Long-term and herd immunity against SARS-CoV-2: implications from current and past knowledge.

Effective herd immunity against SARS-CoV-2 will be determined on many factors: the percentage of the immune population, the length and effectiveness of the immune response and the stability of the viral epitopes. The required percentage of immune individuals has been estimated to be 50-66% of the population which, given the current infection rates, will take long to be achieved. Furthermore, data from SARS-CoV suggest that the duration of immunity may not be sufficiently significant, while the immunity response against SARS-CoV-2 may not be efficiently effective in all patients, as relapses have already been reported. In addition, the development of mutant strains, which has already been documented, can cause the reemergence of the epidemic. In conclusion, the development of an effective vaccine is an urgent necessity, as long-term natural immunity to SARS-CoV-2 may not be sufficient for the control of the current and future outbreaks.