BACKGROUND: General practitioners (GPs) have a vital role in reaching out to vulnerable populations during and after the COVID-19 pandemic. Nonetheless, they experience many challenges to fulfill this role. This study aimed to examine associations between practice characteristics, patient population characteristics and the extent of deprivation of practice area on the one hand, and the level of outreach work performed by primary care practices (PCPs) during the COVID-19 pandemic on the other hand. METHODS: Belgian data from the international PRICOV-19 study were analyzed. Data were collected between December 2020 and August 2021 using an online survey in PCPs. Practices were recruited through randomized and convenience sampling. Descriptive statistics and ordinal logistic regression analyses were performed. Four survey questions related to outreach work constitute the outcome variable. The adjusted models included four practice characteristics (practice type, being a teaching practice for GP trainees; the presence of a nurse or a nurse assistant and the presence of a social worker or health promotor), two patient population characteristics (social vulnerability and medical complexity) and an area deprivation index. RESULTS: Data from 462 respondents were included. First, the factors significantly associated with outreach work in PCPs are the type of PCP (with GPs working in a group performing more outreach work), and the presence of a nurse (assistant), social worker or health promotor. Second, the extent of outreach work done by a PCP is significantly associated with the social vulnerability of the practice's patient population. This social vulnerability factor, affecting outreach work, differed with the level of medical complexity of the practice's patient population and with the level of deprivation of the municipality where the practice is situated. CONCLUSIONS: In this study, outreach work in PCPs during the COVID-19 pandemic is facilitated by the group-type cooperation of GPs and by the support of at least ...