Risk of Blood Clots During Air Travel While Pregnant: Does COVID-19 Vaccination Status Matter?

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In April and May of 2021, just a few months into the COVID-19 vaccine period, when large numbers of people were receiving their first two doses of vaccines, or one dose if they were receiving the Johnson and Johnson (Janssen) vaccine, there was a lot of discussion and concern about rare cases of thrombosis (blood clotting). This was in connection particularly with the Janssen vaccine and the AstraZeneca/Oxford vaccine against SARS-CoV2 (the virus that causes COVID-19).  While immunization with the Janssen vaccine initially required just one shot, it was found to be less effective compared with the mRNA vaccines made by Pfizer-BioNTech and Moderna, so people who received the single dose Janssen vaccine were among the first to be recommended to receive a booster shot, preferably with an mRNA vaccine. While blood clotting, which doctors call thrombosis, actually has not been much of an issue in connection with the mRNA vaccines, it has been an issue with respect to AstraZeneca and Janssen. It’s not a big overall risk, but the risk is significant compared with the mRNA vaccines and those at greatest risk include women in their reproductive years. The issue has been not only thrombosis developing in deep veins, which can lead to a life threatening condition called pulmonary embolism, but also thrombosis developing within cavernous passageways where venous blood passes through the layers of connective tissue that surrounds the brain, called cavernous venous sinuses.

Given the rare, but real occurrence of cerebral venous sinus thrombosis (CSVT) in connection with the AstraZeneca and Janssen vaccines, plus the fact that flying causes people to sit for long periods of time, there was some concern last year about the prospect of any or all of the COVID-19 vaccines possibly causing venous thrombosis in aircraft passengers. While research has not revealed that deep venous thrombosis (DVT) occurs more during flying in passengers overall, there is a concern when it comes to people who have an elevated risk of clotting, for reasons other than flying. This includes women who are pregnant, especially during the final weeks of pregnancy and for a few weeks following delivery.

Unrelated to COVID-19 vaccination, if you are cramped into a seat for more than an hour or two, without moving your legs, the chances of developing a DVT increase, on top of the risk that you have already on account of your pregnancy. Flying while you’re pregnant raises the clotting concern, because two phenomena that contribute to thrombosis combine. One of these two phenomena is stasis of blood, meaning that blood slows and pools in some places, especially deep veins in the legs and pelvis. This happens because your growing womb puts pressure on the pelvic veins and also because you are seated without moving much on the aircraft. The other of the two phenomena is is an increased tendency for your blood clotting system to form clots, which happens during pregnancy, as the body prepares for delivery when it wants to avoid hemorrhage. These two phenomena, plus a third phenomenon, injury to, or abnormalities of, the inner lining of blood vessels, constitute a triad of factors, known as the triad of Virchow, or Virchow’s triad. It is named for the famous 19th century, German physician, Rudolf Virchow (1821-1902), who described these three phenomena and how they cause thromboses, blood clots. During pregnancy, it’s really two of the factors of the triad, venous stasis and increased coagulation that put you at risk. Damage to the inner lining of blood vessels is more of an issue that causes clots to form in arteries, as in coronary artery disease. But the more of the three prongs of the Virchow triad that you have, the more likely you are to form thrombi —blood clots— in the wrong time and place. Normal, healthy pregnant women, generally don’t experience the blood vessel lining injury/disease part of Virchow’s triad.

As we have discussed over and over, here on The Pulse, getting vaccinated and up-to-date in your vaccines against COVID-19 is particularly important, and in fact the risk of blood clots elevates a lot more in those with COVID-19 than in those who receive any kind of COVID-19 vaccine, even the Janssen vaccine, and do not develop COVID-19. But you should be careful when you fly, by getting up from your seat as frequently as possible and moving and stretching out your body and legs. The good news, however, is that, despite the concern and the rationale, research has not revealed that people vaccinated with any kind of COVID-19 vaccine are at increased risk of thrombosis during flight. The research involved general air passenger populations, not pregnant women, but the precaution to do exercises and move around as best you can applies anyway.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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