WASHINGTON — Federal health agencies on Tuesday called for an immediate pause in use of Johnson & Johnson’s single-dose coronavirus vaccine after six recipients in the United States developed a rare disorder involving blood clots within about two weeks of vaccination.

All six recipients were women between the ages of 18 and 48. One woman died and a second woman in Nebraska has been hospitalized in critical condition.

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.