How long do Covid-19 antibodies last?

Cases of coronavirus are surging again across Europe. In France Emmanuel Macron has announced another lockdown; Germany are hot on their heels with a lockdown “lite”, and Belgium currently has the world’s highest per capita rate of coronavirus infections. And England has recently heard news of another lockdown coming into effect on 5th November. The sombre situation looked gloomier still earlier in the week when a study of the population’s Covid antibody levels was published. We had hoped early in the pandemic that testing for antibodies would reveal that many people had already had the new coronavirus, perhaps without even realising it, and that they would remain immune.Unfortunately those tests showed that, so far, fewer than 10% of us appear to have been infected, which is too low to make a difference to how the virus spreads in the community. More damning still were the follow up study results out this week looking at how long people are hanging onto those antibodies afterwards: they fall away fast, meaning people may well become susceptible again to infection within just months. So, alarm bells began to ring for many who were concerned about the implications of the findings for a Covid-19 vaccine. Chris Smith spoke to Wendy Barclay…

Wendy - This study is called REACT. It's a huge study in which we ask people in the United Kingdom to do a home test. So we posted out one of these finger prick blood tests, and people did that in their own home, took a photo and sent it back to us. And over the three rounds that we did this in July, August, and September, more than 350,000 people did that.

Chris - And when you then compare the results that come back, this is really a yes or no answer, "have you got antibodies against coronavirus?" what came back?

Wendy - In the first round we found that 6% scored positive on the test. It's a yes-no answer. As you say, and that means yes, they had antibodies to the SARS-Cov2 virus. We've already checked this test is quite specific. That means that sometime between January and July, they will have been infected with the virus. When we looked again in August and September, the percentage of the random people that had been selected for those rounds who had a positive score in the test was lower. And by the time we got to the beginning of September only 4.4% of the people had those antibodies, which is if you calculate it, about a 26% decrease.

Chris - Does a negative test then therefore mean a person can regard themselves as back to immunological "point-zero", or do you think they still have some degree of immunity? So were they challenged again with the virus, their immune system would actually have a head start?

Wendy - I think there are two questions here to sort of pull apart. One is how much antibody does a person need in their blood to be considered immune. There are some studies in non-human primates where vaccines have been given and then animals have been challenged and you can begin to see how much antibody didn't confer protection. So then you can begin to sort of put a benchmark in. And we actually think that there's quite a good correlation between the point at which you cross the threshold of no longer scoring positive, and the titre of antibody that you would need to have in order to be considered immune. So from that point of view, it's quite worrying because it does say that once a person no longer scores positive, they haven't got the amount of antibody that we think they need to be protected. But on the other hand, as we all know, your immune response has memory, and once you've seen a pathogen, once you are in a better place to respond to it again. And without a doubt that prior exposure to SARS-Cov2 ought to mean that most people, even if they've lost their antibody that's circulating in their blood, have a head start, which probably means they might get infected, but hopefully less severely so.

Chris - What would be the implication then for a vaccine?

Wendy - We have to remain optimistic. Vaccines work in completely different ways from natural immunity, most viruses have evolved ways of evading their hosts immune response. It's a hypothesis that I have that coronaviruses may be quite good at stocking the antibody response working as well as it might otherwise do. And in that case, it might fall away more quickly perhaps than a vaccination with a very powerful vaccine could lead to longer lasting antibodies.