This letter is to inform you that on the 30th of September the Steering Committee of the ETHIC Study decided to terminate the study following a recommendation by the study Data and Safety Monitoring Board (DSMB) after reviewing the unblinded treatment-specific endpoint data. The Steering Committee requested the DSMB review the data because the average event rate (study end point) has been much lower than expected. The observed combined event rate across the treatment and control groups was 11.6% while the study design assumed a combined event rate of approximately 21%. With an event rate of 11.6% for the rest of the study, and assuming a 25% treatment effect, the study would require an increase in the sample size from 1370 to 2,930 in order to maintain the same level of statistical power. The DSMB found no treatment signal in the observed data and the futility probability (the probability that we will not be able to state a significant treatment difference given our current data) is 96%, which is very high. Enrolment is already behind target and with the impact of vaccination on enrolment rates, it is unlikely that the study will be able to achieve the required increased sample size in a feasible time scale. We would like to note the DSMB did not identify any safety concerns with bleeding and their recommendation was exclusively based on efficacy issues. Since the DSMB met, the ACTIV-4B trial results were published (JAMA. doi:10.1001/jama.2021.17272). This trial stopped after 657 of the intended 7000 (9%) patients were enrolled because the event rate was lower than expected. Thus our experience is not unique. After the publication of the ETHIC results, we will collaborate with the OVID study investigators on a meta-analysis of all outpatient thromboprophylaxis trials. The OVID trial is very similar in design to ETHIC so the combination of these two trials alone will provide important information to the field.