There is a “possible link” between Johnson & Johnson’s Covid-19 vaccine and very rare blood clots, the European medicines regulator has said.
But the European Medicines Agency (EMA) concluded the overall benefits of the vaccine “outweigh the risks of side effects”.
The Janssen vaccine is yet to be approved for use in the UK.
Officials have pre-ordered 30 million doses, which are expected to arrive in the second half of 2021 if approved by the Medicines and Healthcare products Regulatory Agency (MHRA).
Prime Minister Boris Johnson said the Government is “confident” about vaccine supply and the news would not derail the UK’s programme to offer a jab to all adults by the end of July.
Asked about the development during a Downing Street press conference, he said: “We’re confident in the security of our supply and that we will be able to get what we said we are going to do by the end of July.”
Dr Nikita Kanani, medical director of primary care for NHS England, said: “It’s not a vaccine that we are using at the moment in this country and as with any vaccine we’d wait for MHRA approval.
“Right now the priority is to get your vaccine if you are offered it because we have the supply to vaccinate you if you are eligible.”
The safety committee of the EMA concluded that unusual blood clots with low blood platelets should be listed as a “very rare” side effect of the Janssen vaccine.
Officials reviewed eight cases in the US, where more than seven million people have received the jab.
All eight occurred in people under 60 within three weeks of vaccination, with the majority in women.
The EMA committee noted that the blood clots occurred mostly at “unusual sites” such as in veins in the brain, in the abdomen and in arteries, together with low levels of blood platelets and sometimes bleeding.
The panel said they were “very similar to the cases that occurred with the Covid-19 vaccine developed by AstraZeneca”.
European officials said they will provide more detail on their review of the Oxford/AstraZeneca jab this week.
Dr Sabrine Straus, chairwoman of the EMA safety committee, told a press briefing: “The AstraZeneca vaccine is an adenovirus-based vaccine, just as the Janssen vaccine, so there are some similarities between the two vaccines but there are also differences.”
She added that it is “too early to draw any conclusions”.
The AstraZeneca and J&J vaccines use viral vector technology where a modified version of a different virus is used to to deliver instructions to the body’s cells to trigger the immune system to begin producing antibodies.
The EMA safety committee said one possible explanation behind the clots could be an immune response similar to one sometimes seen in patients treated with blood thinning medication heparin.
But they said the cause is not yet known.
Dr Straus said healthcare professionals treating people with potential side effects should avoid heparin “out of prudence”.
The EMA said people taking the jab and health workers should be made aware of the potential very rare side effect, adding that prompt specialist medical treatment can help recovery and avoid complications.
Symptoms include: shortness of breath; chest pain; swelling in the leg; persistent abdominal pain; neurological symptoms including severe or persistent headaches or blurred version; skin bruising beyond the site of injection.
Johnson & Johnson previously confirmed it will delay rollout of its single-dose vaccine across Europe after the US paused its administration to investigate reports of potentially dangerous blood clots.
Regulators in the UK said they will examine “any safety reports” during considerations on whether to approve the jab.
Emer Cooke, executive director of the EMA, said: “This weekend the global death toll from Covid-19 surpassed three million and there are still thousands of people dying every day.
“I don’t need to tell you that there is untold human suffering behind all of these cases and these vaccines play an immensely important role in combating this pandemic.
“When vaccines are rolled out to a large number of people it is possible that very rare side effects can occur and these will not necessarily have been identified in the clinical trials
“But because we have a very good pharmaco-vigilance system in place in Europe we can spot these events quickly and we can take action to make sure that healthcare professionals and patients are aware.”
Dr Peter Arlett, head of pharmaco-vigilance and the epidemiology department at the EMA, said global cases of clots with low platelets to April 13 include eight linked to the Janssen vaccine, 287 linked to AstraZeneca, 25 linked to Pfizer, and five linked to Moderna.
Jonathan Ball, professor of molecular virology at the University of Nottingham, said: “These side effects are very similar to those reported for AstraZeneca Covid-19 vaccines, which does suggest that it is the inactivated adenovirus delivery system that might be causing the problems.
“It’s important to remember though that in most people the benefits of these vaccines far outweigh the risks – these are incredibly rare potential side-effects.”