inclusion criteria: * men and women * aged 65-89 years * who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months; * volunteered for a booster dose * had telephone or internet access * were able to provide informed consent * lived \<75 km from the crc (subgroup of 100 participants only)

inclusion criteria: * men and women * aged 65-89 years * who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months; * volunteered for a booster dose * had telephone or internet access * were able to provide informed consent * lived \<75 km from the crc (subgroup of 100 participants only)

inclusion criteria: * men and women, aged 65-89 years, who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a fourth dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived \<75 km from the crc (subgroup of 100 participants only)

inclusion criteria: * men and women, aged 65-89 years, who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a fourth dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived \<75 km from the crc (subgroup of 100 participants only)

inclusion criteria: men and women, aged 65-89 years, who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a fourth dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived <75 km from the crc (subgroup of 100 participants only)

inclusion criteria: men and women, aged 65-89 years, who have received 3 doses of a mrna vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a fourth dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived <75 km from the crc (subgroup of 100 participants only)

inclusion criteria: men and women, aged 65-89 years, who have received 2-3 doses of the same vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a third dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived <75 km from the crc

inclusion criteria: men and women, aged 65-89 years, who have received 2-3 doses of the same vaccine (pfizer-biontech or moderna) for more than 6 months, volunteered for a third dose of vaccine, had telephone or internet access, were able to provide informed consent, and lived <75 km from the crc