Older racial and ethnic minorities and older adults with dementia have an elevated COVID-19 risk, warranting research into the intersection between these two high-risk groups. We examined whether race-ethnicity moderates the association between dementia and COVID-19 diagnosis. Data were retrieved for 3189 respondents from a nationally representative prospective cohort sample of US older adults aged 65+ years. We analysed the effects of the interaction between race-ethnicity and dementia on COVID-19 diagnosis, after adjusting for sociodemographic factors, health and COVID-19 mitigation behaviours. The odds of COVID-19 diagnosis were significantly lower for Black older adults with dementia (adjusted odds ratio [aOR] = 0.07, 95% CI = 0.01–0.78, P = 0.03). In addition, dementia increased the odds of COVID-19 diagnosis among Hispanic older adults (aOR = 1.59, 95% CI = 0.12–21.29, P = 0.72), although this increase was not statistically significant. The interaction between race-ethnicity and dementia should be considered when assessing COVID-19 risk among older adults. Future research is needed to examine pathways through which dementia may interact with race and ethnicity to influence COVID-19 risk.

Serum banks from large, decades-old epidemiological studies provide a valuable opportunity to explore the contributions of in utero vitamin D exposure to fetal origins of adult diseases. We compared 25-hydroxyvitamin D (25(OH)D) by race and season (two powerful predictors of vitamin D status) in sera frozen for ≥  40 years with sera frozen for ≤  2 years to determine whether 25(OH)D is stable enough to test vitamin D-related hypotheses. Data and sera came from seventy-nine pregnant women at 29–32 weeks' gestation in the Boston Collaborative Perinatal Project (CPP; 1959–66) and 124 women at 20–36 weeks' gestation in a 2003–2006 Pittsburgh cohort study. Multivariable linear regression models were used to test main and joint effects of race and season after confounder adjustment. In both cohorts, serum 25(OH)D levels were lower among black than white women (CPP 33·3 v. 46·7 nmol/l, P < 0·01; Pittsburgh 47·1 v. 89·6 nmol/l; P < 0·0001) and in winter than summer (CPP 32·7 v. 47·6 nmol/l, P < 0·0001; Pittsburgh 66·7 v. 89·8 nmol/l, P < 0·001), with no evidence of a race × season interaction in either cohort. Differences remained significant after confounder adjustment. When CPP and Pittsburgh results were compared, there was no significant difference in the race or season effects. The similarity in the relative change in 25(OH)D in these cohorts by two powerful predictors of vitamin D status suggests that, even if 25(OH)D deteriorated somewhat, it did so similarly across samples. Therefore, trends could be obtained from the decades-old serum data that would be relevant in exploring vitamin D-related hypotheses in future studies.