The European drugs regulator has found a “possible link” between the Johnson & Johnson vaccine and very rare blood clots detected in some recipients of the jab.
A warning over the clotting, which is combined with low blood platelet counts in the cases of concern, should be listed as a rare side effect for the one-dose vaccine, the safety committee for the European Medicines Agency said.
However, it insisted that the overall benefit of the jab in providing protection against Covid-19 far outweighed any associated risk.
Last week, Johnson & Johnson halted its European rollout of the vaccine, which is yet to be approved in the UK, after American officials recommended a pause of the jab and launched an investigation into eight reports of clotting detected among seven million people in the US who have received a first dose.
The EMA reviewed these cases and found that all instances of clotting had occurred in adults under 60 years, mostly women, within three weeks of vaccination.
The blood clots largely occurred in veins in the brain, a condition called cerebral venous sinus thrombosis, and the abdomen, together with low levels of blood platelets - otherwise known as thrombocytopenia.
These thromboembolic events were “were very similar to the cases that occurred with the Covid-19 vaccine developed by AstraZeneca,” the EMA said.
Based on the currently available evidence, specific factors that might heighten an individual’s chances of developing the blood clots have not been confirmed, the agency added.
The EMA said people who receive the Johnson & Johnson jab must seek urgent medical attention if they develop the following symptoms three weeks after vaccination: shortness of breath; chest pain; leg swelling; persistent abdominal pain; severe and persistent headaches or blurred vision; and tiny blood spots under the skin beyond the site of the injection.
It also speculated that the rare side effects are a result of an abnormal immune response triggered by the vaccine, leading to a condition similar to one seen sometimes in patients treated with heparin, a blood-thinning medicine.
With recipients of the AstraZeneca vaccine who have developed CVST and thrombocytopenia, scientists in Norway and Germany have suggested that the immune system is forming antibodies that attack the person’s own blood platelets.
It’s not yet clear if there might be a similar mechanism with the Johnson & Johnson vaccine. But both the J&J and AstraZeneca vaccines, as well as a Russian jab and one from China, are made with the same technology.
They train the immune system to recognise the spike protein that coats the shell of Sars-CoV-2. To do that, they use a cold virus, called an adenovirus, to carry the spike gene into the body.
The delayed rollout of the Johnson & Johnson jab on the continent was a further blow to vaccination efforts in the European Union, which have already been plagued by supply shortages, logistical problems and rising hesitancy surrounding the AstraZeneca vaccine.
The EU has ordered 200 million shots of the Johnson & Johnson vaccine for 2021. The likes of Germany, Italy, France and Spain were all set to start administering jabs this week. Supplies were subsequently placed into storage.
South Africa also suspended its use of the vaccine in the wake of the US pause, having already administered more than 289,000 doses to the country’s health workers.
The UK has 30 million doses of the vaccine on order, but it has not yet been signed off for use by the Medicines and Healthcare products Regulatory Agency (MHRA).
These supplies are expected to arrive in the second half of 2021 if approved by the regulator.
The Department of Health and Social Care (DHSC) has previously insisted Johnson & Johnson’s decision to delay the rollout of its vaccine in Europe would not derail the UK’s programme to offer a jab to all adults by the end of July.
Regulators in the UK said that they will examine “any safety reports” during their considerations on whether or not to approve the jab.