In recent months, a new form of selective vaccine hesitancy has made itself known. Reports from across the world, even including such far-off places as Australia, tell us that people normally inclined to accept vaccines are making an exception when it comes to the AstraZeneca vaccine. As bad news travels faster and has more impact than good news, sporadic reports of people developing serious blood clots after Covid 19 vaccination have outshadowed the many potential benefits of the AstraZeneca vaccine. Despite official advice stating that these benefits outweigh the risks of any potential adverse side-effects, offers of this particular vaccine are being turned down and vaccination deferred until an alternative becomes available. And it isn’t just members of the public who have become hesitant. Some governments have responded with caution, either delaying, banning, or suspending the use of the vaccine or restricting its use to lower-risk groups, using age as a cut-off point.
As early as 11 March 2021, the Paul-Ehrlich Institute, an agency of the German Federal Ministry of Health, first reported cases of cerebral venous sinus thrombosis (CVST) that could be linked to the AstraZeneca vaccine, leading to temporary suspension of vaccination in Germany. This was followed by a press release on 7 April, in which the safety committee of the European Medicines Agency listed blood clots with low blood platelets as a very rare side effect of the AstraZeneca vaccine, based on figures from the EuroVigilance surveillance system. Since then, there have been several reports of CVST and other unusual blood clotting disorders following Covid-19 vaccination.
The risk of blood clots and unusual bleeding appears to be higher after the first dose of AstraZeneca vaccine, is seen slightly more often in younger women, and tends to occur between four days and four weeks following vaccination. According to Public Health England, the overall risk is around 1 in 100,000 first doses. There seems to be no link with any specific production site or batch of vaccine. Authorities suggest it is safe to have a second dose of AstraZeneca vaccine provided the first dose was not followed by blood clots.
Post-vaccine blood clots appear to progress rapidly, often causing life-changing complications. These clots often form in unusual locations, such as the venous sinuses of the brain, veins within the abdomen, as well as in various arteries. Abnormal blood clot formation in the presence of low levels of platelets in the circulating blood has been termed vaccine-induced thrombosis and thrombocytopenia, or VITT. A similar condition has previously been recognised as a rare complication of treatment with heparin, an anticoagulant or blood-thinning agent. Platelets are depleted in both instances by an immune response, leading to increased antibody production. In this instance, antibodies to platelet factor 4, a protein that is released from platelets, cause platelets to clump together. Platelet aggregation can then trigger bleeding as platelets are used up, causing pinpoint haemorrhagic spots, or more extensive bruises, in the skin.
The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) is currently recommending a vaccine other than AstraZeneca in people aged 39 years and younger. On 7 May, the Joint Committee on Vaccination and Immunisation (JCVI) advised the offer of alternative vaccines wherever possible, provided this would not lead to substantial delay or prove a barrier in access to vaccination.
CVST has been described mainly in younger women, 50 years of age or under. It normally affects between 5 to 16 new persons per year, per million population, and accounts for up to 1 per cent of all strokes. The condition may be heralded by new, severe and progressive headache that does not respond to the usual painkillers. This headache may get worse upon lying down or bending over. Accompanying symptoms may include blurred vision, nausea and vomiting, difficulty with speech, or weakness, drowsiness or seizures. Confirmation of the diagnosis requires specialised imaging of the venous sinuses, and the specific treatment involves an intravenous infusion of immunoglobulins.
We can conclude, from the available evidence, that the AstraZeneca and Johnson & Johnson adenoviral vector vaccines, which have similar modes of action, appear to be linked to the very rare complication of blood clots with low circulating platelet levels. There seems to be no discernible link with mRNA vaccines such as the Pfizer BioNTech product. It is preferable to follow the guidance of our public health authorities, who are best placed to monitor complications of vaccination and appropriately tailor advice for the people they are responsible for.
Ashis Banerjee