COVID immunity: 10 months after the vaccine

What do we now know about vaccine-induced immunity against COVID-19?
07 September 2021

Interview with 

Sarah Walker, University of Oxford


A needle and bottle of the COVID-19 vaccine.


10 months after the first COVID-19 vaccine was approved in the UK, about 80% of the adult population are now fully vaccinated. What effect has this had on the transmission rates and disease reates from the new coronavirus? Chris Smith spoke to Sarah Walker, who leads the national COVID-19 infection survey, to find out...

Chris – So lots to get our head around: 10 months on what do we now know about the immunity that we get against the new coronavirus from vaccination, or even from infection, and how good are these vaccines at actually protecting us? How long does that immunity last and is our vaccination strategy the right one? Could we even build better vaccines that might improve on our present situation? Well, first let's begin with the situation as it stands. Here in the UK, about 80% of the adult population are now fully vaccinated. So what effect has that had on the transmission rates and the disease rates from the new coronavirus? Sarah Walker leads the national COVID-19 infection survey. What's the impact been, Sarah?

Sarah – I think the impact is really impossible to underestimate. Last week we estimated 1.4% of the English population were COVID positive - that's the same rate as we saw at the end of December and January at the beginning of this year, when over a thousand people were dying every day from COVID. And yet, in the hospitals, there are COVID patients, but things are holding steady. So the vaccines have been enormously effective in preventing severe disease and death.

Chris – The government were quite keen to emphasise that through vaccination the relationship between catching coronavirus and getting severe disease had been weakened, but not completely abolished. It's at least 10 times lower though, isn't it? You're very well protected against severe disease if you've been vaccinated.

Sarah – Yes, absolutely. I would stress, however, that it is something that we do need to keep an eye on. What we're also finding is that your protection from getting infected again does seem to be changing a bit with time. And obviously that's the first step to severe disease. So at the moment there's no cause for concern, but we do need to watch it carefully.

Chris – What are your figures telling you? When you look at a vaccinated population like the UK and the circulation of the infection, what do we think the performance of the vaccines is in terms of stopping the infection spreading and the difference between severe disease and just catching and transmitting on the disease?

Sarah – There are two key components. Firstly, with Delta, the vaccines are not working quite as well as they did against Alpha, but they are still doing a good job, particularly with two doses. But what we do see is that if you do get infected despite being vaccinated, which you have a much lower chance of doing, you have still got as much virus in your nose and throat as people who haven't been vaccinated. And that means, at least potentially, you've got the same chance of passing it on.

Chris – Is that what you're seeing? Do the figures bear out the idea that there are people who are catching it and transmitting it despite vaccination?

Sarah – We see highest positivity rates still in the youngest people who were mostly unvaccinated, but we certainly do see positivity rates rising in older people. And I would stress that if people didn't carry the virus for as long if they'd been vaccinated, we would expect to see lower levels in the survey, because we test people at random. If you've got a high virus level for a shorter period of time, by chance, we should pick you up less likely when you've got a high virus level. And that's not what we see. We see similar levels of virus in people vaccinated and not vaccinated who happened to get infected.

Chris – What do you foresee happening with those sorts of findings as we go into autumn and winter, when traditionally all kinds of infections spread much better because we spend more time indoors together, breathing the same air, the days are shorter, so people tend to be indoors for longer, and it's colder, so people tend to ventilate their spaces less well?

Sarah – Given that we also see that how effective these vaccines are seems to be declining a bit with time, I think it's inevitable that we will continue to see high levels of positivity, but what really matters is keeping people out of hospital and stopping them dying. I don't think anyone now thinks we're going to eliminate COVID - we've got to find a way to live with it. And that probably does mean a reasonable number of infections. We just need to make sure the NHS is still able to function and that people don't end up dying of COVID.

Chris – Do you think, to an extent, that the fact there's high levels of circulation in the population means that some people's immune systems are being reminded, because they're bumping into people who've currently got the infection and that's almost doing the government's booster programme for them?

Sarah – It's a really good point, and we certainly saw that people who had had COVID before did even better if they then got vaccinated. So vaccination and having had COVID before was better than either vaccination alone or having had COVID before alone. I think the really critical question is this issue about making sure that people avoid severe disease, and certainly for the clinically extremely vulnerable, there is still a risk there. There are still 100 people a day dying from COVID, predominantly unvaccinated, but also some people with comorbidities and the elderly. So I think the risk is always going to be there, but there's also potentially a benefit from reminding the immune system what COVID is and giving you that boost.

Chris – When you say that immunity is waning with time, is that just because we're measuring the levels of antibody we can detect in people and we're noting that the levels are dropping? Is that necessarily translating into correspondingly more cases of severe illness? Because we know the immune system works by having a memory that underpins the production of things like antibody, so even though the antibody levels might go down, you've still got that memory as a sort of foundation that your immune system can dip into in a hurry and remobilise its resources against a threat that it meets. So is it not possible that, actually, it doesn't matter that people's antibody levels are coming down a bit? They've got that immune memory, crucially, and that could protect them in the months, to possibly years, ahead.

Sarah – What we found in our study was actually looking at new infections, so it wasn't just about antibodies declining, it was actually about the rates at which people were testing positive again with COVID. But you're absolutely right, the critical thing is to avoid ending up in hospital. If you test positive again, but you avoid hospitalisation, that may well be the best outcome that we can hope for. But it is something that we just need to watch carefully, because if you look at what's happened in Israel, they have seen very abrupt increases in hospitalisations, which is obviously what we all want to avoid.

Chris – Can that be attributed to a decline in immunity? It's not down to the arrival of the Delta agent making people more sick?

Sarah – It doesn't seem to be. Certainly our study done in the UK is when we've had a lot of Delta around. I think there are some really interesting hypotheses about the fact that in Israel they used the very short dosing interval in a lot of people and whether that has also contributed, because studies have shown that actually spacing out the vaccines a bit does give your immune system more time to react better to the second dose. That may be a part of the story.


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