Previous common cold infections could provide protection against Covid-19, a study has shown, offering clues to how medicines should be developed to better tackle the disease.
Exposure to other coronaviruses may also help speed up the clearance of Covid-19, the findings suggest.
Researchers said the next generation of Covid-19 vaccines should aim to induce an immune response against specific proteins that are essential in the earliest stages of the viral cycle.
By designing medicines that activate immune memory cells, known as T cells, to attack infected areas, it may be possible to eliminate Covid-19 at the start of an infection, helping to stop its spread.
This could complement Covid-19 vaccines currently licensed in the UK, which trigger immune responses to a protein.
The study, carried out by University College London and published in the journal Nature, tracked more than 750 healthcare staff, who were working closely with Covid-19 patients and subjected them to regular tests.
A group of 58 people, who never tested positive for Covid-19 , showed a marked increase in T cells, which is a key component of the immune system at work and offers protection against a range of coronaviruses.
"The regions of the virus that these T cells recognise are highly conserved amongst other members of the coronavirus family, such as those that cause common colds every year,” said lead author Dr Leo Swadling.
"Previous common cold exposure may have given these individuals a head start against the virus, tipping the balance in favour of their immune system, eliminating the virus before it could start to replicate."
Researchers said the T cells discovery could lead to the creation of a vaccine for all coronaviruses.
Senior author, Prof Mala Maini, said: "Our research shows that individuals who naturally resisted detectable Sars-CoV-2 infection generated memory T cells that target infected cells expressing the replication proteins, part of the virus's internal machinery.
"These proteins - required for the earliest stage of the virus's life cycle, as soon as it enters a cell - are common to all coronaviruses and remain 'highly conserved', so are unlikely to change or mutate.
"A vaccine that can induce T cells to recognise and target infected cells expressing these proteins, essential to the virus's success, would be more effective at eliminating early Sars-CoV-2, and may have the added benefit that they also recognise other coronaviruses that currently infect humans or that could in the future.”