Challenges in the development of vaccines against COVID-19.

In the face of the urgent need for a COVID-19 vaccine, currently more than 90 vaccine candidates are being developed using different strategies, such as inactivated or attenuated SARS-CoV-2 virus, viral vectors expressing antigens of this virus, nucleic acids or purified viral proteins. These vaccines are in preclinical development and at least six of them have already been injected into volunteers in safety clinical trials. However, the characteristics of the protective immune responses are still unknown; therefore, there is not evidence to indicate that these vaccines will induce protection and if this will be long-lasting. The development of SARS-CoV-2 vaccines is vital; nevertheless, it is also important to unveil the characteristics of the protective immune responses to guide the design of a vaccine that generates a long-lasting protection against COVID-19.