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 study was 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 incidence was 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, 464 were female (59.5%); mean (SD) age was 36.8 (21.8) years. COVID-19 incidence was 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 pandemic were positive for SARS-CoV-2, and only 9 of 456 (2.0%) were positive for SARS-CoV-2 within 6 weeks of the chilblains diagnosis. Test results of 1 of 97 (1.0%) patients were positive for SARS-CoV-2 IgG antibodies. Latinx patients were 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.