To date, there is no effective prophylactic agent to prevent COVID-19. However, the development of symptoms similar to covid19 could be prevented with an aqueous solution of chlorine dioxide (ClO2). This retrospective study evaluated the effectiveness of an aqueous solution of ClO2 (CDS) as a prophylactic agent in 1,163 family members living with positive/suspected COVID19 patients. Prophylactic treatment consisted of 0.0003% chlorine dioxide solution (CDS) orally for at least fourteen days. Family members in whom no reports of the development of covid19-like symptoms were found in the medical history were considered successful cases. The efficacy of CDS in preventing covid19-like symptoms was 90.4% (1,051 of 1,163 relatives did not report any symptoms). The comorbidities, sex and severity of the illness of the sick patient did not contribute to the development of symptoms similar to covid19 (P = 0.092, P = 0.351 and P = 0.574, respectively). However, older relatives were more likely to develop covid19-like symptoms (ORa = 4.22, P = 0.002). There was no evidence of alterations in blood parameters or in the QTc interval in relatives who consumed CDS. The recent findings regarding Chlorine Dioxide justify designing clinical trials to assess its efficacy for preventing SARS-CoV-2 infection.