U.S. health officials are weighing next steps as they investigate a handful of unusual blood clots in people who received Johnson & Johnson's COVID-19 vaccine — a one-dose shot that many countries were eagerly awaiting to help speed virus protection.
It's not clear if the exceedingly rare reports — so far, six cases out of more than 7 million inoculations in the U.S. — really are linked to the J&J vaccine. But the government recommended a pause in J&J vaccinations on Tuesday, just a week after European regulators declared that such clots are a rare but possible risk with the AstraZeneca vaccine, a shot made in a similar way but not yet approved for use in the U.S.
Experts say COVID-19 overall poses a much bigger risk of death and hospitalization than the potential clot threat. The next phase comes Wednesday, when advisers to the Centers for Disease Control and Prevention will debate in a public meeting how to handle the J&J vaccine while authorities try to nail down exactly what's happening.
"Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases as this syndrome may not be easily recognized," CDC Director Rochelle Walensky said ahead of that meeting.
The clot concerns could undermine public confidence in a vaccine many hoped would help some of the hardest-to-reach populations — in poor countries or in places like homeless shelters in the U.S.
"We know we are fighting a war against COVID-19," said Dr. Peter Marks, the Food and Drug Administration's vaccine chief. But when it comes to side effects, "we don't, in the United States, have a lot of tolerance for friendly fire."
Marks said his agency recommended the J&J timeout less because of the number of reported clots than to make sure doctors know how to recognize and treat the unusual condition. One of the six women had the condition complicated by treatment with a blood thinner named heparin, he said — a treatment the government is warning doctors to avoid.
Walensky said the ability to spot such rare reports and take action fast "demonstrates that the safety systems we have in place are working."
The setback for J&J comes as the worldwide death toll from COVID-19 approaches 3 million, including more than 560,000 who perished in the U.S., which continues to report tens of thousands of new infections every day and an average of almost 1,000 deaths.
So far, the J&J vaccine has been a minor player in U.S. vaccinations. More than 122 million Americans have received at least one vaccine dose, the vast majority with shots made by Moderna or Pfizer, and nearly 23% are fully vaccinated.
Both companies are on track to have delivered 300 million doses each by mid- to late July — and federal health authorities stress that there are no signs of the unusual clots with the Moderna and Pfizer vaccines.
Vaccinations are slower in Europe, where many countries have struggled for supply. J&J delayed some of its European deliveries amid the clot evaluation, but Poland said it would use the batch it already has in hand. European medical regulators plan to issue their own evaluation of the J&J clot issue next week.
The U.S. set up intensive systems to track the safety of COVID-19 vaccines as they rolled out, knowing that side effects too rare to have occurred in studies of thousands of people could pop up once millions rolled up their sleeves.
The clots in question are not typical blood clots. They occur in unusual parts of the body, such as veins that drain blood from the brain. And the patients also have abnormally low levels of platelets — cells that help form clots — a condition normally linked to bleeding, not clotting.
When those weird clots were spotted after AstraZeneca vaccinations, scientists in Norway and Germany raised the possibility that some people are experiencing an abnormal immune response, forming antibodies that disable their platelets. That's the theory as the U.S. now investigates the J&J reports.
Health officials caution against confusing the normal flu-like symptoms that occur a day or two after many COVID-19 vaccination with the clot concern. The problematic clot symptoms, such as severe headache or severe abdominal pain, have occurred about a week to three weeks after the J&J shot.
For vaccine recipients, the headlines can be scary. Holli Vrenon, 35, who got her J&J shot three or four weeks ago in Reno, Nevada, had flu-like symptoms after the vaccination and asked her brother, a doctor, what to think.
He told her to monitor her symptoms but not to worry too much, "but obviously it worries you," Vrenon said. "If they're suspending it, obviously people are getting side effects."