Prior studies have suggested that systemic viral infections may increase risks of dementia. Whether this holds true for SARS-CoV-2 virus infections remains uncertain but is of great consequence for predicting future dementia rates. We examined this by comparing changes in plasma biomarkers in UK Biobank participants before and after serology confirmed SARS-CoV-2 infections. We discovered biomarker changes associated with increased AD risk within this population, compared to matched participants with no evidence of infection. SARS-CoV-2 infection was associated with reduced plasma Aβ42:Aβ40 ratios, and in more vulnerable participants, lower plasma Aβ42 and higher plasma pTau-181. These biomarker changes, which have been associated with brain beta-amyloid accumulation in prodromal AD, were associated here with increased brain imaging signatures of AD, poorer cognitive scores, and worse assessments of overall health. Changes were greater in participants who had been hospitalised with COVID-19 or had previously reported hypertension. Our data provide observational biomarker evidence for the hypothesis that SARS-CoV-2 can be associated with accelerating brain pathology related to prodromal AD.