On 5 October 2021, Spain's Public Health Commission approved the recommendation to administer an additional third dose of the COVID-19 vaccine as part of the ongoing response to the pandemic.

WHO WILL RECEIVE THE THIRD DOSE?

The third dose is primarily intended for people aged 70 and over, as age remains the most significant risk factor for developing severe COVID-19. This additional dose is administered at least six months after the second dose.

It is also intended for people aged 65 and over, with the Public Health Commission recommending that this age group begin receiving the additional dose from November 2021.

Previously, the Commission had already approved a third additional dose for people with high-risk medical conditions to strengthen their immune response. These include:

• Recipients of haematopoietic stem cell transplants or CAR-T cell therapy, vaccinated within two years of transplantation or treatment, receiving immunosuppressive therapy, or with graft-versus-host disease (GVHD), regardless of the time since transplantation.

• Solid organ transplant recipients.

• Patients receiving renal replacement therapy, including haemodialysis and peritoneal dialysis.

• Individuals who have undergone chemotherapy or radiotherapy within the previous six months.

• People with primary immunodeficiency disorders.

• People living with HIV with a CD4 count below 200 cells/μL (based on blood tests from the previous six months).

• People with cystic fibrosis.

• Individuals with Down syndrome aged 40 years or older.

• Patients receiving immunosuppressive treatment.

For these high-risk groups, the additional dose should be administered at least 28 days after the second dose.

THIRD DOSE IN CARE HOMES

As care homes are among the settings at greatest risk of COVID-19 outbreaks, a third vaccine dose was also recommended for residents of long-term care facilities.

This additional dose is administered six months after the second dose. In many cases, it may be given at the same time as the seasonal influenza vaccine, although this may vary depending on the epidemiological situation and logistical considerations.

WHAT IS THE DIFFERENCE BETWEEN AN ADDITIONAL DOSE AND A BOOSTER DOSE?

The third doses approved by the Ministry of Health are considered additional doses, intended to complete the primary vaccination schedule in individuals whose immune response may have been insufficient.

A booster dose, by contrast, is administered after immunity has declined over time to reinforce the body's immune memory and improve protection against the virus.

WHICH VACCINE WILL BE USED FOR THE THIRD DOSE?

The additional dose should generally be administered using the same mRNA vaccine received during the initial vaccination course, such as the Pfizer-BioNTech or Moderna vaccine.

The Federation of Spanish Medical Scientific Associations (FACME) has also highlighted the importance of delivering vaccination programmes while taking into account the healthcare needs and circumstances of these particularly vulnerable groups.