[Analysis of vaccination strategy against COVID-19 in Spain. The scientific basics (may 2022).]

After about a year and a half (at the moment these lines are being written) since the start of the massive vaccination campaign in which, thanks to the high coverage achieved in all groups eligible for vaccination, it has been possible to significantly reduce the morbidity and mortality due to COVID-19, it is important to review the scientific basics that have supported the recommendations implemented to date and those that could be adopted in the near future taking into consideration the epidemiological situation. The objective of this article is, therefore, to address the foundations of some of the technical decisions proposed by the Committee on Programme and Registry of Vaccinations (National Immunization Technical Advisory Group in Spain) and the Technical Working Group on Vaccination against COVID-19. Throughout the eleven updates of the Vaccination Strategy against COVID-19 in Spain, several issues pose intense debate as the vaccination intervals between doses, the convenience of using different types of vaccines, the use of heterologous schemes of vaccination, the benefits of hybrid immunity and the use of a fourth dose (second booster dose) for selected populations. All this without forgetting essential aspects of safety of vaccines. This article is divided into the following sections: Vaccination intervals; Heterologous or mixed scheme; Hybrid immunity (vaccination after infection and infection after vaccination [breakthrough]); Second booster dose.