A top government panel delayed making a decision on how to address the Johnson & Johnson COVID-19 vaccine, following concerns about blood clots and low blood platelet levels in at least six women who received the vaccine. 

The move came after an emergency meeting Wednesday of the Advisory Committee on Immunization Practices, which advises the Centers for Disease Control and Prevention. The committee met after the U.S. government advised temporarily halting use of the Johnson & Johnson shot Tuesday. 

It means that the temporary pause will continue, and the committee will reconvene when there is more data collected about the vaccine’s relationship to the blood clots — a process that could take a week to 10 days. 

The United States has administered 6.8 million Johnson & Johnson shots — a fraction of the total 190 million COVID vaccines used so far. The other two vaccines, developed by Pfizer and Moderna, have not raised similar concerns.

The six cases reported so far all emerged in women younger than 48, one of whom has died, and several others are still in hospital care. One more potential case is being investigated, per data presented at the Wednesday meeting. Two similar blood clot cases emerged while the Johnson & Johnson drug was still in clinical trials — one in a 25-year-old man, one in a 24-year-old woman.

Still, it’s not fully clear what is behind the blood clot-low platelet combination. In general, women are at higher risk for this particular kind of blood clot, known as cerebral venous sinus thrombosis (CVST). Adults older than 65 are at lower risk. But CVST is already rare, and it is even less common to see it in conjunction with low blood platelets. 

Typically, pregnancy or being postpartum can elevate the risk for CVST. So can oral contraception. But none of the six reported cases involved someone who was pregnant or postpartum. Only one person was on estrogen or progesterone, according to government data.

Though only 6 out of 6.8 million Johnson & Johnson doses have so far seen the CVST concern emerge, it’s not clear if there is in fact a causal relationship. 

More cases may also be reported, experts note, now that doctors have been alerted to keep an eye out for the symptoms, so it’s difficult to properly assess how common these symptoms are in people who have received the vaccine compared to the general population.

The committee was presented with several options in Wednesday’s meeting: delay making a decision, recommend the Johnson & Johnson vaccine go back into distribution, extend the pause or recommend the vaccine only for men or only for adults of a certain age, who may be less likely to develop CVST and low blood platelet count. 

But CDC advisers worried that there still isn’t enough information to fully understand which populations are at risk — including whether there is in fact a gender-gap for risk. The United States has also already administered at least one shot to almost 80 percent of all adults older than 65, limiting the potential for senior citizens to receive the Johnson & Johnson shot.

In Europe, experts are weighing whether to recommend a vaccine developed by AstraZeneca, which has had similar concerns, exclusively only to adults older than 60. 

Extending a pause on the vaccine, some committee members argued, would give more time to collect data and make sure health care providers are appropriately diagnosing and treating people who develop blood clot-related symptoms. CVST and low platelet count require different treatment from other, more common blood clots known as deep vein thrombosis.

Withdrawing the vaccine or delaying its use could have implications for vaccine equity, both domestically and abroad as experts worry that new, more transmissive variants of the virus could lead to a fourth coronavirus surge.

Johnson & Johnson shot is the only one-dose vaccine authorized in the United States, and the only one that does not require freezer storage — both traits that made it potentially more valuable in reaching rural areas and helping vaccinate people who may not be able to visit a medical site multiple times. 

“Any extension of the pause will invariably result in the fact that the most vulnerable individuals in the U.S., who are prime candidates for the J&J vaccine, will remain vulnerable,” said Dr. Nirav Shah, head of the Association of State and Territorial Health Officials, who is not a member of the committee.

In fact, the pause has already complicated some efforts to vaccinate people experiencing homelessness, affected vaccine supply for mobile vaccine clinics and, in some areas, undercut efforts to vaccinate people who are incarcerated.

“We were planning on using this vaccine in Massachusetts for people who were homebound and not able to get a vaccine,” said Dr. Camille Kotton, an associate professor at Harvard Medical School and voting ACIP member. “…it’s a significant loss.”

Still, it’s unlikely to dramatically slow down the nation’s vaccination effort — at least for now.

But manufacturing problems had forced Johnson & Johnson to back away from its April vaccine delivery targets, though White House officials said efforts were underway to bring the company back up to speed. At a Wednesday briefing, White House coronavirus response coordinator Jeffrey Zients said that the Johnson & Johnson pause would not stop the country from meeting its vaccination targets, which includes having enough supply to immunize the entire adult population by the end of May. 

Concerns could be further-reaching in other countries that have not received robust supplies of Pfizer and Moderna vaccines, and that are more likely to end up relying on a combination of the Johnson & Johnson vaccine and the AstraZeneca shot.

Already, the AstraZeneca blood clot concerns — despite the condition’s rarity — have been cited as a source of vaccine skepticism in other countries, especially in Africa. Some experts worry a similar issue could emerge with the Johnson & Johnson shot.