J&J, AstraZeneca Explore Covid-19 Vaccine Modification in Response to Rare Blood Clots

A J&J spokesman said the company supports “continued research and analysis as we work with medical experts and global health authorities.” AstraZeneca has said it is “actively working with the regulators and scientific community to understand these extremely rare blood-clotting events, including information to drive early diagnosis and intervention, and appropriate treatment.”

Apart from the scientific hurdles, modifications might require changes to ownership rights or regulatory approvals. Still, the unprecedented scale and urgency of the Covid-19 vaccine rollout could fast-track the scientific research and legal issues, scientists say, just as it accelerated the development of the vaccines in the first place.

The risks of clotting combined with low blood platelets—a rare condition termed vaccine-induced immune thrombotic thrombocytopenia, or VITT—following the AstraZeneca shot are roughly 1 to 2 per 100,000 vaccinations, according to U.K. and European tallies of cases.

U.S. government data show a lower rate of around 0.3 instances of the clotting condition per 100,000 doses for the J&J vaccine. The risk of severe clotting is generally higher from Covid-19 itself and has also long been associated with use of the common blood-thinning drug heparin, researchers and regulators say.

Johnson & Johnson’s single-dose Covid-19 vaccine is approved in the U.S., but its deployment was halted temporarily in April over concerns about a link to the blood clotting. After investigating, U.S. health regulators recommended use of the vaccine resume, saying its protection against Covid-19 outweighed the risk of the rare clotting for the most of people. They added a warning about the clotting risk.

The AstraZeneca vaccine isn’t authorized in the U.S., but is widely used in the rest of the world, with more than 600 million doses distributed. In the U.K. and across the Europe Union, regulators have recommended alternative vaccines for younger adults, who are considered more vulnerable to the blood clotting compared with Covid-19.

Both companies and the University of Oxford have said the vaccines’ benefits generally outweigh the risks, echoing guidance from government officials.

The J&J and AstraZeneca shots have more recently been linked by U.S. regulators to a slightly increased risk of Guillain-Barré syndrome, a rare neurological disorder associated with other vaccines. U.S. and European officials have alerted doctors and vaccine recipients to the small increased risk. They said the shots remained valuable tools in the pandemic.

For both shots, minimizing or eliminating the rare blood-clotting issues could be key to boosting confidence in the vaccines and opening doors to wider use. Both vaccine makers are selling their shots without profit. Both can be stored for months at normal refrigeration temperatures, in contrast to ultracold temperatures necessary for longer-term storage of leading vaccines by Pfizer Inc. and Moderna Inc. J&J’s single-dose regimen is also seen as an advantage for protecting hard-to-reach people after just one shot.

Modifications to eliminate clotting reactions, if such changes are possible, could help turn the shots into moneymakers in the long run, especially if booster vaccinations become widely adopted. The two companies could seek to profit from the vaccines in wealthier countries once the pandemic is over.

Scientists say they need to first understand whether the culprit is one or more ingredients in the vaccines, the purification process, or something mechanical in how the vaccines work. Both shots use so-called adenoviruses, modified, harmless viruses that carry genetic instructions into human cells, prompting a buildup of antibodies and immune-defense tools against Covid-19. In the AstraZeneca shot, the vector, or carrier, is an altered chimpanzee cold virus. The J&J shot uses a weakened human cold virus.

“Progress is being rapidly pursued by the companies, and I think their desire to alter the vector or the vaccine is moving in parallel to explaining” the clots, said Mortimer Poncz, pediatric-hematology division chief at the Children’s Hospital of Philadelphia.

AstraZeneca and the team of Oxford scientists who developed the shot are researching how the vaccine in very rare cases triggers the immune response underpinning the clots, according to Sarah Gilbert, an Oxford vaccinologist and co-inventor of the vaccine. Oxford and AstraZeneca are exploring potential changes to the vaccine’s delivery mechanism, according to Prof. Gilbert. She said she can’t predict when a retooled vaccine might be ready. An Oxford spokesman declined to elaborate.

Oxford researchers have said they hope to have a new variant-targeting version of the vaccine ready by autumn. That is probably faster than the vaccine could be modified to address the rare blood clots, according to some of the people familiar with the matter.

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Write to Jenny Strasburg at jenny.strasburg@wsj.com and Parmy Olson at parmy.olson@wsj.com