The FDA granted emergency use authorization (an EUA) for Johnson & Johnson’s coronavirus vaccine. There are *lots* of articles out there about how well the vaccine works, but not nearly as many about *how* it works. It’s an adenoviral-vectored vaccine. What’s that terminology mean? 

An adenovirus is a type of virus that causes some of the illnesses we collectively refer to as “common colds.” Before I started studying science I’d always thought that the “common cold” was an illness caused by a single virus, but there are actually lots of viruses (including several in the coronavirus family) that cause the same symptoms and get lumped together under the term common cold because most people don’t even go to the doctor when they have a cold and, even if they do, doctors don’t usually go around sequencing the snot of every person who comes to them with sniffles. Bottom line, if you have a cold it might be caused by an adenovirus, it might be caused by a (non-SARS-CoV-2) coronavirus, it might be caused by a rhinovirus, etc. So, adenoviruses are one of those types of viruses that can give you the symptoms of a cold. But can it also give you immunity against SARS-CoV-2, the coronavirus that causes the disease COVID-19? It could if you modify it to deliver parts of SARS-CoV-2 (and take away the cold-causing-ness) and this is the idea behind the J & J vaccine as well as the Oxford/AstraZeneca vaccines, although they use different adenoviruses as their genetic delivery vehicles (vectors). 

The 2 coronavirus vaccines previously granted EUA in the US – Pfizer’s and Moderna’s) are both “mRNA vaccines.” They work by delivering the genetic instructions for making the coronavirus Spike protein (S) in the form of messenger RNA (mRNA). This can work because the cells that receive the instructions can make S protein from them – S is the one that juts out from the viral membrane and allows it to bind to and get into human cells. On its own, this protein is harmless, but it is foreign, so the body will learn to mount an immune response against it if it finds that protein again when the virus were to try to attack. However, since naked RNA is fragile and negatively-charged, this mRNA has to encapsulated in lipids (oily molecules) in the form of Lipid NanoParticles (LNPs) in order to get into cells and kept at really cold temperatures before they do so.

Viral vector vaccines therefore take a different approach to delivering S protein instructions, sticking the S gene into a different, harmless, virus and using that virus as a “vector” (vehicle) for the gene delivery. This strategy makes a lot of sense, since viruses offer a lot of protection to the genetic info they contain, and they’re naturally really good at getting genetic info into cells (that’s their M.O.!) 

Why adenoviruses? Adenoviruses (Ads) are double-stranded DNA viruses, whereas coronaviruses are single-stranded RNA viruses. Lengthwise, their genomes are similar. Ads are ~36,000 letters long. SARS-CoV-2 is ~30,000. But coronaviruses have an oily lipid membrane around them, which they acquire when they bud out from cells, whereas adenoviruses don’t have one of these membranes – instead they have a protein “shell” called a capsid. ⠀

All those differences aside, there’s one thing both families of virus are really good at – getting genetic info inside of human cells. Are you thinking what scientists are thinking?… If we could put genetic info we want into those viruses, the viruses would get tricked into delivering them – like in the movies where someone hides a secret message in an egg that the dairy man delivers. ⠀

If this “using-virus-to-deliver-genetic-info” thing could be done, it would offer up a huge range of possibilities, because stretches of that genetic info called genes are like recipes for making proteins. If you could get a virus to deliver those recipes, you could get the cells to make those proteins you wanted. Even if the cell doesn’t normally make that protein. So you could get cells to make proteins that they have typo-ed recipes for (i.e. mutated genes), treating (or even curing depending on the circumstances) genetic diseases – or you could get cells to make viral proteins they could learn to recognize as foreign and produce antibodies against. ⠀

The former (giving human gene to human to compensate for missing and/or mutated gene) is called gene therapy and the latter is called a viral vector vaccine. If you want to learn more about gene therapy uses, check out this post: 

But today I’m going to focus on the viral vector vaccine uses.

quick note: The viruses used for these vectors are usually modified making them “non-replicating.” Normally a virus can use your cells to make more copies of itself which can then infect more cells, but a non-replicating virus can get into your cells but doesn’t multiply once there. Adenoviruses need a viral protein called E1 to replicate. So if scientists cut out the E1 gene, they can make a virus non-replicating. Preventing the virus from making more of itself can make sticking it in people safer (and makes it safer to work with in a lab), but it also poses problems if you want to make more of the virus. So in order to produce it, manufacturers infect cells in a dish/flask that already have a copy of the E1 gene. Those cells can thus make the E1 protein for the virus, the virus will replicate and then burst out and then you can collect that virus (which doesn’t have the E1 gene and thus can’t replicate) and use it.

The use of adenoviral vectors for gene therapies has been challenging in part because they often cause an strong immune response. With gene therapy you *don’t* want an immune response to be generated, but with a vaccine you *do* (in fact, with many vaccines you actually have to include helper chemicals called adjuvants to help stimulate an immune response). So, while adenoviruses might not be good for gene therapy, could they be used for vaccines? Turns out, yes!

To explain this I first want to give a really quick, overly simplified, explanation of part of “adaptive immunity” – how our body learns to recognize and attack a virus or other invader using little proteins called antibodies that specifically bind to parts of that invader (such as pieces of viral proteins). Each time your body is confronted with a new viral infection, it has to develop antibodies specific to that virus. Your body can’t predict what viruses it will need to combat, so it uses a trial and error approach, mixing and matching constant and variable regions of antibodies and seeing what works. ⠀

A certain type of immune cell (antigen-presenting cells) engulfs viral particles, chops up their proteins and then displays those protein parts (peptides) on the surface of the cell. Antibody-making cells (which each produce a different antibody thanks to random recombination) then compete to see which cell has an antibody that can bind this antigen. The “winners” get selected for and more of that antibody is made. The antibody helps the body do things like flag infected cells for destruction, call for backup, and keep watch after the virus has been conquered in case it tries to return (which is why antibody tests can detect past infection). There’s also a branch of adaptive immunity called cell-mediated immunity which involves different types of immune cells called T-cells and I’m not going to go into that here. 

The process of finding and making the right antibodies takes a while, so the goal of a vaccine is to pre-introduce your body to viral parts well before your body actually meets a virus, so you can build up antibodies against them without actually having to get sick. Classically, vaccines do this by using a weakened (aka attenuated) or killed (aka inactivated) version of the virus. Some other vaccines do this by introducing just individual viral proteins that have been made in a lab. But that takes a lot of work (expressing the protein, purifying it, etc.), so the idea of nucleic acid (DNA or RNA-based) vaccines is to get the cells to do the hard work – deliver the recipe for making the viral protein and let the cells make the protein. Much easier. But also not-yet-approved-to-work-as-a-vaccine-in-humans (the only commercial adenovirus vector vaccine is a rabies vaccine for wild animals)

As far as I can tell, prior to the pandemic, there was only one proven-effective human viral vector vaccines approved – for *any* disease *yet*  – but some groups had gotten really close. And one such group is Oxford/AstraZeneca – they’ve been one of the frontrunners in the vaccine “race”, and one reason why is because they were able to “just” modify a vaccine they have in late-stage trials for one of the other coronaviruses, MERS (Middle Eastern Respiratory Syndrome). J&J did the same sort of thing, but with a different adenovirus. Both teams stuck the gene for the SARS-CoV-2 Spike protein into a modified adenovirus. When the virus is injected, it infects the injected-person’s cells, those cells make spike protein, and the immune system makes antibodies against it. Sounds great, and it can be, but this strategy isn’t without some potential problems. 

A lot of times with vaccines, the immunity doesn’t last forever, so you need a follow-up “booster shot.” Problem is, if the initial vaccination causes your body to develop antibodies against the vector, and not just the viral part, when you get a booster shot your immune system will attack the vaccine before it even has a chance to introduce more viral protein. This can also be a problem if you want to mix and match to use the same viral vector to deliver protein from a different virus to vaccinate against a different disease. ⠀

It’s kinda like your viral vector is a creepy white van – and you first use it to get people to learn to attack vampires. So you deliver some vampires – in the creepy white van that gives the people a heads-up that there’s something dangerous so they learn to attack a vampire if they see one again. And then you want to teach people to attack werewolves. So you do the same thing but stick werewolves instead of vampires in the van. Problem is, when people see the van, they attack the van itself without even knowing that a werewolf is inside. So if the werewolf shows up in some other form of transportation, the people won’t know it’s dangerous.

Because adenoviruses are so common, many people have antibodies against various parts of various ones. And this can cause a similar creepy-white-van problem – your body may have already learned to recognize the van before you try to deliver the vampires, so your body never even meets the vampires and won’t know how to greet them in the future. ⠀

This can be seen with failed HIV vaccines and with CanSino’s Ebola vaccine. CanSino got Chinese approval for their Ad5-based Ebola vaccine in 2017, but only for emergency use and they never actually proved it would prevent Ebola infection; additionally, the antibodies made after injection didn’t last very long, potentially because of pre-existing immunity to Ad5 (that white van problem).

CanSino Biologics started human trials of an Ad5-based Covid-19 vaccine in China in March 2020 (the first company to take a Covid-19 vaccine to clinical trial). Because of that pre-existing immunity problem, scientists thought doses given would likely need to be higher which could cause more side effects CanSino recently put out some data and got approval for use in China in February 2021 

Scientists have tried a few different strategies to get around pre-existing immunity problems. One strategy is to modify the adenovirus genes to camouflage it a bit better – like giving it a paint job. Biochemically, they can do this by deleting or altering more adenovirus genes. A company called ImmunityBio is using this approach They’ve also included a second coronavirus gene, N (which has instructions for making the Nucelocapsid protein which normally functions to coat and protects coronavirus RNA and is also very immunogenic (capable of prompting an immune response). Adding this second gene could help the immune system recognize the virus and deploy T-cells even if the S protein evolves to evade antibody binding. ImmunityBio is also experimenting with giving a booster dose of the vaccine orally, which would be a relief for needle-fearers and can also help generate a more expanded immune response. 

Another strategy is to turn to different vans all together – less common coronaviruses, like Ad26, which Johnson & Johnson acquired and made an Ebola vaccine from (it received approval in Europe in July 2020, and I think this makes it the first proven human adenoviral vector vaccine) A benefit of J&J’s vaccine is they’re just giving a single shot (no booster). This makes production and distribution a lot easier, and it avoid the white van problem, except…⠀

Ad26 is a human coronavirus, and lots of people in sub-Saharan Africa & Southeast Asia may have preexisting immunity to it, so some groups are looking to viruses from other animals. And they found some promising ones in chimpanzee poop. This is the strategy taken by Oxford with their ChAdOx1 vector (name comes from CHimpanzee ADenovirus OXford). They took a chimp poop adenovirus, and cut out the E1 – they also removed a gene called E3 which they don’t need in order to free up more space in the genome for sticking in a protein of interest – in this case the SARS-CoV-2 spike protein. They teamed up with AstraZeneca for production and distribution so now you usually hear this vaccine referred to as the Oxford/AstraZeneca vaccine. And I think the ChadOx name has been replaced with “AZD1222” because people didn’t want the chimp association to scare people off. Sorry chimps! 

Russia’s Sputnik V vaccine uses 2 different viral vectors for its first, priming, shot (Ad26) and its second booster shot (Ad5).

In addition to viral vector vaccines and mRNA vaccines, you have the more traditional vaccines. These other strategies include:⠀

  • inactivated (killed) virus⠀
  • weakened (live attenuated) virus⠀
  • viral proteins (aka subunit vaccines)⠀
  • virus-like particles (VLPs)⠀

And you can learn more about them here:  

here’s a nice article/infographic on the J&J vaccine from the New York Times: 

note: adenoviruses are DNA based and do go into the nucleus but they do NOT get into your DNA because they don’t have the integration equipment that is needed to do so). So, SARS-CoV-2 itself can’t get into your DNA.

note: I am not an immunologist nor a doctor. I am a biochemist-in-training. This is my best understanding of how these vaccines work and what the current situation with them is. I apologize if anything is out of date or not nuanced enough, just doing my best here to try to use my platform and background to help people interpret the vast sea of information out there at a level that’s detailed but not super jargonny. So I hope it was somewhat helpful, but if you have specific immune system related questions, I suggest asking an immunologist as I do not pretend to be an expert. 

More on how vaccines are tested:

More coronavirus-related posts:

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉   ⠀

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