Among people with bleeding disorders, vaccination may cause a drop in the small blood cells that help the body form clots and stop bleeding. These blood cells are called platelets. Experts believe that despite a possible drop in platelet levels, the benefits of vaccination outweigh risks. 

One common autoimmune disease is called Immune thrombocytopenic purpura and it is characterized by low platelet counts. Platelets are needed for normal blood clotting, and decreased levels can result in easy bruising or bleeding. Symptoms range from purple bruises or tiny red dots under the skin, to bleeding in the brain. ITP treatment is individualized and may consist of steroid medication, platelet transfusion, or surgical removal of the spleen. Those with pre-existing ITP or taking blood thinners (anticoagulants) are advised to consult a hematologist first for a blood count before vaccination. 

Similarly, vaccines are generally safe for people with blood infections. People living with a chronic infection, like human immunodeficiency virus (HIV), have a suppressed immune system. They have a higher risk of developing serious complications from illnesses, which is why most vaccines are recommended, including any authorized COVID-19 vaccine. However, they may be at risk of developing an infection from some forms of vaccines, such as live attenuated vaccines. Therefore, people with blood infections are recommended to discuss vaccination plans with a physician first.

There have been recent reports of blood clots (thrombosis) with immune thrombocytopenic purpura (ITP) after receiving the AstraZeneca COVID-19 vaccine. Cases occurred primarily among younger people in the UK. Though the AstraZeneca vaccine is not available in the U.S., rare but severe cases of ITP following the Pfizer COVID-19 vaccine, resulting in hospitalization and death, have been reported but a causal link has not been verified. 

Uncertainty over whether the vaccines caused these events led researchers at the University of Edinburgh to assess the risk of bleeding events among 2.53 million adults up to 27 days after receiving the first dose of the AstraZeneca or Pfizer vaccine. The researchers found a small increased risk of ITP after the first dose of AstraZeneca, at a level similar to other common vaccines, such as influenza and measles, mumps, and rubella. They advised that the small risk be interpreted within the context of the vaccine’s greater benefits. The Pfizer vaccine was not associated with any adverse events. 

Evidence from this study, published in Nature Medicine, contributed to the Joint Committee on Vaccination and Immunisation’s (JCVI) advice to the UK government in May 2021. The JCVI noted a preference for people under 40 years of age to receive an alternative to the AstraZeneca vaccine only if supply is available and if doing so does not delay them from becoming fully vaccinated. 

Other health authorities, such as the World Health Organization, the European Medicines Agency, the American Society of Hematology, and the medical board of the Platelet Disorder Support Association, concur that the benefits of receiving the COVID-19 vaccine strongly outweigh the risks. 

Those at risk of developing bleeding or bruising symptoms should seek a hematologist’s guidance before vaccination. All eligible adults without pre-existing bleeding disorders do not need to avoid vaccination out of a concern for excessive bleeding.