Immunity is most commonly associated with antibodies, but they aren’t the body’s only line of defence. For some diseases, the body adopts other mechanisms of immunity, including T-cells.
T-cell responses were found in all 100 test subjects six months after they had contracted the CCP virus, or SARS-CoV2, according to a study for the UK Coronavirus Immunology Consortium (UK-CIC), published on Monday. The study has not yet been peer reviewed.
“To our knowledge, our study is the first in the world to show robust cellular immunity remains at six months after infection in individuals who experienced either mild/moderate or asymptomatic COVID-19,” said professor Paul Moss, one of the study authors from the University of Birmingham.
“Understanding what constitutes effective immunity to SARS-CoV-2 is extremely important, both to allow us to understand how susceptible individuals are to reinfection and to help us develop more effective COVID-19 vaccines.”
The study collected serum and blood samples from over 2,000 health care workers, narrowing down to 100 that tested sero-positive for the virus. They were all asymptomatic or experienced only mild or moderate symptoms and had caught the virus in March or April.
Samples were collected monthly and antibody levels measured. Six months later, blood samples were taken to assess cellular (T-cell) response.
“T-cell responses were present in all individuals at six months after SARS-CoV-2 infection,” according to the study statement. “The cellular immune response was directed against a range of proteins from the virus, including the Spike protein that is being used in most vaccine studies.”
The study result is “promising news,” said professor Fiona Watt, executive chair of the 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,” she said.
Shamez Ladhani, a consultant epidemiologist at Public Health England and another author of the study, said that cellular immunity is a potentially “very significant piece of the COVID-19 puzzle”.
He said that more research was needed, but that early results indicate T-cell response may outlast initial antibody response.
In a statement accompanying the results, UK-CIC said that immunity is extremely complex with many different potential routes to immunity.
A study published last week found that antibodies across the population appear to be dropping—but scientists say it didn’t prove the case against stable natural immunity.
Researchers from Imperial College London used DIY finger-prick tests for antibodies on 365,000 people at home over three rounds.
In the first round, 6 percent had antibodies. Three months later, that had declined to 4.4 percent.
Helen Ward, one of the lead authors of the report, said, “This very large study has shown that the proportion of people with detectable antibodies is falling over time.” She added, “We don’t yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19.”
But some scientists say the study may indicate more about the sensitivity of that test than provide meaningful data on numbers of people with antibodies.
Alexander Edwards, an associate professor in biomedical technology at the Reading School of Pharmacy at the University of Reading, said that these rapid tests are only able to detect high levels of antibodies compared to labs, and only give a binary result: positive or negative. Lab tests, on the other hand, can indicate a sliding scale.
“If you use a single cutoff value at a higher signal in these [lab] tests, this would potentially lead to the same conclusion as seen in [the Imperial study], i.e. fewer patients detected,” Edwards said in a statement. “When people are ill, antibody levels rise, and when you heal, antibody levels do drop naturally—this is not exactly the same as losing immunity.”