Vivek P. . Vora . Lalitkumar K. . Pandya . Anjali K. . Patravale . Vandana B. . November 2021 . Intranasal vaccines for SARS-CoV-2: From challenges to potential in COVID-19 management . Drug Discovery Today . en . 26 . 11 . 2619–2636 . 10.1016/j.drudis.2021.07.021. 34332100 . 8319039 .
  48. Rauch . Susanne . Jasny . Edith . Schmidt . Kim E. . Petsch . Benjamin . 2018-09-19 . New Vaccine Technologies to Combat Outbreak Situations . Frontiers in Immunology . 9 . 1963 . 10.3389/fimmu.2018.01963 . 30283434 . 6156540 . 1664-3224. free .
  49. de Gruijl . Tanja D. . Ophorst . Olga J. A. E. . Goudsmit . Jaap . Verhaagh . Sandra . Lougheed . Sinéad M. . Radosevic . Katarina . Havenga . Menzo J. E. . Scheper . Rik J. . 2006-08-15 . Intradermal Delivery of Adenoviral Type-35 Vectors Leads to High Efficiency Transduction of Mature, CD8+ T Cell-Stimulating Skin-Emigrated Dendritic Cells . The Journal of Immunology . en . 177 . 4 . 2208–2215 . 10.4049/jimmunol.177.4.2208 . 16887980 . 25279434 . 0022-1767 . 2023-01-05 . 2023-02-02 . https://web.archive.org/web/20230202191043/https://journals.aai.org/jimmunol/article/177/4/2208/1486/Intradermal-Delivery-of-Adenoviral-Type-35-Vectors . live . free .
  50. Liebowitz D, Gottlieb K, Kolhatkar NS, Garg SJ, Asher JM, Nazareno J, Kim K, McIlwain DR, Tucker SN . 6 . Efficacy, immunogenicity, and safety of an oral influenza vaccine: a placebo-controlled and active-controlled phase 2 human challenge study . The Lancet. Infectious Diseases . 20 . 4 . 435–444 . April 2020 . 31978354 . 10.1016/S1473-3099(19)30584-5 . 210892802 .