Third-grade reading proficiency predicts later reading achievement and high school graduation, yet third-graders from historically marginalized groups experience differential learning opportunities and sometimes perform poorly in reading. Similar to the effects of summer learning loss, lost instructional opportunities due to COVID-19 have translated to disproportionate declines in reading skills for low-income, Black, and Latinx students. We analyzed archival reading data for 68 rising third-graders from historically marginalized populations, who read below grade level and participated in summer reading intervention in 2018, 2019, or 2020. Intervention involved evidence-based reading instruction with program elements aligned with high-impact tutoring; however, format (virtual), curriculum, and dosage were modified in 2020 due to COVID-19. We examined differences across years in oral reading fluency words correct per minute (WCPM) and word reading accuracy (WRA) intervention change scores, change scores adjusted for differences in dosage, and post-intervention scores. The 2020 cohort grew less and ended summer performing lower on WCPM and WRA compared to prior years, with 2019 vs. 2020 contrasts reaching statistical significance for WCPM (p = 0.03). After accounting for differences in dosage across years, no contrasts were statistically significant nor practically meaningful across year nor outcome. This study showed heightened risk via slower growth and lower end-of-summer scores for students receiving SRI in summer 2020, potentially driven by the decreased dosage in 2020. Implications for design of post-COVID summer programming focus on ensuring adequate dosages are achieved, even for programs with elements that otherwise align strongly with recommendations for high-impact tutoring.