COVID-19 vaccination has hit arguably its biggest hurdle so far. The rollout of two vaccines – AstraZeneca’s and Johnson & Johnson’s – has been paused, restricted or completely stopped in a number of countries as regulators and governments respond to signs that they may be linked to rare abnormal blood clots in a small number of recipients.
The European Medicines Agency has noted that the link between the blood clots and the AstraZeneca vaccine is probable but unproven. But it is up to individual member states to decide how they wish to use the vaccine. The effect is that some European countries have introduced strict limits, allowing the jab to be given only to older age groups who are at significant risk from the virus, writes Anthony Cox, Reader in Clinical Pharmacy and Drug Safety at the University of Birmingham.
Denmark has even chosen to withdraw the product completely, and Norway’s public health institute has said it should do the same. However, in Britain the response has been more measured. The UK’s Joint Committee on Vaccination and Immunisation has recommended that under-30s should be offered an alternative vaccine — Pfizer and Moderna shots are now both being administered in the country — but that otherwise its use should continue as for all other groups it benefits outweigh the risks.
In the US, the same safety signal of a rare, unwanted type of blood clotting has been reported more frequently than normally would be expected among people who have taken the Johnson & Johnson vaccine. As a result, the vaccine’s rollout has been paused in the US while officials, like their European counterparts, try to uncover whether there is a link.
The decisions of these western nations’ regulators are having a knock-on effect elsewhere. The withholding of AstraZeneca and Johnson & Johnson vaccines in Europe and the US has dented both official and public confidence in these jabs, in some instances in countries where there are currently no alternatives.
South Africa, having stopped using the AstraZeneca vaccine because of concerns that it may be less effective against the country’s dominant variant of coronavirus, has now also paused giving the Johnson & Johnson vaccine. In other African countries still offering the AstraZeneca jab, public fear is mounting and doses are going unused. Considering how efforts to deliver vaccines to lower-income countries are already faltering, declining faith in the few doses that have been provided is worrying – this compounds the issue that coverage in low- and middle-income countries was already likely to be low for the foreseeable future.
The specific type of blood clotting reported with these vaccines is quite rare. It’s called cerebral venous sinus thrombosis, explains Adam Taylor, Director of the Clinical Anatomy Learning Centre at Lancaster University, and is accompanied by a lack of platelets in the blood – what’s known as thrombocytopenia. The rarity of this type of clotting suggests that the same thing might be going wrong in recipients of both vaccines, which has led people to look at their similarities.
One theory, which Kylie Quinn of RMIT in Australia outlines, is that the clotting may be related to the vaccines’ design. Both stimulate an immune response by getting the body to make copies of the coronavirus’s spike protein, and they do this by delivering some of the coronavirus’s genetic material into the body using a different, harmless adenovirus as a carrier. There’s no evidence yet, but it could be that this adenovirus-based system is triggering an adverse immune response that affects the blood in a small minority of people.
Meanwhile, a second blood-related disorder – capillary leak syndrome – also may be surfacing more frequently than it would usually among people who have taken the AstraZeneca vaccine. However, drawing a link between this condition and the vaccine may be even harder than with the blood clotting, as so little is known about it, writes Peter Abel, Senior Lecturer in Haematology, Immunolgy, Genetics and Evolution at the University of Central Lancashire. There’s also no evidence so far that it and the blood clotting issues are linked.
