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New research has identified unusual antibodies that appear to have caused, in rare cases, serious and sometimes fatal blood clots in people who received the Covid vaccine made by AstraZeneca.

Exactly why the rare reactions to the vaccine occurred is still a mystery.

Scientific teams from Germany and Norway found that people who developed the clots after vaccination had produced antibodies that activated their platelets, a blood component involved in clotting. The new reports add extensive details to what the researchers have already stated publicly about the blood disorder.

WASHINGTON — Supplies of Johnson & Johnson’s one-dose coronavirus vaccine will be extremely limited until federal regulators approve production at a Baltimore manufacturing plant with a pattern of quality-control lapses, the White House’s pandemic response coordinator said on Friday.

With allocations of the company’s vaccine set to plunge by 86 percent next week, governors across the country warned that the loss of supplies they had been counting on would set back their vaccination drives.