Washington, April 16 : People who have recovered from Covid-19 require only one vaccine dose. A second jab is important for those who have not had Covid-19 to reach strong immunity, suggests a study. The study, led by researchers from the University of Pennsylvania in the US, found that Covid survivors had a robust antibody response after the first mRNA vaccine dose, but little immune benefit was seen after the second dose. On the other hand, those who did not have Covid-19 -- called Covid naive -- did not have a full immune response until after receiving their second vaccine dose, reinforcing the importance of completing the two recommended doses for achieving strong levels of immunity. Two doses is optimal to induce strong antibody and B cell responses in patients who are immunologically naive for SARS-CoV-2, and antibodies induced by the vaccination could protect against the more infectious and deadly South African variant, said the researchers. The findings, published in the journal Science Immunology, provides more insight on the underlying immunobiology of mRNA vaccines, which could help shape future vaccine strategies. "These results are encouraging for both short- and long-term vaccine efficacy, and this adds to our understanding of the mRNA vaccine immune response through the analysis of memory B cells," said E John Wherry, director of the Penn Institute of Immunology. The human immune response to vaccines and infections result in two major outcomes -- the production of antibodies that provide rapid immunity and the creation of memory B cells, which assist in long-term immunity. For the study, the team included 44 healthy individuals who received either the BioNTech/Pfizer or Moderna mRNA Covid-19 vaccine. Of this cohort, 11 had a prior Covid-19 infection. Blood samples were collected for deep immune analyses four times prior to and after vaccine doses. They confirmed that the mRNA vaccines induced antibodies that could neutralise the D614C and B1351 variants. These responses peaked one week after the second vaccine dose in naive patients but peaked two weeks after the first dose in recovered patients, with similar patterns in B cell responses for both groups. /IANS