The first wave of the pandemic brought a sharp, unexpected rise in fatal blood clot complications in COVID-19 patients, such as pulmonary embolism, breakthrough clots clogging dialysis lines, and even emergent strokes in the young and healthy.
Now with Omicron dominating U.S. cases, clotting complications appear to have declined. Although it's still too early to know for sure whether Omicron has altered thromboembolism risk compared with other variants, Omicron may cause less severe disease and therefore carry lower risk of clots.
Anecdotally, clinicians contacted by 鶹ý universally reported seeing fewer deep vein thrombosis/pulmonary embolism (DVT/PE) cases in recent months.
"We have not seen, particularly with this surge, the same level of blood clotting complications that we had been during that initial wave," said Peter Faries, MD, a vascular surgeon at Mount Sinai in New York.
Looking at studies from the initial waves of the pandemic, a systematic review and meta-analysis in February 2021 found that 16.5% of all COVID patients examined had PE and 14.8% had DVT. Those admitted to the ICU had a higher risk of PE than those who were not (24.7% vs 10.5%, respectively).
Additionally, another meta-analysis found that the overall venous thromboembolism incidence was 9% overall among hospitalized COVID-19 patients and 21% among those admitted to the ICU.
While patients in critical care still appear to be at a high risk of clotting, Michael Streiff, MD, medical director of the Johns Hopkins Special Coagulation Laboratory in Baltimore, said that he hasn't seen any recent patients return with PE/DVT after hospitalization, which had happened in earlier waves. "It seems like the clot risk is a little bit less, but we don't know," Streiff said.
"Almost all of the clotting complications we've seen are really in the more acute phase," Faries agreed.
What's Different About Omicron?
The risk of clotting may have changed for a number of reasons, Faries told 鶹ý, such as less severe illness with Omicron, the availability of vaccines, or widespread use of anticoagulation therapy among at-risk patients.
Also, previous strains of SARS-CoV-2 caused an inflammatory response in the body that increased risk of blood clots – one that has yet to be seen as frequently with Omicron, said Alex Spyropoulos, MD, director of anticoagulation and clinical thrombosis services at Northwell Health in new Hyde Park, New York.
The majority of pulmonary embolisms during the first and second waves of the pandemic were in situ lung clots rather than clots that broke off and traveled to the lungs, Spyropoulos noted. Some evidence shows that Omicron may be more limited to the upper respiratory airway, meaning it would be less likely to cause clots directly in the lungs.
"Anecdotally, we're not seeing the amount of in situ pulmonary embolus that we know we saw in the first wave of the pandemic," Spyropoulos told 鶹ý.
However, he cautioned: "Let's not downplay the importance of the Omicron variant as a cause for decompensation of an otherwise very comorbid and sick patient." It could push such patients to a "tipping point" in which they could develop more severe COVID-19 and face a greater risk of clotting, he suggested.
Prevention and Treatment
Monitoring patients who might be at an increased risk of blood clots remains critical with Omicron.
It's important to avoid a "false sense of security" with Omicron, especially when case numbers are so high and could be driving a greater number of severe cases, said Behnood Bikdeli, MD, MS, a cardiologist with Brigham and Women's Hospital in Boston who has been involved in COVID-19 clot prophylaxis trials.
Some factors that may put patients at a greater risk of blood clots include advanced age, a history of cardiopulmonary disease, a history of cancer, immobility, and a history of sepsis or severe infection, Spyropoulos said. Also, a patient with Omicron who presents with D-dimer levels that are extremely elevated, he said, should be monitored closely due to elevated risk of thrombosis and death.
Another factor is vaccination status, Faries noted, as patients who are unvaccinated present with more severe illness, and thus are more prone to blood clots.
Even if clotting shows up less often in this wave of the pandemic, the protocols for prophylaxis haven't changed at most centers, Streiff said. Identifying patients at risk, conducting routine D-dimer tests, and administering anticoagulation therapy has remained in place, he said.
The prophylactic dosing of anticoagulation medication for patients who are hospitalized with COVID-19. Among critically ill patients, low molecular weight heparin or unfractionated heparin is preferred over oral anticoagulants, because they have shorter half-lives, can be administered intravenously or subcutaneously, and have fewer interactions with other drugs.
Considering that Omicron may cause less severe illness, it may be time to reassess the use of full-dose anticoagulants -- and their inherent bleeding risk -- in moderately sick hospitalized patients, said Stephan Moll, MD, of the division of hematology at the University of North Carolina at Chapel Hill. But more epidemiological studies and prospective trials will be needed to evaluate the risks and benefits of those therapies.
"I think it's enough to know that we have seen blood clots in patients who have Omicron," said Bikdeli "Now the question of, do we see more of it or less of it? And when we see it, are [patients] having more ominous outcomes or not? Those questions need adjusted analyses."
"So for now, I'm thinking of it more or less along the same lines of prior variants until we get more high quality data to inform us," he added.