Epidemiologic Analysis of Chilblains Cohorts Before and During the COVID-19 Pandemic

IMPORTANCE Beginning in March 2020, case reports and case series linked the COVID-19 pandemic with an increased occurrence of chilblains, but this association has not been evaluated in an epidemiologic study. OBJECTIVE To assess whether a correlation exists between COVID-19 incidence and chilblains incidence. DESIGN, SETTING, AND PARTICIPANTS A retrospective cohort studywas conducted within the Kaiser Permanente Northern California system from January 1, 2016, to December 31, 2020; health plan members of all ages were included. EXPOSURE COVID-19 incidence in 207 location-months, representing 23 geographic locations in northern California across 9 months. MAIN OUTCOME AND MEASURES Chilblains incidencewas the main outcome. The association of chilblains incidence with COVID-19 incidence across the 207 location-months was measured using the Spearman rank correlation coefficient. RESULTS Of 780 patients with chilblains reported during the pandemic, 464were female (59.5%); mean (SD) agewas 36.8 (21.8) years. COVID-19 incidencewas correlated with chilblains incidence at 207 location-months (Spearman coefficient 0.18; P=.01). However, only 17 of 456 (3.7%) patients with chilblains tested during the pandemicwere positive for SARS-CoV-2, and only 9 of 456 (2.0%) were positive for SARS-CoV-2 within 6weeks of the chilblains diagnosis. Test results of 1 of 97 (1.0%) patientswere positive for SARS-CoV-2 IgG antibodies. Latinx patientswere disproportionately affected by COVID-19 but not by chilblains. CONCLUSIONS AND RELEVANCE This cohort study found that in northern California, the incidence of chilblains increased during the pandemic but was correlated weakly with the incidence of COVID-19 across 207 location-months. These findings may have resulted from a causal role of COVID-19, increased care-seeking by patients with chilblains during the pandemic, or changes in behavior during shelter in place.