COVID vaccines: delayed booster may help

It's been a week of Covid contrasts. On the one hand we mourned the passing of Captain Sir Tom Moore, whose 100 laps of his care home raised millions for the NHS; and at the same time we marked a vaccine milestone. More than 10 million people across the UK have received at least one vaccination for coronavirus. And one of those vaccines, AstraZeneca's, seems - based on a preliminary report - to do more than just prevent severe disease: it stops people spreading Covid as well. The report claims that nearly 70% fewer infections were picked up in people who'd had their first dose of the jab; which is good news because it suggests that as well as being protected themselves, vaccinated people are also helping to protect others by being less likely to pass on the infection.

Also this week, the government’s vaccine minister Nadhim Zahawi announced a new £7M study to look at the effect of using two different vaccines together. Mixing doses is likely to become a priority in the future, not least because - owing to the way they interact with the body - some of the vaccines we're using might not work more than once. The other big question that still remains unanswered is whether we've got the gap between doses right; the UK has introduced a policy of waiting 12 weeks to maximise the number of people who can be protected in the first instance. This was the first question Chris Smith put to immunologist Peter Openshaw when they spoke about the vaccine initiative earlier this week. He thinks that giving jabs too close together would be a bad idea...

Peter - Sometimes by giving too much of a thing, you can actually drive down the immune response. The new evidence coming out generally shows that you do get quite good protection from that first dose, which lasts all the way through until the three months' second dose. And indeed you may end up with a better immune response as a result of having delayed the second dose.

Chris - The data that we've got to go on - that's informing what you're saying - is based on AstraZeneca's vaccine. Does the same apply then to the Pfizer vaccine? Because one criticism people are leveling at the current strategy is that the Pfizer vaccine and the AstraZeneca vaccine are two quite different things, and therefore to assume that what goes for one will apply to the other is a misstep.

Peter - Some of the investigators have said that they don't think it's going to make any difference whether you make the spike protein of the coronavirus one way, or make it another. I think that's a reasonable hypothesis, but it absolutely needs to be tested. The other point to make is that the vaccines were developed at great speed. The very short interval between the first dose and the second dose was decided on largely in order to make sure that the vaccine studies could be done very fast. That doesn't mean to say that that was necessarily the best interval between the first dose and the second dose. And I've certainly... in vaccine studies that I've done myself, I've found that sometimes by giving the second dose too quickly, you can actually get a much weaker immune response.

Chris - A question which is also surfacing a lot is...people have got an eye on what's been happening in Europe, they've got an eye on the fact that the current bottleneck with vaccination appears to stem from a supply problem. We just can't get enough vaccines in the door fast enough. Therefore people are legitimately saying, "well, if I can't access a second dose of the thing I had the first time, and I have to use a different vaccine product, is that necessarily a bad thing?"

Peter - That actually might be a perfectly good strategy. There's many examples of vaccination regimes where a different vaccine has been used in the first and the second dose. And indeed some of the vaccines can't actually be repeated again and again, because they’re all within an adenovirus vector. So with some of these vaccines, it is actually a big advantage to be able to prime with one and boost with another.

Chris - So what you're saying is that because, say, AstraZeneca's vaccine is delivered by a disabled virus, to actually get the message about the spike of the coronavirus into the body in the first place, you're going to get an immune response to that disabled virus that's the Trojan horse. Are the government potentially in trouble then, if they need to revaccinate people, they wouldn't be able to use it - in people who've already had AstraZeneca vaccine, they couldn't use it again?

Peter - Yes, that is a potential problem. And I think it illustrates the reason why it's so good to have a rich pipeline of different types of vaccines.

Chris - How is it looking in terms of how the vaccines are thought to protect against the current slew of variants that we're seeing emerging in various geographies around the world?

Peter - Well, at the moment, things are looking quite good in terms of our own homegrown variant. It still seems to be susceptible to the immune response, but I think we're all expecting that over time, the main driver for evolution of the virus will become the host immune response, either due to vaccination or due to previous exposure to the virus. When that becomes the driving force, then we are going to have to actually reformulate the vaccines to match the currently circulating strains.