Hopes of lasting Covid-19 immunity were raised today after scientists found SARS patients still have crucial disease-fighting cells.
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 research, by scientists in Singapore, offers hope the same may be true for SARS-CoV-2 — the name of the coronavirus behind the pandemic.
With some viruses, such as chickenpox, protection is life-long and it not possible to become sick again. But others, like the common cold, protection is short lived.
Experts are still baffled as to how long immunity against Covid-19 lasts for because it has only been around since December 2019. Antibody studies have suggested it may only offer short-lived immunity, like other coronaviruses.
But 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.
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.
Singaporean researchers have found patients who had SARS have immunity for up to 17 years. It offers hope the same could be true for the closely related coronavirus which is causing the current global pandemic, called SARS-2. Pictured, an illustration of a coronavirus being attacked by immune cells
They collected blood samples and tested whether they still harboured any immune cells that were effective against SARS.
All patients had T cells, which suggest they are an important part of fighting off this infection.
T cells are a type of white blood cell that are a key component of the immune system and help fight off disease.
WHAT ARE T CELLS?
When the body is invaded by bacteria, a virus, such as SARS-CoV-2, or parasites, an immune alarm goes off, setting off a chain reaction of cellular activity in the immune system.
T lymphocytes (T cells) are white blood cells that are a major part of the immune system.
They are part of the adaptive immune system, which is considered the more specialised response. They use past interactions to remember foreign threats and how to attack them.
The adaptive immune system kicks in after the innate immune system, which is the immediate response to a virus, or 'first line of defense'.
There are several different types of T cells, some of which their functions are not understood.
Killer T cells directly kill cells that have already been infected by a foreign invader, while helper T cells stimulate other parts of the immune response - such as B cells.
B cells are also part of the adaptive immune system. They help make antibodies which are specific to each pathogen.
Antibodies are able to block the novel coronavirus by attaching to it and marking it for destruction by other immune cells.
'Killer T cells' fight off a pathogen, while 'helper T cells' signal for action in other parts of the immune system when they see a cell has been invaded.
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.
It couldn't be ruled out that the T cells could come from other animal coronaviruses — mainly thought to originate in bats — rather than human coronaviruses.
Lead researcher Dr Antonio Bertoletti told The Telegraph these T cells may be more common in Asia than the rest of the world.
He did not divulge why, but he may have been referring to the fact zoonotic diseases — those that jump from animals to humans — have emerged more frequently in Asia.
It could explain why the continent has been relatively unscathed compared to Europe and the US during the Covid-19 pandemic.
The team said strong immunity towards closely-related viruses could reduce susceptibility or severe disease in relation to SARS-2.
But it is not known for sure that these T cells would protect someone if they were exposed to Covid-19.
Either way, the researchers said this 'pre-existing' protection should be considered on a population scale, because it could mean less people are at risk of getting sick with Covid-19 than thought.
Scientists at the La Jolla Institute for Immunology in California have also previously said it is 'tempting to speculate' that having had a cold could offer some form of immunity.
They found blood samples from people donated in 2015-2018 contained T cells that recognised SARS-Cov-2 and responded to it, publishing their findings in the journal Cell.
Researchers at the La Jolla Institute for Immunology in California analysed blood samples from recovered COVID-19 patients (left) and frozen blood samples from people two years ago. They found both had T cells that recognise SARS-CoV-2 and some that are able to kill it
T cells have become an increasingly discussed topic during the Covid-19 pandemic, once overshadowed by the interest in antibodies.
T cells are part of adaptive immunity, the first line of defence against a foreign substance. On the other hand, antibodies, triggered into action by B cells, don't start helping until a few days later.
They may not be needed if T cells and other parts of the immune system have already wiped the virus out.
For that reason, some people may not have antibodies against Covid-19 in their blood despite being infected with the coronavirus in the past, scientists say.
The Singaporean team said their previous research discovered antibodies for SARS-1 were undetectable after two or three years.
Therefore, antibody testing would suggest they could be infected again, when actually they are protected by T cells.
In terms of SARS-2, Professor Karol Sikora, an advisor to the World Health Organisation, previously told MailOnline he believes only 10 per cent of people who are infected with the coronavirus develop antibodies.
The Karolinska Institutet and Karolinska University Hospital, in Sweden, recently showed that many people with mild or asymptomatic Covid-19 demonstrate T-cell immunity to the disease and do not test positive for antibodies.
The research would suggest that immunity among the population is higher than antibody testing reveals.
Some parts of the immune response remain a complete mystery to scientists and are unable to measure.