Patients vaccinated against COVID-19 had a 59 percent lower risk of both blood clots and death within 30 days of infection, according to a study published in the journal Research and Practice in Thrombosis and Haemostasis and reported by Drug Topics on 2 November. The finding comes from a large retrospective study of patients in the United States, and readers should note it shows an association, not proof of cause and effect.
The researchers studied 756,217 patients with COVID-19 and 678,747 patients with other acute respiratory infections, using the National Clinical Cohort Collaborative database, according to the journal paper. Compared with patients hospitalised with flu and other respiratory infections, those with COVID-19 had an increased risk of venous thromboembolism — blood clots in the veins — with an odds ratio of 1.45, and of death, with an odds ratio of 1.80, according to the study.
Vaccination changed that picture substantially. Among COVID-19 patients, vaccination was associated with a reduced odds of blood clots of 0.41 and of death of 0.41 — a 59 percent reduction in both — according to the study, as reported by Drug Topics. The authors conclude that vaccination significantly reduced the risk of blood clots and mortality in COVID-19 patients. Separate data from the IVY Network, cited in the Drug Topics report, found that completing an original monovalent mRNA vaccination series was 84 percent effective in preventing COVID-19 hospitalisations involving acute blood clots.
The study also traced how the risk has changed over time. The elevated risk of clots and death was most acute during the early ancestral and Delta variant waves, according to Drug Topics, but by the contemporary Omicron period, from May 2022 to November 2024, the clinical risk profile of COVID-19 had converged with that of other acute respiratory infections, particularly among vaccinated people. The post-infection risk was heavily concentrated in the first 60 days after acute illness; beyond eight weeks, event rates in vaccinated and unvaccinated people alike levelled off towards the levels seen with other respiratory infections.
The findings land as COVID-19 vaccine guidance shifts in the United States: the Centres for Disease Control and Prevention and its Advisory Committee on Immunization Practices have moved from universal recommendations to a shared clinical decision-making framework for adults and children, according to Drug Topics. The study's authors say the results offer useful guidance for doctors monitoring patients for lingering clotting complications after infection, including those linked with long COVID.