The ministry said the dose would be given five months after a third dose.
The decision affects 120,000 people in Spain.
For the population as a whole, a third dose will from now on be available for those aged 18 and above rather than the previous age floor of 40, and from five months after the second dose rather than six as previously.
Countries such as Chile and Israel have already started administering a fourth dose to the general population under the premise that it increases by five the number of antibodies against the virus, but in Spain some virologists are not convinced by this approach.
“From a basic immunological standpoint, it makes no sense that in little more than a year a fourth dose is being considered,” immunologist Alfredo Corell told Spanish broadcaster RTVE.
“The aim of being vaccinated is to generate memory, not to generate antibodies.
“As long as we have T cells, which are the ones that coordinate the immune response, there isn’t a problem. At most it will take five days, in which we will experience a mild or moderate reaction, a cold”.
Spain has been in the global vaccination vanguard with 90.5 percent of the population aged 12 and above jabbed to date.
In addition, Spain, which has logged 90,508 deaths and 7.7 million cases during the pandemic, has given at least one dose to 38.5 percent of children aged five to 11.
In terms of the reinforcement dose, around 34 percent of the eligible population in Spain has so far received a Covid booster shot to date.