How do vaccines work?

What are vaccines and how do they protect us?
21 May 2019

Interview with 

Clare Bryant, University of Cambridge


Vaccine injection


These days, a lot goes into a vaccine, and understanding all the different kinds of vaccines, as well as how they work can be a really difficult point to get across, and given the resurgence of diseases we have vaccines for, understanding this is more important than ever. Izzie Clarke spoke to Clare Bryant, from the Department of Veterinary Medicine at the University of Cambridge, about what vaccines are and how they actually give us immunity.

Clare - As we just heard, a vaccine is a medicine which is designed to train the body to generate a protective immune response against a disease it hasn't seen before. So it's a medicine that mimics the disease to produce an immune response but doesn't make you sick.

Izzie - How does it actually work? How does that give you immunity?

Clare - Okay. To do this you need to understand exactly what the body does when it sees an infection, and it involves a complex interplay between different series of cells. So the cells that initially pick up the bacterium or the virus when it comes into the body, these cells will take bits of the pathogen and express it on the surface of the cell. It will also become activated by the infection and it will move then to the tissues of the body that contain lymph nodes (lymphoid tissues they're called), and in these lymphoid tissues reside other cells. There are B cells and what B cells do is they produce antibodies, and there are T cells and T cells are the killer cells take out infected cells within the body. And so when the white blood cells rocks up with the activated state with the antigen on its cell surface, it will interact with the B cell and that will cause it to become activated, expand and produce lots and lots of antibodies becoming an antibody killing machine. And then the antibody binds to the virus or binds to the bacteria and disables it and kills it. At the same time, it can interact with T cells; the T cells are the sort of killer cells of the body, they will go along and they will take out any cells that are infected. So these two cell populations are really important in generating the response against a pathogen.

Now the point of a vaccine though is that it actually needs to work way down the line, so some of the B cells and some of the T cells will actually go into a sort of dormant state. And these cells are then lurking in your body, they are very very very specific because they'll know exactly what the bug looks like, so next time the bug comes along or when the proper bug comes along, it will recognise the bug and generate lots and lots and lots of the cells and that will produce a massive immune response to take out the bug immediately. And you won't know it's going on, hopefully.

Izzie - Fingers crossed. And is that what we call our immune system memory?

Clare - Yeah. That's exactly what immune system memory is. It's the B and T cells that have been vaccinated for that are sitting in the body that recognise that particular bug so when it appears they kick-off and they're very active to take out the bug.

Izzie - Essentially, they send in the cavalry?

Clare - Indeed, indeed.

Izzie - But there are different types of vaccines aren’t there so can you explain how that works?

Clare - Yeah. Over the years, the initial types of vaccines, as we heard with respect to smallpox, was what's called an attenuated vaccine. So that is basically the disease in a very low grade form, so it really is a real live infection, but it's a real live infection that's not very toxic so you can give that people, they may get mild sickness but not very much - it's very safe. But the bug will proliferate and will generate really a pretty natural immune response against it so they confer the most specific and are usually the longest lasting type of immunity.

But we have moved on because vaccines have to be safe and this is the real concern. You can get vaccines that are dead vaccines so that's where you take the bugs and you kill them so some of the early underst and at all (03.42) so they were whooping cough vaccines were made from this. And at this point you inject lots and lots of dead bugs and that's your vaccine.

Now we've become even more sophisticated so we can take what are called the antigens, so that proteins from the different pathogens, for example the tetanus toxoid, one of the toxoid proteins from the toxin. Diphtheria, one of the diphtheria proteins from the diphtheria toxin. Or the flu vaccine, the flu vaccines are all based on proteins. And what they are, they’re proteins that are in the surface of the pathogen so it's actually tricking the body into thinking that the pathogen is there because it’s showing the cell surface protein to the cells.

Izzie - Why do we have all these different versions, essentially?

Clare - What we're trying to do, the key thing with a vaccine is you want to get great immunity but you don't want to cause any harm to the host because you're choosing to vaccinate healthy people, and what you don't want to do is make those healthy people sick. So the problem with the attenuated vaccines is although there great, they generate the best immunity, you are actually giving somebody a low dose of the disease. Whereas the dead pathogen or the toxoid or the antigen vaccines, they can't cause disease because the pathogen is not able to grow, it's a dead version of the bug. So that's why we've developed those to make it as safe as possible because that's the whole point, you're vaccinating healthy people and you don't make them sick.

Izzie - So it's the safest way possible?

Clare - Yep.

Izzie - Now, how do we protect people who don't have an immune system or they're compromised and they can't get vaccines?

Clare - This is the absolute necessity of what's called the herd vaccination, which makes as all sound like a bunch of cows.

Izzie - As we heard earlier.

Clare - As we heard earlier. What that means is if you can get enough people in the population vaccinated you can stop the transmission of that pathogen so measles is a cracking example of this. You need to vaccinate 95% of the people, that means that the pathogen is no longer circulating in the population which means that immunocompromised people, people on cancer chemotherapy and so forth will not be able to pick up the bug because the bug is not actually there because so many people are vaccinated the bug can't be transmitted any more.

Izzie - Now one thing that I would like to know is why do we need boosters? Sometimes you can have one vaccination and that's all you need, there are a few other examples where you need to come back at later points in life, why is that?

Clare - Depends upon the bug, and depends upon the vaccination. Live attenuated vaccinations are often really good at generating lifelong immunity. Not always but generally pretty good. With the dead bugs or the antigen type vaccinations, because they're not totally mimicking the disease, you need to have boosters to keep on boosting your immune system. Some things like tetanus, for example, you need to have every 10 years and it's just because it's not as efficient as giving a low dose of the disease.

Izzie - I see. And so how would you know if you needed a booster or your vaccination was doing the job that it should be?

Clare - Well, hopefully you won't. Because if your vaccination is working then you won't know that you've got the disease. With respect to things that need boosting, if you're on the ball enough you should know that every 10 years you need to go and get a vaccination. Quite often we forget, but if you're going to different parts of the world where the disease is endemic, when you go to the GP surgery to get your travel vaccinations then you'll be reminded. If you have an accident in the garden where you cut yourself for example, and you come into contact with soil, you will then think about your tetanus and go and have a booster. These are the routine things that happen. Normally you're not reminded unless you're going somewhere or you going to hospital with an injury or something like that.

Izzie - And you mentioned a word there that I think's quite important - endemic. So that is something where it's in an area essentially and so what's the difference say we talk about eliminated, eradicated, how do they work?

Clare - Eradicated means the disease is not there anymore and that’s smallpox, okay. Things like measles, we were getting towards being eradicated but now, because not so many children are being vaccinated, the disease is beginning to come up in the population again and this is then causing problems. In some parts of the world they don't actually vaccinate children against diseases; polio is an example of this. In Pakistan for example, it's a widespread disease in Pakistan because people are not vaccinated against it, so if you go there you need to make sure your vaccination is up-to-date for that particular reason.


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