Introduction: During the pandemic, it has been recommended that vaccination against COVID-19 be a priority for patients with cancer; however, these patients were not included in the initial studies evaluating the available vaccines. Objective: To define the impact of vaccination against COVID-19 in preventing the risk of complications associated with the infection in a cohort of patients with cancer in Colombia. Methods: An analytical observational cohort study, based on national registry of patients with cancer and COVID 19 infection ACHOC-C19, was done. The data was collected from June 2021, until October 2021. Inclusion criteria were: Patients older than 18 years with cancer diagnosis and confirmed COVID-19 infection. Data from the unvaccinated and vaccinated cohorts were compared. Outcomes evaluated included all-cause mortality within 30 days of COVID-19 diagnosis, hospitalization, and need for mechanical ventilation. The estimation of the effect was made through the relative risk (RR), the absolute risk reduction (ARR) and the number needed to treat (NNT). Multivariate analysis was performed using generalized linear models. Results: 896 patients were included, of whom 470 were older than 60 years (52.4%) and 59% were women (n=530). 172 patients were recruited in the vaccinated cohort and 724 in the non-vaccinated cohort (ratio: 1 to 4.2). The cumulative incidence of clinical outcomes among the unvaccinated vs vaccinated patients were: for hospitalization 42% (95% CI: 38.7%-46.1%) vs 29%; (95% CI: 22.4%-36.5%); for invasive mechanical ventilation requirement 8.4% (n=61) vs 4.6% (n=8) and for mortality from all causes 17% (n=123) vs 4.65% (n=8). Conclusion: In our population, unvaccinated patients with cancer have an increased risk of complications for COVID -19 infection, as hospitalization, mechanical ventilation, and mortality. It is highly recommended to actively promote the vaccination among this population.