inclusion criteria: criteria for entry into the study: * any kidney transplant recipient from mayo clinic who has received the mrna vaccine (two or three dose mrna vaccine-moderna or pfizer) and are \>28 days after most recent vaccination at the time of spike protein assessment. * recipients of a kidney transplant, including those transplanted with other solid organ transplants in addition to the kidney. subjects may have received more than 1 kidney transplant. * more than 90 days since any transplant including a kidney transplantation. •≥18 years of age on the day of consent. criteria for entry into segment i * must have a roche elecsys® anti-sars-cov-2 s level of \<250 u/ml to be eligible for segment i. * platelet count of \>75,000/µl on the day of vaccination with the janssen ad26.cov2.s vaccine. * contraceptive (birth control) use should be consistent with local regulations regarding the acceptable methods of contraception for those participating in clinical studies. * before randomization, participants must be either: * not be of childbearing potential * of childbearing potential and practicing an acceptable effective method of contraception. subject must agree to remain on contraception from date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. use of hormonal contraception should start at least 28 days before the 1st administration of the janssen ad26.cov2.s vaccine. the sponsor-investigator should evaluate the potential for contraceptive method failure (for example, noncompliance, recently initiated) in relationship to the janssen vaccination. acceptable effective method a for this study include: * hormonal contraception: * combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal) * progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable) * intrauterine device * intrauterine hormone-releasing system * bilateral tubal occlusion/ligation procedure * vasectomized partner (the vasectomized partner should be the sole partner for that participant) * sexual abstinence (defined as refraining from heterosexual intercourse from the date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. the reliability of sexual abstinence needs to be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the participant.). note: use of condoms is not considered as an acceptable contraceptive barrier method due to the failure rate of female and male condoms (centers for disease control and prevention. reproductive health: contraception. https://www.cdc.gov/reproductivehealth/contraception/index.htm. accessed 23 november 2020) * if subject is female and of childbearing potential, she must: * have a negative highly sensitive serum pregnancy test prior to vaccination. * participant agrees to not donate bone marrow, blood, and blood products from the first janssen ad26.cov2.s vaccine administration until 3 months after the last dose of the janssen ad26.cov2.s vaccine.

inclusion criteria: criteria for entry into the study: * any kidney transplant recipient from mayo clinic who has received the mrna vaccine (two or three dose mrna vaccine-moderna or pfizer) and are \>28 days after most recent vaccination at the time of spike protein assessment. * recipients of a kidney transplant, including those transplanted with other solid organ transplants in addition to the kidney. subjects may have received more than 1 kidney transplant. * more than 90 days since any transplant including a kidney transplantation. •≥18 years of age on the day of consent. criteria for entry into segment i * must have a roche elecsys® anti-sars-cov-2 s level of \<250 u/ml to be eligible for segment i. * platelet count of \>75,000/µl on the day of vaccination with the janssen ad26.cov2.s vaccine. * contraceptive (birth control) use should be consistent with local regulations regarding the acceptable methods of contraception for those participating in clinical studies. * before randomization, participants must be either: * not be of childbearing potential * of childbearing potential and practicing an acceptable effective method of contraception. subject must agree to remain on contraception from date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. use of hormonal contraception should start at least 28 days before the 1st administration of the janssen ad26.cov2.s vaccine. the sponsor-investigator should evaluate the potential for contraceptive method failure (for example, noncompliance, recently initiated) in relationship to the janssen vaccination. acceptable effective method a for this study include: * hormonal contraception: * combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal) * progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable) * intrauterine device * intrauterine hormone-releasing system * bilateral tubal occlusion/ligation procedure * vasectomized partner (the vasectomized partner should be the sole partner for that participant) * sexual abstinence (defined as refraining from heterosexual intercourse from the date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. the reliability of sexual abstinence needs to be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the participant.). note: use of condoms is not considered as an acceptable contraceptive barrier method due to the failure rate of female and male condoms (centers for disease control and prevention. reproductive health: contraception. https://www.cdc.gov/reproductivehealth/contraception/index.htm. accessed 23 november 2020) * if subject is female and of childbearing potential, she must: * have a negative highly sensitive serum pregnancy test prior to vaccination. * participant agrees to not donate bone marrow, blood, and blood products from the first janssen ad26.cov2.s vaccine administration until 3 months after the last dose of the janssen ad26.cov2.s vaccine.

inclusion criteria: criteria for entry into the study: any kidney transplant recipient from mayo clinic who has received the mrna vaccine (two or three dose mrna vaccine-moderna or pfizer) and are >28 days after most recent vaccination at the time of spike protein assessment. recipients of a kidney transplant, including those transplanted with other solid organ transplants in addition to the kidney. subjects may have received more than 1 kidney transplant. more than 90 days since any transplant including a kidney transplantation. •≥18 years of age on the day of consent. criteria for entry into segment i must have a roche elecsys® anti-sars-cov-2 s level of <250 u/ml to be eligible for segment i. platelet count of >75,000/µl on the day of vaccination with the janssen ad26.cov2.s vaccine. contraceptive (birth control) use should be consistent with local regulations regarding the acceptable methods of contraception for those participating in clinical studies. before randomization, participants must be either: not be of childbearing potential of childbearing potential and practicing an acceptable effective method of contraception. subject must agree to remain on contraception from date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. use of hormonal contraception should start at least 28 days before the 1st administration of the janssen ad26.cov2.s vaccine. the sponsor-investigator should evaluate the potential for contraceptive method failure (for example, noncompliance, recently initiated) in relationship to the janssen vaccination. acceptable effective method a for this study include: hormonal contraception: combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal) progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable) intrauterine device intrauterine hormone-releasing system bilateral tubal occlusion/ligation procedure vasectomized partner (the vasectomized partner should be the sole partner for that participant) sexual abstinence (defined as refraining from heterosexual intercourse from the date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. the reliability of sexual abstinence needs to be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the participant.). note: use of condoms is not considered as an acceptable contraceptive barrier method due to the failure rate of female and male condoms (centers for disease control and prevention. reproductive health: contraception. https://www.cdc.gov/reproductivehealth/contraception/index.htm. accessed 23 november 2020) if subject is female and of childbearing potential, she must: have a negative highly sensitive serum pregnancy test prior to vaccination. participant agrees to not donate bone marrow, blood, and blood products from the first janssen ad26.cov2.s vaccine administration until 3 months after the last dose of the janssen ad26.cov2.s vaccine.

inclusion criteria: criteria for entry into the study: any kidney transplant recipient from mayo clinic who has received the mrna vaccine (two or three dose mrna vaccine-moderna or pfizer) and are >28 days after most recent vaccination at the time of spike protein assessment. recipients of a kidney transplant, including those transplanted with other solid organ transplants in addition to the kidney. subjects may have received more than 1 kidney transplant. more than 90 days since any transplant including a kidney transplantation. •≥18 years of age on the day of consent. criteria for entry into segment i must have a roche elecsys® anti-sars-cov-2 s level of <250 u/ml to be eligible for segment i. platelet count of >75,000/µl on the day of vaccination with the janssen ad26.cov2.s vaccine. contraceptive (birth control) use should be consistent with local regulations regarding the acceptable methods of contraception for those participating in clinical studies. before randomization, participants must be either: not be of childbearing potential of childbearing potential and practicing an acceptable effective method of contraception. subject must agree to remain on contraception from date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. use of hormonal contraception should start at least 28 days before the 1st administration of the janssen ad26.cov2.s vaccine. the sponsor-investigator should evaluate the potential for contraceptive method failure (for example, noncompliance, recently initiated) in relationship to the janssen vaccination. acceptable effective method a for this study include: hormonal contraception: combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal) progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable) intrauterine device intrauterine hormone-releasing system bilateral tubal occlusion/ligation procedure vasectomized partner (the vasectomized partner should be the sole partner for that participant) sexual abstinence (defined as refraining from heterosexual intercourse from the date of consent until 3 months after the last dose of the janssen ad26.cov2.s vaccine. the reliability of sexual abstinence needs to be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the participant.). note: use of condoms is not considered as an acceptable contraceptive barrier method due to the failure rate of female and male condoms (centers for disease control and prevention. reproductive health: contraception. https://www.cdc.gov/reproductivehealth/contraception/index.htm. accessed 23 november 2020) if subject is female and of childbearing potential, she must: have a negative highly sensitive serum pregnancy test prior to vaccination. participant agrees to not donate bone marrow, blood, and blood products from the first janssen ad26.cov2.s vaccine administration until 3 months after the last dose of the janssen ad26.cov2.s vaccine.