Use of adenovirus type-5 vectored vaccines: a cautionary tale

Authors: The Lancet

We are writing to express concern about the use of a recombinant adenovirus type-5 (Ad5) vector for a COVID-19 phase 1 vaccine study,1 and subsequent advanced trials. Over a decade ago, we completed the Step and Phambili phase 2b studies that evaluated an Ad5 vectored HIV-1 vaccine administered in three immunizations for efficacy against HIV-1 acquisition.23 Both international studies found an increased risk of HIV-1 acquisition among vaccinated men.24 The Step trial found that men who were Ad5 seropositive and uncircumcised on entry into the trial were at elevated risk of HIV-1 acquisition during the first 18 months of follow-up.5 The hazard ratios were particularly high among men who were uncircumcised and Ad5 seropositive, and who reported unprotected insertive anal sex with a partner who was HIV-1 seropositive or had unknown serostatus at baseline, suggesting the potential for increased risk of penile acquisition of HIV-1. Importantly for considering the potential use of Ad5 vectors for COVID-19 infection, a similar increased risk of HIV infection was also observed in heterosexual men who enrolled in the Phambili study.4 This effect appeared to persist over time. Both studies involved an Ad5 construct that did not have the HIV-1 envelope. In another HIV study, done only in men who were Ad5 seronegative and circumcised, a DNA prime followed by an Ad5 vector were used, in which both constructs contained the HIV-1 envelope.6 No increased risk of HIV infection was noted. A consensus conference about Ad5 vectors held in 2013 and sponsored by the National Institutes of Health indicated the most probable explanation for these differences related to the potential counterbalancing effects of envelope immune responses in mitigating the effects of the Ad5 vector on HIV-1 acquisition.7 The conclusion of this consensus conference warned that non-HIV vaccine trials that used similar vectors in areas of high HIV prevalence could lead to an increased risk of HIV-1 acquisition in the vaccinated population. The increased risk of HIV-1 acquisition appeared to be limited to men; a similar increase in risk was not seen in women in the Phambili trial.4Several follow-up studies suggested the potential mechanism for this increased susceptibility to HIV infection among men. The vaccine was highly immunogenic in the induction of HIV-specific CD4 and CD8 T cells; however, there was no difference in the frequency of T-cell responses after vaccination in men who did and did not later become infected with HIV in the Step Study.8 These findings suggest that immune responses induced by the HIV-specific vaccine were not the mechanism of increased acquisition. Participants with high frequencies of preimmunisation Ad5-specific T cells were associated with a decreased magnitude of HIV-specific CD4 responses and recipients of the vaccine had a decreased breadth of HIV-specific CD8 responses,9 suggesting that pre-existing Ad5 immunity might dampen desired vaccine-induced responses. 

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From adenoviruses to RNA: the pros and cons of different COVID vaccine technologies

Authors: Joel Abrams

The World Health Organisation lists about 180 COVID-19 vaccines being developed around the world.

Each vaccine aims to use a slightly different approach to prepare your immune system to recognise and fight SARS-CoV-2, the virus that causes COVID-19.

However, we can group these technologies into five main types. Some technology is tried and trusted. Some technology has never before been used in a commercial vaccine for humans.

As we outline in our recent paper, each technology has its pros and cons.

1. DNA/RNA-based

DNA and RNA vaccines use fragments of genetic material made in the lab. These fragments code for a part of the virus (such as its spike protein). After the vaccine is injected, your body uses instructions in the DNA/RNA to make copies of this virus part (or antigen). Your body recognises these and mounts an immune response, ready to protect you the next time you encounter the virus.


  • these vaccines can be quickly designed based on genetic sequencing alone
  • they can be easily manufactured, meaning they can potentially be produced cheaply
  • the DNA/RNA fragments do not cause COVID-19.


  • there are no approved DNA/RNA vaccines for medical use in humans, hence their alternative name: next-generation vaccines. So they are likely to face considerable regulatory hurdles before being approved for use
  • as they only allow a fragment of the virus to be made, they may prompt a poor protective immune response, meaning multiple boosters may be needed
  • there’s a theoretical probability vaccine DNA can integrate into your genome.

The speed at which these vaccines can be designed, needing only the genetic sequence of the virus, is why these vaccines were among the first to enter clinical trials.

An RNA vaccine, mRNA-1273, being developed by Moderna and the US National Institute of Allergy and Infectious Diseases, advanced to clinical testing just two months after the virus was sequenced.

2. Virus vectors

These vaccines use a virus, often weakened and incapable of causing disease itself, to deliver a virus antigen into the body. The virus’ ability to infect cells, express large amount of antigen and in turn trigger a strong immune response make these vaccines promising.

Examples of viruses used as vectors include vaccinia virus (used in the first ever vaccine, against smallpox) and adenovirus (a common cold virus).


  • highly specific delivery of antigens to target cells and high expression of antigen after vaccination
  • often a single dose is enough to stimulate long-term protection.


  • people may have existing levels of immune protection to the virus vector, reducing the effectiveness of the vaccine. In other words, the body raises an immune response to the vector rather than to the antigen.

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Here’s Why Viral Vector Vaccines Don’t Alter DNA

— It’s pretty simple — they can’t

Authors: by Veronica Hackethal, MD, MSc, Enterprise & Investigative Writer, MedPage Today March 12, 2021

Adenoviral vector vaccines have been in development for decades, but very few have been approved for use in humans. What does the history of adenoviral vector vaccine development tell us about their safety and their potential to alter DNA?

How Do Adenoviral Vector Vaccines Work?

Essentially, these types of vaccines act like delivery shuttles. They use an adenovirus — which has been engineered to be incapable of replicating and causing disease — to deliver the genes for making the antigen; in this case, that’s the SARS-CoV-2 spike protein. That in turn elicits an immune response and provides protection against the coronavirus.

Adenoviruses are basically common cold viruses that can cause illnesses ranging from cold-like symptoms to bronchitis, gastroenteritis, and conjunctivitis.

“I think people are unfortunately familiar with adenoviruses … [A]t far too many points, you know, you’ve had the sniffle. You’ve had the cough. You felt crummy. If it’s a cold it’s often adenovirus,” Daniel Griffin, MD, PhD, said on a recent episode of MedPage Today‘s “Track the Vax” podcast. Griffin is chief of infectious disease at ProHEALTH Care, an Optum unit.

Humans are infected with multiple different types of adenoviruses throughout their lifetimes. Most serotypes cause mild illness, although adenovirus serotype 7 has been associated with more severe illness. Older adults and people who are immunocompromised or have pre-existing respiratory or cardiac disease may have worse illness.

Precisely because adenoviruses are so common, one problem with using them in vaccines is that people may already have antibodies to them, overwhelming them before they can do their assigned work. Researchers get around that issue by using adenoviruses that humans are unlikely to have encountered before.

Currently, five adenovirus vector vaccines for COVID-19 are in use worldwide.

Each works on the same basic principle, although delivery platforms differ. The AstraZeneca/Oxford vaccine uses the ChAdOx1 platform, which is based on a modified version of a chimpanzee adenovirus.

The Johnson & Johnson vaccine uses a proprietary AdVac platform, made up of a recombinant human adenovirus (adv26). It’s the same platform used in the company’s Ebola virus vaccine (which is approved in Europe) and its investigational Zika, RSV, and HIV vaccines.

Russia’s Sputnik V uses recombinant human adenoviruses Ad26 and Ad5 for the first and second doses, respectively. Finally, China’s CanSino vaccine uses the recombinant human adenovirus Ad5.

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What Do We Really Know About Adenovirus Vectors for Vaccines?

Authors: By Serena Marshall and Lara Salahi February 24, 2021

— The newest COVID shot uses an existing technology but one with lingering questions

As the U.S. hits the half-million death mark from COVID-19 — a grim milestone that is equal to roughly the entire population of Atlanta and more than that of Miami — a new weapon is being added to the COVID-19 vaccine arsenal.

Johnson & Johnson is seeking emergency use authorization for what would become the U.S.’s first one-dose and non-mRNA COVID vaccine. It employs adenovirus vectors, a technology that has been used in labs for decades and was approved for the Ebola vaccine by the FDA in December 2019. It’s the same technology that AstraZeneca/Oxford and Sputnik V use.

Still, questions remain on how these vaccines may be different than mRNA or similar enough to other existing shots to encourage vaccine uptake. To explain how adenovirus vectors work and what to expect from the new products.

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Pros and Cons of Adenovirus-Based SARS-CoV-2 Vaccines

Authors: Eric J. Kremer1,∗

Main Text

Most of us might be surprised by the rudimentary scientific rationale prevalent in the field of vaccine research just 50 years ago. For over a century after Louis Pasteur’s vaccine against rabies, approaches usually consisted of inactivating a virus, injecting it, and seeing if it protected the host. Unlike today, interactions between vaccinologists and immunologists to improve vaccine efficacy were marginal.

With the rise of molecular biology, vaccine designs became more nuanced and the use of viral vectors emerged. An example is the evolution and checkered history of vaccines based on adenoviruses (Ads). Live Ad types 4 (Ad4) and 7 (Ad7) have been used in North American military recruits since the 1950s to prevent severe respiratory illness.1 Similarly, dogs in western countries are vaccinated with an attenuated canine Ad type 2 (CAV-2) to prevent infection of the more virulent CAV-1.

Many of the first replication-defective Ad “vectors” in the early 1980s were vaccines. The original Ad vaccine design was relatively simple: delete a region of the viral genome that the virus needs to propagate, provide these functions via transcomplementing cells (e.g., Frank Graham’s 293 cells) so that one could grow the vaccine, and then insert into the virus genome an expression cassette encoding the targeted epitopes.

Fast forward to 2020. The SARS-CoV-2 pandemic may be headed toward historic proportions—although still far from the 1918 Spanish flu (50 million deaths) and AIDS (35 million deaths)—inflicting havoc on families, communities, and economies and overwhelming health care facilities. Clearly, we need a vaccine. Are Ad-based vaccines targeting the SARS-CoV-2 spike and capsid proteins our best bet? After almost 70 years of working with Ads, their biochemical properties are well characterized: Ads are simple to make (in ∼2 weeks a graduate student could generate enough of a novel Ad vaccine to treat a thousand mice and dozens of monkeys), easy to purify to high titer, genetically stable, easily stockpiled, relatively inexpensive, and can be delivered via aerosol, oral, intradermal, and intramuscular routes. The aerosol route is particularly relevant when targeting a respiratory virus because inducing protective immune responses that home to the tissue where infections will occur is strategically important. It is also worth noting that Ad-based vaccines tend to induce B cell and T cell responses.

Hundreds of millions of euros, dollars, and yen have been invested in advancing Ad-based vaccines. These advances include production and purification methods, genetic incorporation of epitopes into the capsid so that mononuclear phagocytes present these antigens via major histocompatibility complex (MHC) class I and II pathways, cloaking the capsid with polymers/shields or using Ad types with a lower level of seroprevalence to prevent neutralization by antibodies (NAbs) to common types found in many individuals, retargeting the vector to professional antigen-presenting cells, using helper-dependent vectors (so that the vector-infected cell only expresses the target epitopes and not Ad antigens), and single-cycle replication of vaccines to produce massive amounts of antigens. Each tweak, alone or in combination with others, has improved vaccine efficacy in preclinical trials.

As SARS-CoV-2 became a pandemic, it is astonishing that, in the case of the Ad-based vaccine frontrunners, little has changed from the basic design of 40 years ago. Some used the well-trodden path of an Ad5-based vaccine, while others switched to human (e.g., Ad26) or simian (monkey and gorilla) Ads that have low seroprevalence in Europe and North America (but not necessarily in Africa or Asia).2 Conceptually, Ad type switching to avoid NAbs is at least 30 years old. The advent of simian Ad vaccines was not developed following a rigorous testing of all of the >200 different Ad types but was most likely the result of intellectual property issues and the ability to produce simian Ads in good manufacturing practice (GMP)-compliant cells. One presumes that subsequent rounds of Ad-based coronavirus disease 2019 (COVID-19) vaccine candidates will be more sophisticated.

Should we go “all in” on an Ad-based vaccine against SARS-CoV-2? The first issue is safety. There are few drugs or biologicals that do not have side effects or cause adverse reactions. Weighing the advantages versus disadvantages during the current pandemic can be idiosyncratic, and the strength of the reasoning varies by population, culture, religious beliefs, and bizarrely (for those of us outside the USA) even political affiliation. Current criteria limit the window to identify adverse reactions to 2 months. In addition to swelling and pain at the injection site, common to some vaccines, Ad-based vaccine adverse effects include fever, pneumonia, diarrhea, transient neutropenia and lymphopenia, fatigue, labored breathing, headaches, liver damage, and fasting hyperglycaemia. Rare but grave adverse reactions include neuropathies such as Bell’s palsy, Guillain-Barré syndrome, gait disturbance, and transverse myelitis, an inflammatory condition in the spinal cord.

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What are Adenovirus-Based Vaccines?

Authors: By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Sophia Coveney, B.Sc.

Adenoviruses are considered excellent vectors for delivering target antigens to mammalian hosts because of their capability to induce both innate and adaptive immune responses. Currently, adenovirus-based vaccines are used against a wide variety of pathogens, including Mycobacterium tuberculosis, human immunodeficiency virus (HIV), and Plasmodium falciparum.

What are adenoviruses?

Adenoviruses are non-enveloped, double-stranded DNA viruses (genome size: 34-43 kb; virion size: 70-90 nm), first discovered in the human adenoid tissue in 1953 by Rowe and his colleagues. In humans, adenoviruses generally cause mild respiratory and gastrointestinal tract infections; however, adenovirus-induced infections can be life-threatening in immunocompromised people, or people with pre-existing respiratory or cardiac disorders.

These viruses are isolated from a wide variety of mammalian species, ranging from simians to chimpanzees to human beings. In humans, more than 50 adenovirus serotypes (no cross-neutralization by antibodies) have been identified, which are divided into 7 subgroups (A – G) based on red blood cell agglutination properties and sequence homology.

Adenoviruses express two types of genes: early genes and late genes. Early genes (E1A, E1B, E2, E3, and E4) are necessary for supporting viral replication inside host cells; whereas, late genes are required for host cell lysis, viral assembly, and virion release. Recombinant adenoviruses that are generated in the laboratory as vectors can be either replication-deficient or replication-competent.

Because the E1 gene is essential for viral replication, experimental depletion of the E1 gene generates adenoviruses that are capable of infecting the host cells but cannot grow in numbers because of defective replication. However, some specialized cells, such as HEK 293, can facilitate the replication of E1-deficient adenoviruses by providing E1 functions in trans.

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Adenovirus-vectored Vaccines for COVID-19 – How Do They Work?

Authors: Heather D. Marshall, PhD | Vito Iacoviello, MD

Viruses are genetic material surrounded by a protein shell with the ability to infect cells, highjack cellular machinery, and make copies of themselves. Viruses are also detected by the immune system and stimulate innate and adaptive immune responses and quite often, potent immunological memory that protects the host from reinfection. Therefore, researchers have been using viruses as vectors or carriers of vaccine antigens in order to protect against infectious diseases.

Adenoviruses are nonenveloped DNA viruses with a range of properties making them suitable vaccine vectors. Adenoviruses infect a wide range of cell types, they are easy to genetically modify, and in nature are known to cause mild or asymptomatic respiratory infection (thus, they are relatively safe). Further, gene swapping (replacing an essential adenovirus gene required for replication with the vaccine gene) renders the vectors replication incompetent, further contributing to safety. Many adenovirus-vectored vaccines target viral diseases because the adenovirus mimics a potent antiviral immune response, and its use as a SARS-CoV-2 vaccine is no exception.

One consideration with the use of adenovirus-vectored vaccines is the level of pre-existing immunity to the vector. Because adenoviruses are naturally occurring respiratory pathogens, most people are exposed to a variety of adenoviruses during childhood and have some immunity to the vector, which can blunt the response to the vaccine. To circumvent this issue, rare adenovirus serotypes such as Ad26 and Ad35 and non-human adenoviruses including chimpanzee, gorilla, and rhesus macaque viruses may be used as vaccine vectors.

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8 tools that helped us tackle the coronavirus

Authors: Ryan Cross Laura Howes Megha Satyanarayana


Viruses might be nature’s most efficient gene-delivery vehicles. At its essence, a virus is simply a package that shuttles genes into a host cell, which it then hijacks to make more viruses. About 4 decades ago, scientists started hijacking the viruses themselves in an attempt to make novel vaccines. The fruits of that labor have led to some of the most advanced vaccines for fighting COVID-19.

In early 2020, scientists around the world—most prominently, those at the University of Oxford (later joined by AstraZeneca) in the UK, Johnson & Johnson in the US, CanSino Biologics in China, and the Gamaleya National Center of Epidemiology and Microbiology in Russia—began using an old but largely experimental vaccine technology to make vaccines for COVID-19.

The technology relies on adenoviruses, perhaps best known for causing the common cold. Scientists began tinkering with adenoviruses in the 1980s, first removing genes that the adenoviruses relied on for replication and then inserting those genes into special cells designed to grow the viruses in the lab.

The gutted adenoviruses, called adenoviral vectors, are versatile tools into which scientists can slip new genes at will. To make COVID-19 vaccines, researchers insert DNA encoding the SARS-CoV-2 spike protein into the vectors. The modified viruses are then grown, isolated, and packaged into vaccines, which contain about 50 billion vectors per shot. Once injected, the vectors slip into our cells and trick our bodies into producing coronavirus spike proteins, which trigger the immune system to develop antibodies and T cells that target the coronavirus.

Before last year, several groups had tried, and failed, to use adenoviral vectors to make vaccines for some of medicine’s toughest problems: HIV infection, malaria, and cancer. In July 2020, success for the technology finally arrived when Johnson & Johnson’s adenoviral vector vaccine for Ebola virus disease was approved in Europe. This year could bring several authorizations for COVID-19 vaccines based on the technology. Even if these vaccines turn out to be less effective than the messenger RNA vaccines, they might make a bigger global impact because they are cheaper to produce and easier to distribute.

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