Immunity after mild Covid-19 infection has the potential to last for years, a small study in the US suggests.
Early evidence from US researchers shows that while initial serum antibody levels may wane in someone who has been infected by Covid-19, cells in the bone-marrow may be primed to keep producing them in the longer-term.
In response to some studies suggesting that protective immunity against Covid-19 may be transient, the research team at Washington University in St Louis measured antibody production over time in 77 people who had recovered from mild illness.
Using blood samples, they found – as others have also shown – that Covid-19 antibodies dropped off in the four months after infection before declining more slowly with some still detectable 11 months later.
In a smaller group of participants they then collected bone marrow and found low numbers of bone marrow plasma cells formed after the Covid-19 infection.
In 15 of the 18 bone-marrow samples they found ultra-low but detectable populations of specific bone marrow plasma cells formed after coronavirus infections from eight months earlier.
There were none in the 11 control participants, the researchers reported in Nature.
These cells are a ‘persistent and essential source’ of protective antibodies, they explained.
‘Overall, we show that SARS-CoV-2 infection induces a robust antigen-specific long-lived humoral immune response in humans,’ the researchers concluded.
But they said that having a persistent source of antibodies is not the only factor in a durable immune response.
Dr Lizzie Mann, a research fellow at the University on Manchester who has been working on a project looking at long-term immunity, said the study ‘was good news’.
She said: ‘You can have quite low levels of antibodies and have some immunity and then your body starts to produce more when needed.’
But she added: ‘In this T cells have been largely ignored and that is the other half of this protective immunity.’
Dr Mann added that ongoing vaccination would likely still be needed even though this work suggests the body is able to keep mounting an immune response when needed to deal with variants.
She said: ‘In terms of boosters, I suspect even if we have protective immunity for years and years we will need to keep having boosters at least annually and there is already work in progress to alter vaccines. I suspect they will need to be updated regularly.’
This month the Government announced a new UK clinical trial to test seven different vaccines for a booster Covid jab.
The study, which will be led by University Hospital Southampton NHS Foundation Trust, will inform any potential booster jab campaign that will be rolled out later in the year, the Government said.
Data from the Office for National Statistics published this week show that more than three-quarters of adults across most of the UK are likely to have antibodies to Covid-19 either through infection or vaccination.
The ONS estimates, based on blood test samples up to 3 May, suggest 75.9% of adults in England, 76.6% in Wales and 75% in Northern Ireland have antibodies, with 68.6% for Scotland.
Latest figures from 27 May show 73.3% of the adult population have had one vaccine dose in the UK with 45.6% having had two doses.