Brazilian pediatric doctor Monica Levi, one of the volunteers who received the COVID-19 vaccine, works at the Specialized Clinic in Infectious and Parasitic Diseases and Immunizations (CEDIPI), in Sao Paulo, Brazil, on July 24, 2020. The doctor is one of the 5,000 volunteers participating in Brazil of the phase 3 trials – the last before homologation – of the ChAdOx1 nCoV-19 vaccine, developed by the University of Oxford together with the British pharmaceutical company AstraZeneca.Nelson Almeida | AFP | Getty ImagesLONDON — The preliminary findings of a phase two trial show that the coronavirus vaccine being developed by the University of Oxford and AstraZeneca is safe and triggers a similar immune response among all adults.The promising early-stage results were published in The Lancet, one of the world’s top medical journals, on Thursday.The study of 560 healthy adults, including 240 over the age of 70-years-old, found the vaccine to be safe and produced a similar immune response among people aged over 56-years-old and those aged between 18 and 55.British pharmaceutical giant AstraZeneca, which is working in collaboration with the University of Oxford, has previously said interim data showed their experimental vaccine had produced an immune response in older and younger adults.A safe and effective vaccine is seen by many as a game-changer in the battle against the coronavirus pandemic, which has claimed the lives of over 1.3 million people worldwide.Huge challenges remain before a vaccine can be rolled out, however. The global battle to secure prospective supplies has raised alarm about equitable access, while questions remain over logistics, distribution, and cost.The Oxford vaccine candidate was found to cause few side effects and triggered immune responses in both parts of the immune system in all age groups and at low and standard doses.The preliminary results showed that the vaccine — ChAdOx1 nCoV-19 — prompted what’s known as a “T-cell response” within 14 days of the first dose, and an antibody response within 28 days of the booster dose. Scientists expect T-cell responses to play a role in long-term immunity against the virus.Dr. Maheshi Ramasamy, a co-author of the study at the University of Oxford, said the antibody and T-cell responses among older adults were “robust” and “encouraging.””The populations at greatest risk of serious COVID-19 disease include people with existing health conditions and older adults,” Ramasamy said.”We hope that this means our vaccine will help to protect some of the most vulnerable people in society, but further research will be needed before we can be sure.”Study limitationsThe authors of the Oxford study said their results could be encouraging if the immune responses are found to be associated with protection against Covid-19 infection. The phase two trial did not assess vaccine efficacy, however, and phase three trials are ongoing to confirm this.Results are expected later this year depending on the rate of infection within clinical trial communities.The authors noted some limitations to their study, including that participants in the oldest age group had an average age of 73 to 74 and few underlying health conditions, and almost all participants were White and non-smokers.People from a range of backgrounds, countries, and ethnicities were said to be included in the phase three trial.The study comes days after two other vaccine makers announced encouraging results from phase three trials. They said their experimental vaccines were found to be highly effective in protecting against the coronavirus, boosting optimism at a time when health systems in Europe and the U.S. are once again being pushed to breaking point.Pfizer and BioNTech said on Wednesday a final analysis found their vaccine candidate was 95% effective in preventing Covid-19 and appeared to fend off severe disease. Earlier this week, Moderna had said preliminary phase three trial data showed its vaccine was 94.5% effective.