This competitive revision aims to increase access and uptake of COVID-19 testing and vaccination among people who inject drugs (PWID), a population highly vulnerable to COVID-19. The proposed RADxUP project, LinkUP, will be nested within the San Diego component of our parent grant, La Frontera, which was funded by NIDA in 04/20. To date, 31% of La Frontera PWID have tested SARS- CoV-2 antibody+, of whom 65% had not previously been tested for COVID-19. Only 3% had been vaccinated by 05/2021 and 45% were vaccine hesitant. Our specific aims are: AIM 1. To prospectively evaluate the prevalence, predictors and barriers to COVID-19 testing among PWID. AIM 2. To prospectively assess incidence, predictors and barriers to COVID-19 vaccination among PWID. AIM 3. To adapt and pilot an intervention offered at a mobile syringe services program (SSP) to improve uptake of COVID-19 testing and vaccination among PWID. AIM 4. To compare COVID testing and vaccination rates before and after the state introduces a) COVID-19 rapid testing and b) COVID-19 vaccination at the SSP, which represents a natural experiment. AIM 5. To prospectively assess the impact of COVID-19 testing and vaccination on PWIDs’ behaviors. To meet Aim 1 and 2, we will conduct interviews with 400 PWID in La Frontera, results which will be available in Fall 2021. Based on these findings and input from our Community and Scientific Advisory Board (CSAB), we will select and adapt a brief intervention to improve COVID-19 testing and vaccination uptake among PWID at the mobile van of our community partner, OnPoint. The LinkUP intervention will then be piloted among a subgroup of 150 La Frontera PWID who have not received COVID-19 vaccine or testing and will be implemented by OnPoint peer counsellors, who will also offer on-site rapid COVID-19 antigen testing and confirmatory PCR and referrals to the closest COVID-19 vaccination center (until vaccine is offered at OnPoint by the state). We will evaluate COVID-19 vaccine uptake independently through ongoing La Frontera study visits that will include self-reported interview data, linkage of electronic health records with the San Diego County COVID-19 database, and by analyzing SARSCoV-2 antibody patterns to differentiate between natural infection and vaccine-induced immunity. Results will be used to estimate effect sizes for a future efficacy trial and shared with the RADxUP consortium, policymakers and program planners. Since there are 185 SSPs across the country, our study will inform efforts to enable SSPs to become important ‘touchpoints’ to reach marginalized PWID, strengthening the nation’s pandemic preparedness infrastructure to reduce COVID-19 health disparities.