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Concerns about Johnson & Johnson's Covid-19 vaccine rose after the US Food and Drug Administration (FDA) and Centers of Disease Control and Prevention (CDC) recommended for states to pause administering the inoculation so the agencies could review "extremely rare" blood clots.
The United States has administered more than 6.8 million single-doses of the vaccine to residents. Of those administered, six women aged 18 to 48 have developed a severe disorder that caused rare blood clots.
In each case, symptoms formed six to 13 days after the recipients received the dose, the FDA and CDC revealed in a joint statement.
The cases were isolated and statistically very low with the women developing a condition called cerebral venous sinus thrombosis, or CVST, that caused the rare blood clots. But the federal health agencies recommended the vaccine pause "out of an abundance of caution" so healthcare providers could have enough information to treat these rare cases.
"These events appear to be extremely rare. However, Covid-19 vaccine safety is a top priority for the federal government and we take all reports of adverse events following vaccination very seriously," said Dr Janet Woodcock, the acting FDA commissioner, during a press briefing on Tuesday.
The CDC’s Advisory Committee on Immunization Practices (ACIP), an independent group of experts, met on Wednesday to discuss the Johnson & Johnson vaccine pause and what it means. It failed to come up with a definitive decision on whether to lift the pause on administering the vaccine pending further investigations.
How rare is CVST in people who received the vaccine?
It is important to note that the six cases being investigated are extremely rare out of the more than 6.8 million single-dose vaccines that have been administered to Americans.
"This is a really rare event. If you look at what we know so far, there have been six out of the 6.85 million doses, which is less than one in a million. So remember, this is something that we always – really out of an abundance of caution ... to see if we can get further information," said Dr Anthony Fauci during a White House press briefing on Tuesday.
The rare disorder, CVST, occurs when a blood clot forms in the brain, which then prevents blood from flowing out of the area. As a result, blood cells may break and leak blood into the brain, causing a haemorrhage, according to Johns Hopkins Medicine.
Similar blood clots have been seen in Europe and the United Kingdom after residents received AstraZeneca's Covid-19 vaccine, which was not authorised for emergency use in the US. Both vaccines are different from the inoculations created by Pfizer and Moderna because they use common cold viruses called adenoviruses to transfer genetic material from Covid-19 into the body to create an immune response.
During the ACIP’s emergency meeting, the CDC"s Dr Beth Bell said the agency did not yet know the link between the two vaccines and blood clots.
"The extent to which the cases seen after each of these adenoviral vector vaccines represent exactly the same syndrome is not entirely clear at this time," she said.
Part of the reason why the FDA and CDC have recommended a pause for the vaccine was so the agencies could inform healthcare providers across the country about the rare blood clots and the proper way to treat them.
"Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given," the agencies said in a joint statement.
Do health officials know why these cases are occurring?
Part of the Johnson & Johnson vaccine pause was in part so the FDA and CDC could investigate the six cases and determine how they related to the inoculation.
To date, though, there was no evidence of a causal relationship between the Johnson & Johnson vaccine and the "extremely rare" blood clots. This point has been highlighted by the pharmaceutical giant in a statement on Friday.
"Our close tracking of side effects has revealed a small number of very rare events following vaccination. At present, no clear causal relationship has been established between these rare events and the Janssen Covid-19 vaccine," the statement read. Janssen is the vaccine arm of the pharmaceutical giant.
Dr Aran Maree, chief medical officer for pharmaceuticals at J&J's Janssen vaccine arm, said that they believed the Covid-19 vaccine benefits outweigh any blood clot risks when speaking during the ACIP’s meeting on Wednesday.
"I'd like to reiterate that based on the current data, Janssen believes the overall benefit-risk profile for our vaccine is positive across the population for which it's authorised," Dr Maree said.
"We strongly support ensuring awareness of the signs and symptoms of this very rare event, as well as recommendations to ensure the correct diagnosis, treatment, and reporting by healthcare professionals," she added.
At this time, the FDA and CDC have no details on whether specific subgroups of the American population were at risk of developing blood clots after receiving the Johnson & Johnson vaccine. Dr Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said the clots likely formed due to an "immune response that occurs very, very rarely after some people receive the vaccine."
"That immune response leads to activation of the platelets and these extremely rare blood clots," Dr Marks said during a media briefing on Tuesday.
Are people who previously received the vaccine at risk?
Of the six reported cases, the women developed severe symptoms to the Johnson & Johnson vaccine between six to 13 days after inoculation.
Experts from the FDA and CDC emphasised that anyone who received the vaccine more than one month ago should feel confident in knowing their risk of developing blood clots was extremely low.
However, people who received the vaccine within the last three weeks were asked to monitor for any adverse effects. Symptoms to look out for include severe headache, abdominal pain, and shortness of breath.
If anyone experiences any of the above symptoms, they are asked to contact their healthcare provider immediately.
Should people still get the Johnson & Johnson vaccine if it's available?
At the moment, most states have halted the use of Johnson & Johnson's vaccine at vaccination sites amid the pause. Instead, health officials were offering residents with Johnson & Johnson appointments the opportunity to receive Moderna and Pfizer shots instead.
Dr Anne Schuchat, principal deputy director of the CDC, said that the federal health agencies were not seeing blood clotting in Moderna or Pfizer shots, so unvaccinated individuals should still get vaccinated from one of the two available vaccines.
"People who have vaccine appointments with the other two vaccines should continue with their appointments," Dr Schuchat said.
Following the investigation launched by the FDA and CDC, the public will have a better idea whether it's safe to receive the Johnson & Johnson vaccine. The pause was only expected to last for a "matter of days:" as the agencies reviewed the available information, Dr Woodcock said.
"The message to the patients would be to those who haven't been vaccinated, to continue to get the vaccines that are available to them because the risks from the pandemic are significant," Dr Woodcock added.
Johnson & Johnson received emergency use authorisation for its Covid-19 vaccine in February following its clinical trials. The vaccine was found to be 66 per cent effective against preventing symptomatic disease, which was a lower efficacy rate compared to that of Pfizer's and Moderna's shots.
Clinical trials showed the vaccine was 85 per cent effective at preventing severe disease, and 100 per cent effective at preventing hospitalisations and deaths.