'Killer cells' may guard against COVID-19

The T-cell response to COVID-19 infection remains active for as long as six months in most adults, new research from the United Kingdom has shown, raising hopes that this aspect of immunity to the novel coronavirus may outlast the more transient response seen from antibodies.

Scientists in the UK looked at levels of T-cell activity in 100 people who had tested positive for COVID-19 back in March and April. The researchers detected this key immune mechanism six months after infection in all of the study participants. None of the participants had been hospitalized, and some were asymptomatic.

In terms of the longevity of immune response, the results compare favorably to several separate studies that suggest COVID-19 antibodies linger for just a few weeks or months after infection is cleared.

Two forms of white blood cells known as T-cells and B-cells control important immune mechanisms in the body. B-cells produce antibodies that engage the virus itself, while T-cells earn the name "killer cells" by attacking infected cells.

Both T-cells and antibodies can help prevent reinfection and are important indicators of immunity to disease.

"Early results show that T-cell responses may outlast the initial antibody response, which could have a significant impact on COVID-19 vaccine development and immunity research," said Shamez Ladhani, who is a consultant epidemiologist at Public Health England and an author on the new study.

The research was published this week on the preprint server bioRxiv and carried out by Public Health England, the Manchester University NHS Foundation Trust, and the UK Coronavirus Immunology Consortium, or UK-CIC.

Paul Moss, who is the UK-CIC lead at the University of Birmingham, said the study is the first in the world to show that robust cellular immunity remains for an extended period after infection in individuals who experienced either mild, moderate or asymptomatic COVID-19.

"Interestingly, we found that cellular immunity is stronger at this time point in those people who had symptomatic infection compared with asymptomatic cases," said Moss. "We now need more research to find out if symptomatic individuals are better protected against reinfection in the future."

In September a major study of 365,000 people in England revealed that around a quarter of those who had antibodies against novel coronavirus during the summer were no longer positive, "suggesting antibodies reduce in the weeks or months after a person is infected", according to a statement from the UK Department of Health and Social Care.

Researchers behind this antibody study and others provide the caveat that the immune system is complex, and cite T-cell response as a potential avenue through which the body might achieve longer lasting protection against the virus.