By Thomas M. Burton 

WASHINGTON -- Clinical trials are medicine's way of deciding whether vaccines and drugs work, and how safe they are.

But the studies have limits, as was shown Tuesday when U.S. health authorities called for temporarily sidelining Johnson & Johnson's Covid-19 vaccine while they investigate blood clots that six women developed after getting the shot.

The Food and Drug Administration approved an emergency use authorization for the J&J vaccine following positive results in clinical trials involving 44,000 people, half of whom got the vaccine, with the rest getting a placebo.

Since then, nearly seven million people in the U.S. have gotten the J&J shot. Scientists say it isn't surprising that other side effects occurred as the vaccine was given to a much larger population than during trials.

"We have to keep reiterating that we can't pick up a one-in-a-million event in clinical trials," said Arnold Monto, a University of Michigan epidemiologist and public-health professor who chairs the independent panel that evaluated the J&J clinical study for the Food and Drug Administration.

Paul Offit, a vaccine authority and pediatrics professor at the Children's Hospital of Philadelphia, agreed that a trial of 44,000 people would be "unlikely to pick up an instance of adverse events at the rate of one in a million."

Both Dr. Offit and Dr. Monto noted that the cases of blood clots did turn up in the FDA's post-study safety net once vaccination commenced in the broader population.

Dr. Offit said he thinks the phenomenon of cerebral blood clots among younger women linked to the Covid-19 J&J vaccine is real, although rare, and added that federal officials were wise to pause the use of the vaccine.

Even beneficial vaccines can have side effects -- for example, flu vaccines have been linked to possibly causing rare cases of Guillain-Barre syndrome, a rare neurological disorder.

The oral polio vaccine, now largely discontinued, led to one polio case in about 2.4 million doses -- a ratio considered acceptable given how many cases it prevented.

Write to Thomas M. Burton at


(END) Dow Jones Newswires

April 13, 2021 14:57 ET (18:57 GMT)

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