People testing negative for Covid-19 antibodies may still have some immunity, a new study has suggested. Researchers at the Karolinksa Institute in Sweden tested 200 people for both antibodies and T-cells. For every person testing positive for antibodies, two were found to have specific T-cells which identify and destroy infected cells. However, it is likely that some did mount an antibody response, because either it had faded or was not detectable by the current tests. The findings add to a growing body of evidence that "antibody testing alone underestimates immunity". It is also not yet clear whether these T-cells just protect that individual, or if they might also stop them from passing on the infection to others.