Though roughly 90% of NBA players are vaccinated as training camp approaches Tuesday, tension exists between those around the league mandated to be vaccinated and the nearly 40 unvaccinated players, league sources told ESPN.

In some instances, vaccinated staffers say they're concerned about the health risks of being exposed to unvaccinated players. In others, staffers say they're upset that players aren't facing the same vaccine requirements as most team staff and referees. In others, there's animosity toward the league itself for not imposing such a mandate.

"Everyone who is vaccinated should be pissed at those who aren't," a veteran assistant coach said, adding, "Not requiring NBA players to be vaccinated is horse s--t."

The league has pushed for mandatory vaccinations for players, league sources said, but as ESPN first reported, the NBPA has refused to budge on a vaccine mandate for players, referring to it as a "non-starter" in negotiations between the league and the player's union.

One vaccinated Western Conference strength and conditioning coach said they're concerned about a potential breakthrough case that could affect family members.

"For me, it's a problem because my parents are very sick, and I'm in close contact with these guys and I would hate to bring this home and my parents pass away from it," the coach told ESPN.

Said the strength and conditioning coach of NBA leaders, "They need to hold the players to the same standards they hold us. This is a disease that doesn't differentiate between a player and a staff member."

A vaccinated Western Conference head athletic trainer echoes the concern about the rise in breakthrough cases.

"Say you have a small child with asthma, and you're doing everything you can, but you bring something home to your family and children of a certain age aren't able to be vaccinated yet," the head athletic trainer said. "It's real. It's a real thing. Breakthrough infections are a real thing."

This month, the Centers for Disease Control and Prevention reported results of a new study that examined vaccination data from 11 states and two major metropolitan counties.

From April 4 to June 19, fully vaccinated people accounted for 5% of cases (23,503), 7% of hospitalizations (2,025) and 8% of deaths overall (428).

But these same percentages increased from June 20 to July 17 -- rising to 18% of all cases (22,809), 14% of hospitalizations (951) and 16% of deaths overall (188).

In the April 4-June 19 data, the CDC noted an average of 10.1 breakthrough cases for every 100,000 fully vaccinated people across all ages. In the later period, such cases rose to an average of 19.4.

"There are more breakthrough infections happening than there were before -- that's a real phenomenon," Heather Scobie, an epidemiologist at the CDC and lead author of the largest of the studies, told The New York Times in mid-September. "But for the most part, people are not going to hospitals if they've been vaccinated."

Most team staff, as well as NBA referees, are required to be vaccinated. A spokesperson for the NBA Coaches Association declined to comment. A message seeking comment from the National Basketball Referees Association was not returned. The NBA and NBPA didn't immediately respond to requests for comment.

Health and safety protocols for vaccinated and unvaccinated players have not yet been finalized, league sources said. But it's expected, as ESPN previously reported, that unvaccinated players will face more testing and be asked to sit in separate areas of team meetings, team meals, locker rooms, on the team plane and bus.

One general manager told ESPN that the issue of tension between those who are mandated to be vaccinated and those who aren't "'just speaks to selfishness run amok. But the NBA is comprised of human beings and we are seeing the same thing in the public at large."

Washington Wizards star Bradley Beal said Monday during his team's media day that he isn't vaccinated for "personal reasons."

Beal added, "I'll definitely think about it, for sure. With the guidelines that the league makes and everything that the protocols are doing, they kind of make it difficult on us to where they kind of force us in a way to want to get it.

"But at the end of the day, [I'll] talk it over with my family and we make a group decision that we feel is best for us, just like the rest of the world."

"People want to draw it as a personal health decision, but it's not," said the Western Conference head athletic trainer. "It's a public health decision."

One league health source tied to athletic training staffs downplayed concerns, noting that 90% of players being fully vaccinated -- a figure recently provided to ESPN by a league spokesperson -- represented considerable progress and that the number would probably increase.

But a second league source also tied to training staffs noted that many peers "believe the league is prioritizing the athletes' lives over their own. On the opposite side, some members don't want to force anyone to vaccinate if they feel uncomfortable with it, but it should be a standard set across the board instead of the league one way and the players the other."

"It's very concerning to everybody involved," said a second general manager: "I'm out of energy trying to convince somebody to save themselves and their loved ones."

Just in the last week, reports have surfaced that two NBA stars are unvaccinated in markets that require vaccinations to play. Golden State Warriors wing Andrew Wiggins has remained unvaccinated, the San Francisco Chronicle recently reported, despite recently meeting with a doctor to address his concerns. Wiggins' decision could prevent him from playing in home games this season due to a recently passed law in San Francisco that requires people to be fully vaccinated to be allowed indoors for entertainment.

Wiggins' request for a religious vaccine exemption was denied, the NBA announced Friday. "Wiggins will not be able to play in Warriors home games until he fulfills the city's vaccination requirements," the league said in a statement.

Brooklyn Nets guard Kyrie Irving also hasn't been vaccinated, and could miss home games because New York passed a similar law in August. Irving missed the team's media day on Monday, though he was made available on a Zoom call.

"I know that I'll be there every day no matter what and just be present for my teammates as one of the leaders on the team and be there for my growing tribe off the court," Irving said.

Orlando Magic power forward Jonathan Isaac, who expressed vaccine hesitancy in a recent Rolling Stone story, tweeted Sunday, "I believe it is your God given right to decide if taking the vaccine is right for you! Period!"

"It makes the health performance staff's jobs so much harder," a third general manager said of unvaccinated players. "Not having everyone vaccinated can be a disaster."