A crucial type of defensive blood cell persists for at least six months in people after Covid-19, even in those who had no symptoms, in a new study that may ease concern about waning immunity and its implications for a vaccine.

The research on 100 people shows that all had T-cell responses against a range of the coronavirus’s proteins, including the spike protein used as a marker in many vaccine studies, after half a year. Those who experienced symptoms had levels that were at least 50% higher than those who didn’t. As a handful of vaccines nears the finish line, it’s still unclear how long any protection they afford would last. A small number of patients have fallen ill with Covid-19 twice.

“This is promising news,” said Fiona Watt, executive chair of the UK’s Medical Research Council. “If natural infection with the virus can elicit a robust T-cell response, then this may mean that a vaccine could do the same.”

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A previous study, published last month by Imperial College London, raised concern that immune defenses may wane, as it showed that the percentage of Britons with antibodies declined over time. T cells aren’t antibodies. They are white blood cells that can remember past diseases, kill virus-infected cells and rouse antibodies to marshal defenses when they are needed. People infected with another coronavirus that was responsible for the SARS epidemic in 2003, for example, still have a T-cell response to the disease 17 years later.

The study, from the a group of immunologists from 17 universities called the UK Coronavirus Immunology Consortium, has not yet been peer reviewed. It may be the first to show that a robust cellular memory against the virus persists for this long, the authors said.

None of the patients whose blood and serum samples were studied had been hospitalized with Covid-19.

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