Up to 10,000 active-duty Marines will not be fully vaccinated against the coronavirus when their deadline arrives in coming days, a trajectory expected to yield the U.S. military's worst immunization rate. 

While 94 percent of Marine Corps personnel have met the vaccination requirement or are on a path to do so, according to the latest official data, for the remainder it is too late to begin a regimen and complete it by the service's Nov. 28 deadline. Within an institution built upon the belief that orders are to be obeyed, and one that brands itself the nation's premier crisis-response force, it is a vexing outcome.

The holdouts will join approximately 9,600 Air Force personnel who have outright refused the vaccine, did not report their status, or sought an exemption on medical or religious grounds, causing a dilemma for commanders tasked with maintaining combat-ready forces - and marking the latest showdown over President Joe Biden's authority to impose vaccination as a condition of continued government service.

"Marines know they're an expeditionary force, and pride themselves on discipline and being first to fight," said David Lapan, a retired Marine Corps officer and former communications chief for the service. Leadership, he said, should be alarmed that the Marine Corps ethos of always being ready for the next mission appears to be tarnished in this case. "Why," Lapan asked, "did they decide not to follow a direct order?"

Answering that question will be essential, he added, "if this is somehow indicative of a problem" that could arise again in the future.

The Marine Corps made no secret it has struggled with vaccine hesitancy in the ranks. Late last month, officials issued an ultimatum: get vaccinated, apply for an exemption or get kicked out.

Then, as the cutoff to be in compliance drew near, the Marines' top general, Commandant David H. Berger, and his senior enlisted adviser, Sgt. Maj. Troy E. Black, distributed a video message to the force imploring those who had not been vaccinated to get it done. They appealed to Marines' sense of fidelity and calmly explained that the Marine Corps would be less capable unless everyone met the requirement.

"When something bad happens around the world and the president says, 'I need to know how long it's going to take to get Marines there,' it's too late then to get vaccinated," Berger said in the video.

"It's challenging for us to be able to continue the mission," Black added, "if we're not ready to go."

Berger spoke last: "We need every single Marine in the unit to be vaccinated. We don't have extra Marines. We're a pretty small force, and we have to make sure that everybody on the team is ready to go all the time. That's our job."

The Marine Corps is the U.S. military's least-populous branch of service. Numbering about 183,000, it's roughly one-third the size of the active-duty Army but fills a significant role within the Defense Department's portfolio. Whenever there's a high-stakes emergency overseas - such as the hasty evacuation from Afghanistan this past summer - Marines are often among the first U.S. personnel to set foot in harm's way.

Importantly, the service's coexistence within the Navy Department means Marines routinely operate from ships at sea, living in close, enclosed spaces where the virus can spread readily. Navy data shows that 99.7 percent of sailors have received at least one shot of the coronavirus vaccine ahead of the same Nov. 28 deadline - the top figure among all military services.

The general's message, circulated Nov. 8, appears to have made little impact. At that time, the Marine Corps' partial vaccination rate - an indicator of newly obtained shots - was 94 percent and remained unchanged as of Wednesday, according to official data. The rate slowed in recent weeks overall, indicating the pool of Marines who intended to comply has all but dried up.

A spokesperson for Berger declined to comment, pending a final tally.

Capt. Andrew Wood, a Marine Corps spokesman at the Pentagon, would not address questions about the service's vaccination rate. He issued a written statement instead. "The Marine Corps has always recognized the threats posed by the COVID-19 pandemic as a readiness issue, which is why we have consistently emphasized the importance of receiving the vaccine," it said. "We are still ready to fight and win our nation's battles should we be called."

It's unclear how many unvaccinated Marines have requested medical or religious exemptions, or how many of those requests have been granted, but such cases are expected to be exceptionally rare. The Navy has granted six permanent medical exemptions and no religious exemptions - for any vaccine - in the last seven years, officials said. The Army has granted just one permanent medical exemption and, like the Navy, zero religious exemptions, the service said. The Army's compliance deadline is Dec. 15 - the last for active-duty personnel - and 95 percent of soldiers have received at least one dose of the vaccine, service data show.

Air Force officials are processing about 4,800 religious exemption requests but so far have not approved any. Nearly 1,400 airmen have received medical exemptions, but most are temporary, said Ann Stefanek, an Air Force spokeswoman. Temporary medical exemptions can be issued when someone has a current coronavirus infection and is awaiting a doctor's authorization to be vaccinated. Its vaccination deadline was Nov. 2.

Wood said Marine Corps data on refusals and exemptions would be made available after the deadline passes.

Guidance from the military services outlines escalating punishment for vaccine refusers, starting with counseling from commanders and moving onto letters of reprimand and ultimately dismissal from the service. Navy Secretary Carlos Del Toro told reporters Thursday that each Marine refusing a vaccine will be addressed on a case-by-case basis.

"We're just not going to all kick them out on the day of the deadline itself," he said, predicting "minimal impact on our overall readiness."

The military's vaccine mandate has energized conservative politicians eager to challenge Biden's directives covering private businesses as well as government workers. Some predicted an exodus from the ranks and warned of crippling strains on the defense industrial base.

Earlier this month, Oklahoma Gov. Kevin Stitt, a Republican, took the extraordinary step of firing the state's National Guard commander and ordering his replacement to make vaccination optional while personnel are under state control. Other states have expressed interest in enacting similar policies.

Vaccination rates throughout much of the National Guard and reserves remain well below those of the active-duty force. These units tend to be less connected to the rigid top-down environment that governs daily life on active duty. Military analysts also associate vaccine hesitancy among service members with the circulation of false information online, broader political and societal attitudes, and cultural traits unique to each service.

Marines, on average, are younger, predominantly male and - like many enlisted personnel throughout the armed forces - generally don't have four-year college degrees, according to 2018 Pentagon data. All of those factors contribute to lower vaccination rates in the broader U.S. population by some degree, according to government data and surveys. And compared to civilians in the same age range, Marines generally are more physically fit and thus may simply doubt the need to be vaccinated.

For Marine Corps leadership, much of the concern now will turn to whether unvaccinated Marines are congregated in certain units, Lapan said. That would affect how drastically commanders may need to rebalance their personnel to ensure deployment standards.

Some highly deployable units appear to have overcome those issues. The crisis-response unit that supported the emergency evacuation from Kabul in August was 98 percent vaccinated months before then, officials said.

But overall, the disparity in Marines' vaccination rate compared to those of other services "seems to indicate a failure of leadership to get ahead of the vaccine hesitation within their own ranks," said Rachel E. VanLandingham, a former Air Force lawyer and president of the National Institute of Military Justice.

Berger, speaking at a national security conference earlier this month, appeared to acknowledge the behavioral pattern is worrisome.

"We're challenged by disinformation. ... I'm concerned about it because we have to be ready to go every day," he said. "We are taught that your unit is more important than you are."