Covid-19 patients have cellular immunity six months after infection

Covid-19 patients still have cellular immunity against the coronavirus six months after infection, encouraging study shows

  • A group of more than 2,000 people working for PHE volunteered for the study 
  • 100 people tested positive, 56 had symptoms, and none were hospitalised  
  • All infected participants had a detectable T cell response six months later
  • Scientists say the findings are encouraging that people who have been infected before at less at risk of getting Covid-19 again 

Covid-19 patients maintain a form of immunity against the coronavirus for at least six months after infection, a new study shows. 

The findings may mean people who have already had the virus are less likely to get reinfected if they come into contact with the virus again. 

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. 

The study focused on a specific type of immune response, called T cells, which are created by the body following infection. They are different to antibodies but are just as pivotal in fighting disease. 

The scientists behind the research call their findings encouraging and are ‘cautiously optimistic’ there is long-lasting and robust immunity following coronavirus infection. 

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This graph shows the cellular immune response in 100 infected people after six months, The blue bars are level of T cells targeting the viral spike. Blue and purple show other proteins which induce an immune response 

A T cell response was detected in all patients, with people who expressed symptoms creating around 50 per cent more T cells than in asymptomatic patients. Pictured, a graph showing the heightened immune response in symptomatic patients 

The UK Coronavirus Immunology Consortium (UK-CIC) worked with PHE on the study and asked colleagues to be tested for the virus and take part in the study. 

It has not yet been peer-reviewed and is due to be published to the server bioRxiv soon.  

Of the 100 people who tested positive for the virus, 77 were women and the average age was 41. None of the infected participants were older than 65. 

The blood samples were taken directly to a lab where they underwent a range of tests to determine the level of immune cells to various proteins found on the virus.

This included cells which target the viral spike on SARS-CoV-2 which allows the coronavirus to infect human cells as well as other proteins found in and on the virus. 

All 100 people in the study had detectable T cells after six months. However, the level of antibodies in some participants had dropped below detectable levels. 

Speaking today at a press briefing, Professor Paul Moss, study author from the UK CIC and Dr Shamez Ladhani, study author and a consultant epidemiologist at Public Health England, say this does not necessarily mean antibodies have vanished.  

Instead, it is more likely the antibody levels have simply dropped below what the current assays can detect. 

The researchers add that the level of antibodies needed to fight off a repeat infection remains a mystery.

Antibody concentration below the detection sensitivity of the current tests may well be sufficient, they say. 

A study from PHE found T cells – a type of white blood cell in the immune system – are produced by everyone infected with the coronavirus (stock)

Until it is know what specific level of antibodies is needed to fight off infection, it will be impossible to say if this decline and gradual plateau of antibody levels is a cause for concern or not. 

‘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,’ says Dr Ladhani. 


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.

However, a T cell response was detected in all patients, with people who expressed symptoms creating around 50 per cent more T cells than in asymptomatic patients. 

The reason for this is unclear. One explanation could be that people who had symptoms created a greater immune response and therefore may be less likely to get infected again. 

However, another plausible reason is that asymptomatic individuals are inherently better at fighting off the virus and are therefore less likely to get reinfected. 

‘To our knowledge, our study is the first in the world to show robust cellular immunity remains at six months after infection in individuals who experienced either mild/moderate or asymptomatic COVID-19,’ Professor Moss says. 

‘Interestingly, we found that cellular immunity is stronger at this time point in those people who had symptomatic infection compared with asymptomatic cases.

‘We now need more research to find out if symptomatic individuals are better protected against reinfection in the future.’ 

The findings have implications for public health protocols but also for vaccine development. 

Of the T cells the study looked for, those which target the viral spike were very common. This spike is the crux of most vaccines in development. 

However, the research also revealed other proteins which are abundantly targeted by the T cells, indicating vaccines that target other aspects of the virus could be effective. 

Although the impetus is on the viral spike for many vaccines, there are some potential vaccines looking at other aspects of the virus. 

‘I want to emphasise that vaccination trials are studying cellular immunity and data so far shows that this is achieved with the current regimes that are in use,’ Professor Moss says. 

‘It is not the case that this is neglected by vaccine trialists,’ Professor Moss says. 

The researchers say that while this study is encouraging and implies people have come cellular immunity to the virus, it does not mean people can not contract Covid-19 twice.

‘It does not mean you can not get re-infected…’ Professor Moss says.

‘This can not be taken as confirmation of an immunity passport. Absolutely can not do that.’ 

Professor Charles Bangham of Imperial College London, who was not involved in the research says it is an excellent study which ‘provides strong evidence that T-cell immunity to SARS-CoV-2 may last longer than antibody immunity’.

‘These results provide reassurance that, although the titre of antibody to SARS-CoV-2 can fall below detectable levels within a few months of infection, a degree of immunity to the virus may be maintained,’ he says. 

‘However, the critical question remains: do these persistent T cells provide efficient protection against re-infection?’

The study authors hope to address this question in future, with larger and more tailored studies which can determine if the cellular immunity is effective at fending off infection.


Scientists have found SARS patients still have crucial disease-fighting cells 17 years after infection. 

SARS — another type of coronavirus very similar to the one that causes Covid-19 —was behind an epidemic that predominantly struck Asia in 2003. No cases have been identified for 15 years.

But some infected during the outbreak still have key white T cells, suggesting they would be protected from ever getting re-infected.   

The study led by Duke-NUS Medical School, in Singapore, published in the scientific journal Nature this week,  involved 23 SARS patients.

They collected blood samples and tested whether they still harboured any immune cells that were effective against SARS.

Investigations showed all patients ‘still possess long-lasting memory T cells’ reactive to the virus.

The findings ‘support the notion that Covid-19 patients will develop long-term T cell immunity,’ the researchers wrote. 

This could be significant for vaccine research because it helps scientists understand how long a vaccine would protect a person before before another booster shot is needed. 

In further experiments, the scientists mixed blood samples with fragments of SARS-CoV-2 to see what happened. 

The cells showed ‘robust’ reactivity against SARS-2 in all patients by latching on to them.

To explore the subject of immunity further, 37 volunteers who had never been infected with either SARS-1 or SARS-2 were recruited.

They wanted to see if infection with other human coronaviruses that have been around for centuries offered some kind of protection against Covid-19.

The researchers found ‘remarkable’ levels of T cells able to latch on to the Covid-19 virus in 50 per cent (19) of the participants.  

‘Surprisingly, we also frequently detected SARS-CoV-2 specific T cells in individuals with no history of SARS, Covid-19 or contact with SARS/Covid-19 patients,’ the authors wrote. 

They believe these T cells may exist due to previous infection with another coronavirus — of which there are seven that can infect humans.

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