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COVID-19: People with antibodies ‘ARE protected against reinfection’


People who have previously caught the coronavirus and have antibodies are unlikely to get infected again within six months, a NHS study shows. 

Research from Oxford University Hospitals looked at 12,180 healthcare workers in Oxford hospitals from April through to November. 

Antibody tests revealed 1,246 workers had already caught the virus by the time the study started, but none had symptoms. Only three (0.24 per cent) of these people later tested positive, and none of them went on to develop symptoms. 

Researchers say the study indicates ‘prior SARS-CoV-2 infection offers protection from reinfection, in the short term’. 

Antibody tests revealed 1,246 workers had already caught the virus by the time the study started, but none had symptoms. Only three of these people later tested positive, and none had symptoms 

Regular testing also revealed 89 of the 11,052 people who had not been infected before later developed an infection and symptoms. 

A further 76 members of staff who had not previously been infected then tested positive but were asymptomatic. 

So in total, 168 (1.49 per cent) of those with no prior infection went on to either test positive for the disease or exhibit symptoms.

The study, published online today as a pre-print, shows that people who have already been infected are highly unlikely to catch the virus again.

One of the authors of the paper, Professor David Eyre of the University of Oxford’s Nuffield Department of Population Health, said: ‘This ongoing study involving a large cohort of healthcare workers has shown that being infected with COVID-19 does offer protection against re-infection for most people for at least six months.

A study had previously found that antibody levels drop rapidly following infection and reach half their original number after just 85 days (pictured). However, the latest study is encouraging that the base level of antibodies is sufficient to prevent reinfection


Antibodies are substances produced by the immune system which store memories of how to fight off a specific virus. 

They come in different forms and may attack viruses and destroy them themselves, or may force the body to produce other kinds of immune cells and white blood cells to do the dirty work for them. 

They can only be created if the body is exposed to the virus by getting infected for real, or through a vaccine or other type of specialist immune therapy.

Once antibodies have been created once – the body essentially moulds them around a virus when it encounters one in the blood – the body usually retains a memory of how to make them and which ones go with which virus. 

Generally speaking, antibodies produce immunity to a virus because they are redeployed if it enters the body for a second time, defeating the bug faster than it can take hold and cause an illness. 

Scientists are still unsure on the truth on immunity because Covid-19 has only been around since January – meaning its long-term effects are still unclear.

So far cases of people getting infected more than once have not been numerous nor convincing.

With some illnesses such as chickenpox, the body can remember exactly how to destroy it and becomes able to fend it off before symptoms start if it gets back into the body. But it is so far unclear how long Covid-19 patients are protected for. 

Evidence is beginning to suggest that antibodies disappear in as little as eight weeks after infection with the coronavirus, scientifically called SARS-Cov-2. 

However, antibodies are only one type of substance that can produce immunity. The immune system is a huge web of proteins that have different functions to protect the body against infection. 

T cells — which can’t be detected by the ‘have you had it’ antibody tests — made in response to the infection may offer a form of immunity that lasts several times longer.

T cells are a type of white blood cell that are a key component of the immune system and help fight off disease. 

Other scientific studies have shown people who have had a common cold in the past two years have T cells that show ‘cross-reactive protection’ against Covid-19.

‘We found no new symptomatic infections in any of the participants who had tested positive for antibodies, while 89 of those who had tested negative did contract the virus. 

‘This is really good news, because we can be confident that, at least in the short term, most people who get COVID-19 won’t get it again.’

A study had previously found that antibody levels drop rapidly following infection and reach half their original number after just 85 days. 

The decline continues and the amount of antibodies falls below detectable levels in most people 137 days after their peak reading.

But Professor Eyre says the latest finding shows ‘there is some immunity in those who have been infected’. 

After the previous study, concerns were raised that antibody levels may drop too low and recovered Covid-19 patients would be vulnerable to reinfection. 

However, it is possible the reduction in antibody levels doesn’t mean they vanish, just that they drop below the detectable level. 

Also, if the concentration of antibodies is extremely low, the immune system has a memory function, called B cells, which remembers how to make the antibodies and churn them out rapidly if it recognises the virus again. 

The Oxford clinicians say it is too soon to say whether or not there is long-term protection beyond six months, but call the findings ‘encouraging and exciting’. 

Dr Katie Jeffery, Director of Infection Prevention and Control for Oxford University Hospitals said: ‘This is an exciting finding, indicating that infection with the virus provides at least short-term protection from re-infection – this news comes in the same month as other encouraging news about COVID vaccines. 

‘I would like to thank all our staff who have shown great commitment in attending our clinics for repeated swab and antibody testing in order to keep our patients and each other safe.’ 

A separate study, published earlier this month, looked at the complete cellular response in Covid-19 patients following infection.  

A group of more than 2,000 people working for Public Health England volunteered to take part in the study and donate blood every month, with the first people recruited in early March, before lockdown was announced. 

A total of 100 people tested positive for SARS-CoV-2, the virus which causes Covid-19, but none were hospitalised. More than half (56 per cent) had symptoms. 

All 100 people had high Covid-specific T cells six months later, mirroring the findings from the latest antibody study, but the antibody number in this cohort had dropped below detectable levels. 

Dr Shamez Ladhani, co-author of this paper, said at the time: ‘Early results show that T-cell responses may outlast the initial antibody response, which could have a significant impact on COVID vaccine development and immunity research.’

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