Antibodies developed against Covid-19 may 'decline sharply' a month after leaving the hospital, a new study from China's Nanjing University Medical School has claimed. For the study, antibody responses were observed in 7 severe Covid-19 patients and 19 non-severe ones for a duration of 7 weeks from the onset of the infection. They concluded that over 80% of recovered patients had varying levels of antibodies, only a small portion had a 'potent' response to the virus if it was reintroduced. In addition, the levels of antibodies declined significantly three to four weeks later. One in five patients' antibodies were incapable of fighting the virus after leaving hospital. This may help develop effective vaccines or treatment against Covid-19. The study also stressed on the importance of carefully selecting blood samples from recovered patients for transfusion to other Covid-19 patients.