Leading health experts have raised questions about what if the world never sees a Covid-19 vaccine. They raised their concerns as what the world has not witnessed any vaccine in the case of HIV and dengue even after years of research."There are some viruses that we still do not have vaccines against. We can't make an absolute assumption that a vaccine will appear at all, or if it does appear, whether it will pass all the tests of efficacy and safety," Dr David Nabarro, World Health Organization's Covid-19 special envoy, was quoted as saying in a CNN report.