Surveillance data indicate that food security rates increased among Supplemental Nutrition Assistance Program (SNAP) participants during the COVID-19 pandemic (2020 and 2021) compared with pre-pandemic (2019), but this could have been due to increased participation from better resourced households. Our objective was to examine if demographic differences between SNAP-participating households in each year were responsible for the increased prevalence of food secure households. We calculated the observed 30-d food security prevalence among SNAP-participating households for each year. We used indirect standardisation to produce expected 2020 and 2021 prevalences with 2019 as the standard population using household size, income, age, sex, race, Hispanic ethnicity, presence of children, single parent household, metropolitan status and census region. We calculated standardised prevalence ratios (SPRs) to understand if the observed prevalence was higher than expected given any changes in the demographic profile compared to 2019. The Current Population Survey data were collected by the United States Census Bureau and Department of Agriculture. Our sample included 5,245 SNAP-participating households. The observed prevalence of food secure households increased by 3⋅6 percentage points comparing 2019 to 2020 (SPR = 1⋅06, 95 % confidence interval = 1⋅00, 1⋅11) and by 8⋅6 percentage comparing 2019 to 2021 (SPR = 1⋅13, 95 % confidence interval = 1⋅07, 1⋅18). The greater prevalence of food secure SNAP households during the pandemic did not appear to be attributable to socio-demographic differences compared to pre-pandemic. Despite hesitance among policymakers to expand or enhance social safety net programmes, permanently incorporating COVID-19-related policy interventions could lessen food insecurity in years to come.