AstraZeneca and Johnson & Johnson are investigating how to tweak their vaccine to preclude the risk of blood clots.
The companies are understood to be conducting laboratory work to understand the molecular mechanism that may cause the complication, with a view to potentially producing a modified version.
It comes after data showed a probable causal link between the jabs and an extremely small chance of fatal blood clots.
The risk has prompted watchdogs to restrict the AstraZeneca jab to those aged 40 and older.
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.
Meanwhile 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.
AstraZeneca confirmed it was actively investigating its current vaccine to try to pin down potential cause of the blood clots.
However, a modified vaccine programme has not yet been launched.
“Astounding” success of vaccines show they are almost as effective against Indian variant
A spokesman said: “We are 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.”
A J&J spokesman said the company supports "continued research and analysis as we work with medical experts and global health authorities."
It came as the US Food and Drug Administration updated its warning labels for the one-shot Johnson & Johnson jab to include information about an observed “increased risk” of a rare neurological disorder called Guillain-Barre Syndrome.
The vaccine was approved for use in the UK in May, and the first of 20 million ordered doses are expected to arrive later this year.