People with high levels of T cells from common colds are less likely to catch COVID, according to a new peer-reviewed study.
Researchers said the findings could help provide the blueprint for the production of new vaccines which give longer-lasting immunity and would protect against current and future coronavirus variants such as Omicron and Delta.
Imperial College London researchers say the high levels of T cells and the role in fighting COVID is an "important discovery" - but warned "no one should rely on this alone" and insisted people should still get vaccinated as the "best way" to protect against COVID.
T cells are a type of white blood cell that help protect the body from infection.
Dr Rhia Kundu, first author of the study, from Imperial's National Heart & Lung Institute, said: "Being exposed to the SARS-CoV-2 virus doesn't always result in infection, and we've been keen to understand why.
"We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses like the common cold, can protect against COVID-19 infection.
"While this is an important discovery, it is only one form of protection, and I would stress that no one should rely on this alone.
"Instead, the best way to protect yourself against COVID-19 is to be fully vaccinated, including getting your booster dose."
The study started in September 2020 when most people in the UK had not been infected with COVID-19.
It included 52 people who lived with someone who had been exposed to COVID, which had been confirmed through a PCR test.
The participants did PCR tests at the outset and then at four and seven days later to determine if they developed an infection.
T cells target internal proteins
Blood samples from the 52 people were taken within six days of them being exposed to the virus to enable the researchers to analyse the levels of pre-existing T cells induced by previous common cold coronavirus infections that also cross-recognise proteins of COVID.
The researchers found that there were significantly higher levels of these cross-reactive T cells in the 26 people who did not become infected, compared to the 26 people who did contract COVID.
The study - published in Nature Communications - said the T cells target internal proteins within COVID, rather than the spike protein on the surface of the virus.
Current vaccines do not induce an immune response to these internal proteins.
Professor Ajit Lalvani, senior author of the study and Director of the NIHR Respiratory Infections Health Protection Research Unit at Imperial, said: "Our study provides the clearest evidence to date that T cells induced by common cold coronaviruses play a protective role against SARS-CoV-2 infection.
"These T cells provide protection by attacking proteins within the virus, rather than the spike protein on its surface.
"New vaccines that include these conserved, internal proteins would therefore induce broadly protective T cell responses that should protect against current and future SARS-CoV-2 variants."