Johnson & Johnson and AstraZeneca are conducting early-stage research into whether potential modifications of their Covid-19 vaccines could reduce or eliminate the risk of rare, but serious, blood clots associated with the shots, The Wall Street Journal reported Tuesday, citing unnamed sources.

The blood clotting issue, which some scientists are calling vaccine-induced immune thrombotic thrombocytopenia, or VITT, has been linked to both shots. Scientists have been discovering clues into how the clots form, boosting hopes of possibly reengineering the shots, the Journal reported.

J&J said it supports “continued research and analysis as we work with medical experts and global health authorities.”

Johnson & Johnson remains committed to helping end this deadly pandemic as quickly as possible, and we continue to believe in the positive benefit-risk profile of our vaccine,” the company said.

AstraZeneca 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.”

The Food and Drug Administration in April said it was adding a warning label to J&J’s Covid vaccine, citing the blood clotting disorder as a rare side effect.

On Monday, the FDA said it was adding yet another warning to J&J’s label, saying the shot has been linked to a serious, but rare, autoimmune disorder known as Guillain-Barre syndrome.

About 100 preliminary reports of Guillain-Barre have been detected after 12.8 million doses of the J&J vaccine were administered, according to the CDC. The cases reported after receiving the J&J shot largely occurred about two weeks after vaccination and mostly in males, many aged 50 years and older, according to the CDC. Available data do not show a similar pattern with Pfizer’s or Moderna’s vaccine, the agency said.

The FDA and CDC on April 13 asked states to temporarily halt using J&J’s vaccine “out of an abundance of caution” while it investigated six women, ages 18 to 48, who developed cerebral venous sinus thrombosis, or CVST, in combination with low blood platelets within about two weeks of receiving the shot.

They recommended resuming use of the shot 10 days later after the CDC determined that the benefits of the inoculations outweighed their risks.

The Journal report Tuesday noted that modification of the vaccines might require changes to ownership rights or regulatory approvals.