Antibody tests showed no positive results in former COVID-19 patients even after having recovered after a few months. This raised the question of to what extent people build up long-term immunity after having been infected with the novel coronavirus. Or whether it is possible to become infected more than once. Researchers from the UK Coronavirus Immunology Consortium (UK-CIC), the Public Health England, and the Manchester University NHS Foundation Trust have now conducted a study to examine the role of T cells in immunity to SARS-CoV-2 six months after becoming infected.
In this study (which was published in a preprint on bioRxiv,) the scientists collected blood samples from more than 2,000 clinical and non-clinical healthcare workers, including 100 people who tested seropositive for SARS-CoV-2 in March/April 2020. All 100 people showed either mild to moderate symptoms or were asymptomatic. None needed hospital treatment.
T cells present after six months
Antibody levels were measured in serum samples that were collected on a monthly basis. After six months, blood samples were taken to assess the cellular (T-cell) response to SARS-CoV-2 and various proteins of the virus. These cellular analyses are much more complex to perform than antibody studies. This study is one of the largest ever in the world in this field and involved 100 people.
The researchers found that T-cell responses were present in all people six months after a SARS-CoV-2 infection. “The cellular immune response was directed against a number of proteins in the virus, including the spike protein used in most vaccine trials,” the researchers explain. “However, a comparable immune response was also found against other proteins, such as the nucleoprotein, suggesting that these could be useful for inclusion in future vaccine protocols.” All this appears as if a robust cellular memory against the virus persisted for at least six months.
However, there were significant differences in the extent of T-cell responses between individuals. It was 50% higher in people who had experienced a symptomatic illness six months earlier at the time of infection. The researchers stress that more research is needed in order to determine the significance of this finding. It is possible that elevated cellular immunity in people with a symptomatic initial infection might provide greater protection against a recurrence of the infection. On the other hand, asymptomatic people might be able to fight off the virus without having to trigger a major immune response.
According to the scientists, the results nevertheless indicate a robust cellular (T-cell) immune response against SARS-CoV-2 six months after an infection. “Cellular immunity is a complex yet potentially very important piece of the COVID-19 puzzle, and it is vital that more research is done in this area,” said Dr. Shamez Ladhani, epidemiologist consultant at Public Health England and author of the study. “However, initial results show that T-cell responses may outlast the initial antibody response, which could have a significant impact on COVID vaccine development and immunology research.”
The scientists point out that this paper is a preprint with preliminary data that has not yet been peer-reviewed.