ATHENS – Greece has selected the Pfizer-BioNTech vaccine as the preferred version for third booster shots for the elderly and most vulnerable to continue being protected during the ongoing COVID-19 pandemic.

National Vaccination Committee chief Maria Theodoridou said those who received the Astra Zeneca vaccine in the first vaccination phase will be given the Pfizer vaccine as a booster, without explaining why.

There's been no decision on what version would be given those who had received the single-shot Johnson & Johnson from the United States as Greece has offered boosters but hasn't made shots mandatory.

The program to offer the third shot had already begun among people with immunosuppressed systems and will be extended to those in nursing homes, those over 60 years old and health care workers for whom shots are required.

Greece's Eleftheria (Freedom) vaccination program has seen little more than 56 percent of people fully protected, far below the 70 percent that health officials said is needed to beat back the pandemic.