Blood clot fears spread to J&J’s Covid-19 vaccine

The US pauses use of JNJ-78436735 after six cases of cerebral venous sinus thrombosis.

Until today, fears over unusual blood clots had been limited to one of the approved Covid-19 jabs, Astrazeneca’s Vaxzevria (AZD1222). Now, use of Johnson & Johnson’s JNJ-78436735 has been paused in the US while the FDA investigates six cases of cerebral venous sinus thrombosis (CVST).

The agency stressed that the events were very rare – CVST has been seen in less than one in a million people vaccinated with JNJ-78436735 – but the latest development will not help with vaccine hesitancy.

And it did not help J&J’s share price either: the group has pledged not to make a profit on its vaccine during the pandemic, but nevertheless slid 2% this morning.

Meanwhile, the stock of the mRNA vaccine makers was up: Moderna climbed 8% while Biontech rose 6%.

J&J also said it was "proactively" delaying the rollout of its one-shot vaccine in Europe, where it is approved but not yet available. On Friday the EMA began a review of blood clots seen in JNJ-78436735-vaccinated people, citing four serious cases of clotting with low platelets.

Six CVST cases

All six CVST cases seen with with JNJ-78436735, which the FDA is now investigating, occurred in women between 18 and 48. In this respect this is similar to the experience with AZD1222, where most CVST cases have been in women under 60.

One thing the two vaccines have in common is that they both use an adenoviral vector, and theories are emerging about how such vectors could trigger the body’s immune response. One is that virus particles could break apart and release their DNA, which could bind to platelet factor 4 and spur the production of antibodies. These remain just theories for now, however.

JNJ-78436735 uses a human vector, adenovirus 26, while AZD1222 uses a chimpanzee adenoviral vector. Other vaccines using similar vectors are Cansino's Ad5-nCoV, which employs adenovirus type 5, and the Gamaleya Institute's Sputnik V, which uses an adenovirus 26 primer followed by an adenovirus 5 booster.

Peter Marks, the head of the FDA's CBER division, was asked during a media call today whether he thought this was a class effect with the adenoviral vector-based vaccines. "I hesitate to call it a class effect, but I think it’s plainly obvious to us already that what we’re seeing with the Janssen vaccine looks very similar to what’s being seen with the AZ vaccine," he stated.

He added that no cases of CVST combined with low platelets had been seen with either of the approved mRNA vaccines, with over 180 million doses of these given so far.

The CDC has convened a meeting of the independent Advisory Committee on Immunization Practices tomorrow to discuss JNJ-78436735, and perhaps things will become clearer then.

Cases of CVST with selected Covid-19 vaccines
Vaccine Developer CVST cases adjudicated Implied CVST incidence Source
AZD1222/Vaxzevria  Astrazeneca/University of Oxford/Vaccitech 169 in 35 million vaccinated (both figures for EEA + UK) 4.83 per million EMA statement, April 7
JNJ-78436735 Johnson & Johnson 6 in 6.8 million vaccinated (both figures US) 0.88 per million FDA statement, April 13
mRNA-1273 Moderna 5 cases WW; 4 million vaccinated in EEA + UK - EMA call, April 7
Comirnaty/BNT162b2 Pfizer/Biontech 35 cases WW; 54 million vaccinated in EEA + UK  - EMA call, April 7

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