On 23 June, the UK government announced that three in five adults had received both doses of COVID-19 vaccine. Now, thanks to the rise of variants of concern, such as the Delta variant, they are urging the remaining members of the public who are yet to receive their second doses to do so as soon as they become available.


But why is getting a second dose of vaccine so important? We spoke to Dr Jeremy Rossman, Honorary Senior Lecturer in Virology at the University of Kent, to get the lowdown.

Why do we need two doses of the vaccine?

We need two doses of the vaccines because the first dose primes the immune system. You start to build that immunity to the coronavirus. But it's not quite enough to offer a very robust level of protection against infection. And so you need that second booster jab in order to build and strengthen the immunity to a level that's really good at protecting people from infection. That first dose does help, but you just need a little bit more.

It’s really as simple as that. This happens with a lot of other vaccines, sometimes you need two doses, sometimes you need a whole series, and sometimes you just need one. It does vary from disease to disease and vaccine to vaccine.

The UK government now says I can get my second dose eight weeks after the first. How long should I wait? Is it okay to have your second dose earlier or later than advised?

We want to stick with the timelines that were evaluated in the clinical trials because this is what we know works. The issue is that if you start to shorten that interval between the doses or lengthen it, you run the risk that the vaccines aren't going to induce quite the same level of immunity and you run the risk of a less effective immune response and a less effective vaccination campaign.

It is possible that changing that interval between the doses might actually strengthen the response. But we need to clinically evaluate it. We can't just guess. And so we have to stick to that timeline, which, depending on the specific vaccine, is about three to four weeks. And that's because you need that time to really build that robust immune response from that dose. And then you come back and you amplify it. If you shorten that interval between the doses, you might not have had enough time to build that real immune response.

And so you don't get as much amplification. And conversely, if you came in too late, some of that immune response might have started to fade. And you don't get that same level of amplifying amplification. And it's not hard and fast, there's flexibility in there, but you don't want to shorten it too much and you don't want to lengthen it too much.

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Will I get the same vaccine for my second dose?

This is a really important thing because people have been really wondering how this is going to work, and if it is safe, and if it is effective. We have good reason to think that it would be. But of course, we want to make sure that that is clinically evaluated and that we know that for sure, instead of just guessing that that is what's occurring.

Thankfully, we just had some research that was published the other day that was actually looking at this. And this was a paper that came out in the in The Lancet looking at what happens if you had a first dose of AstraZeneca and the second dose of the Pfizer vaccine. And what they saw was that actually worked very well. And it looked like people built a very strong immune response.

Also the vaccine was quite safe. So it looks like, at least in that combination of starting with one dose of AstraZeneca and then coming back with a booster shot of Pfizer, that that does work quite well. There's still a lot more evaluation that needs to occur. But the evidence so far is that that does work well.

How long after the second dose will I have full protection?

It's about two weeks. You start to get some pretty good protection after the second dose, about one week after you've had it. But really, it's about two weeks in order to get that full amplification of that dose and start to see that robust protection.

How do the side effects of the second dose compare with the first dose?

Typically the side effects are worse on the second dose. Most of the side effects are all about your body building the immune response to the vaccine that is then protective. So things like the pain at the injection site, the fever, the tiredness, the headache, all of this is because your body is having this very large immune response to the vaccine.

It's that immune response that your body's building that then ends up protecting you, so it's a sign that the vaccines are working. But because one dose gives you a little bit of a response and then the second dose amplifies that response that means often for a lot of people, that amplifies the side effects. And it's not side effects as in something that is going wrong, it's just a consequence of building a really strong immune response.

What happens if I caught coronavirus after my first jab?

What people are recommending is that you still get the full vaccine series, even if you've had a coronavirus infection. The reason for that is that we know that if you're infected with the virus and recover you build antibodies, you build an immune response and then that can be protective.

But that response can fade and it varies from person to person as to how protective it is. And so that's why there is this recommendation to continue the vaccine series. Now, if you had one dose and then got infected you have to make sure that you've fully recovered from that before you go and get the second dose.

However, we do know that people who were infected that then get one dose in that order do build a very strong immune response. There is some evidence that they might be able to actually get by with just one dose and the natural infection.

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How about the risks of blood clots following the second dose?

Most of what we were seeing was with the first dose, because in some people the body's response to the vaccine sort of goes a little bit skewed compared to most people, because they have this reaction that causes this clotting disorder.

So typically, if you're going to see that, you would see that relatively early on. And when you start to get large numbers like we're seeing with how many people have been vaccinated worldwide, you do start to have individual instances of people having unusual responses. But these are incredibly low numbers. Typically, if you're going to see something, you will see it on the first dose. But that doesn't necessarily mean that can't happen. It's just not typical.

Will a second dose help to protect me from the Delta variant and other variants of concern?

It's going to vary from variant to variant. We know that each one is slightly different. With one dose, we know that the vaccines are significantly reduced in efficacy against the Delta variant, but with two doses, it's much better. So there are two components to this. There's the efficacy that is preventing you from getting infected and having any sort of symptomatic disease. But there's also the efficacy of preventing severe or fatal COVID disease.


The vaccines are extremely good at preventing severe and fatal COVID disease, even though they are slightly reduced in the protection against getting infected. So, for instance, the AstraZeneca vaccine is still about 92 per cent effective against hospitalisation for the Delta variant. So that's really good news. The efficacy of preventing any infection at all is lower, down in the in the 70 per cent range, but it's still decent. But the important thing is that the vaccines are still working really well at preventing that severe and fatal COVID illness.

About our expert, Dr Jeremy Rossman

Jeremy is an Honorary Senior Lecturer in Virology and the President of Research-Aid Networks, University of Kent. His research focuses on the process of infectious disease outbreaks.

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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.