inclusion criteria: 1. age: volunteer must be at least 18 years of age, at the time of signing the informed conset. 2. sex: male or female. all female volunteers of reproductive age will be requested to use contraceptive measures for the two months following enrolment. 3. received 3 doses of bnt162b2 with the 3rd dose at least 4 months previously. 4. have a serology test within the previous 3 months of 700 bau or less. 5. responded to the previous vaccine doses, i.e. at least one igg\>100. 6. medical conditions: volunteers with any medical condition are allowed, as long as they adhere to the criteria above. 7. agreed to attend all visits and signed the informed consent -

inclusion criteria: 1. age: volunteer must be at least 18 years of age, at the time of signing the informed conset. 2. sex: male or female. all female volunteers of reproductive age will be requested to use contraceptive measures for the two months following enrolment. 3. received 3 doses of bnt162b2 with the 3rd dose at least 4 months previously. 4. have a serology test within the previous 3 months of 700 bau or less. 5. responded to the previous vaccine doses, i.e. at least one igg\>100. 6. medical conditions: volunteers with any medical condition are allowed, as long as they adhere to the criteria above. 7. agreed to attend all visits and signed the informed consent -

inclusion criteria: age: volunteer must be at least 18 years of age, at the time of signing the informed conset. sex: male or female. all female volunteers of reproductive age will be requested to use contraceptive measures for the two months following enrolment. received 3 doses of bnt162b2 with the 3rd dose at least 4 months previously. have a serology test within the previous 3 months of 700 bau or less. responded to the previous vaccine doses, i.e. at least one igg>100. medical conditions: volunteers with any medical condition are allowed, as long as they adhere to the criteria above. agreed to attend all visits and signed the informed consent -

inclusion criteria: age: volunteer must be at least 18 years of age, at the time of signing the informed conset. sex: male or female. all female volunteers of reproductive age will be requested to use contraceptive measures for the two months following enrolment. received 3 doses of bnt162b2 with the 3rd dose at least 4 months previously. have a serology test within the previous 3 months of 700 bau or less. responded to the previous vaccine doses, i.e. at least one igg>100. medical conditions: volunteers with any medical condition are allowed, as long as they adhere to the criteria above. agreed to attend all visits and signed the informed consent -