Vaccines are not necessarily the best or only therapy against a viral disease. We still do not have a vaccine against HIV 36 years after its discovery and a prediction made by the US Department of Human Health and Human Services that it would be available in two years after the cause of the disease was known. Second, there is a phenomenon called antibody-dependent enhancement (ADE) where a (non-neutralising) vaccine could make a patient more susceptible to a second attack by that virus, as exemplified by the Sanofi vaccine, Dengvaxia, against dengue virus. Sanofi, the company that manufactured the vaccine, noted that it should be used prophylactically by people who have not been infected with dengue virus. Finally, in the past, live, attenuated virus strains have also been effective as vaccines, as seen for measles, mumps, rubella and polio.
Gene-based vaccines also have many advantages, as well as some disadvantages, depending on the case at hand. For SARS-CoV2, which is an enveloped virus, we know that the viral Spike protein is responsible for allowing viral entry into human cells carrying the ACE2 receptor. So, there is a sound rationale in trying to create a vaccine that blocks the ability of the virus to enter host cells. However, there is the possibility that for a virus whose surface proteins are changing rapidly (fortunately not the case for SARS-CoV2) such a strategy may not work. Additionally, immunity is effective only when there is a strong, neutralising and preferably long-lasting antibody response against the protein used. Another issue with a vaccine is whether the virus will become resistant or change and therefore require annual vaccines, as is the case for influenza. To mitigate these risks, other viral proteins are also being targeted as vaccine candidates.