The race to untangle the secrets of rare, severe blood clots after Johnson & Johnson vaccination

When an otherwise healthy 48-year-old Nebraska woman arrived at an emergency room after three days of abdominal pain and malaise, doctors discovered a life-threatening puzzle. Her platelets, the colorless blood cells that clump to form clots, had plummeted. But a CT scan of her abdomen and pelvis revealed extensive blood clots.

Her medical team raced to untangle the seemingly paradoxical combination of symptoms. Even as they treated the patient with a common blood thinner, more clots appeared — in her brain and in the blood vessels around her liver and...