US federal agencies may need two weeks or more to know whether Johnson & Johnson's coronavirus vaccine is linked to rare blood clots, two senior White House health officials told Politico on Saturday.

US regulators recommended a pause in the distribution of J&J's shot last Tuesday due to six reports of clotting among women who'd recently received the vaccine.

The particular blood clot in question, central venous sinus thrombosis (CVST), forms in the brain — so it can lead to headaches or stroke. In an average year, the condition occurs in about five people out of every million.

The Centers for Disease Control and Prevention's vaccine advisory panel is scheduled to convene Friday to discuss whether to lift the vaccine pause. But two health officials told Politico that the CDC still might not have enough data by then to determine if J&J's vaccine indeed causes rare clots.

US regulators may ultimately consider placing age- or gender-based restrictions on the shot, which has been authorized for people ages 18 and older. Alternatively, regulators could simply deliver stronger warnings about possible blood clots in unusual cases.

Many political leaders and medical experts worry that if regulators take too long to evaluate the potential blood clot link, an increasing share of Americans will lose trust in J&J's vaccine.

"The longer the pause is, the longer it's going to take for us to convince people that this particular vaccine is safe again," Arkansas Gov. Asa Hutchinson told Politico.

Peter Gulick, an associate professor of medicine at Michigan State University, said the J&J pause could delay the prospect of herd immunity — the threshold beyond which the virus won't be able to pass easily from person to person.

"The fear is that, hearing all this, the anti-vaxxers and even the ones on the fence are falling off the fence now into the arena of 'I don't think I want to get any vaccine until things are known a little more,'" Gulick told Insider. "We may have taken two steps backwards as far as our wanting to get herd immunity."

Searching for blood clots in a 'muddied water' of data

The CDC's vaccine advisory panel has already met once to review the rare blood clot cases. At a meeting last Wednesday, the panel recommended continuing the pause on J&J's vaccine until more data could be gathered.

"It's important from the perspective of the public: When we say rare, what does that mean?" Dr. Beth Bell, a professor of global health at the University of Washington, said during the meeting. "I want to be able to feel comfortable with my family members and myself getting this vaccine."

US regulators are now encouraging doctors to report any post-vaccination CVST cases over the last few weeks. Regulators are also working with Johnson & Johnson to find out more about the six reported cases — in particular, whether the women had underlying health problems or were taking any medications that could have predisposed them to clotting.

So far, regulators have noticed a few patterns: The women were between the ages of 18 and 48. They also had a rare combination of CVST and low levels of platelets — colorless blood cells that help clots form.

Before the vaccines were authorized, this combination was primarily seen in association with the blood-thinning drug heparin. In rare cases, people taking the medication develop antibodies that bind to a specific platelet, which can make them more susceptible to clots.

"This observation of the low platelet count is part of the mystery and something that has to be worked through to see if that's connected or not," Namandjé Bumpus, director of the pharmacology and molecular sciences department at Johns Hopkins Medicine, told Insider.

But medical experts stressed the need for more data before associating the clots with any particular group yet.

"Everything is just like a big muddied water and then you just try to clear things out as much as you can to try to evaluate what is going on," Gulick said.

Vaccinations may slow for the homeless, prisoners, or rural Americans

Yvonne Gibbs, 72, receives Moderna's COVID-19 vaccine, at the TCF Center in Detroit.

Shortly after US regulators announced a pause in J&J's vaccine, White House COVID-19 coordinator Jeff Zients said the recommendation wouldn't affect the pace of the US vaccine rollout.

"We have more than enough supply of Pfizer and Moderna vaccines to continue the current pace of about 3 million shots per day," Zients said at a Tuesday press briefing.

Indeed, many health departments, pharmacies, and vaccine clinics that planned on administering Johnson & Johnson were able to quickly procure other shots so people didn't lose their appointments.

But some vaccination sites — particularly those in rural areas — were forced to shut down temporarily. A state-run mass vaccination site in Aurora, Illinois, was canceled earlier this week, terminating appointments of 1,000 people. Around the same time, a Johnson & Johnson clinic in Jefferson County, Illinois, put vaccinations on hold.

The J&J pause has also slowed the pace of vaccinations for homeless people, prisoners, and those unable to leave their homes due to illness or old age. J&J's vaccine is the only single-dose shot authorized in the US, so it's the easiest to administer. It's also easier to store than the Pfizer or Moderna vaccines (it can be kept in standard refrigerators rather than freezers).

On top of that, people may gravitate toward J&J's vaccine if they're afraid of needles or have difficulty taking time off work to get vaccinated.

"We're actually seeing that some people opt for the Johnson and Johnson shot just because of their circumstances — it's one dose, it's available, and so on," Johan Bester, director of bioethics at the University of Nevada, Las Vegas School of Medicine, told Insider.

Without the J&J option, medical experts said, US health officials may have a harder time convincing more Americans to get vaccinated — even as new, more contagious variants drive up cases across the country.