The NBA has raised the level of urgency regarding getting booster shots against the coronavirus, telling players and coaches that it is no longer advisable to wait before receiving the additional dose.

The booster shots should be received "as soon as possible, particularly in light of the current coronavirus situation and increasing cases," the league told teams Friday in a memo, a copy of which was obtained by ESPN.

Earlier in the week, the NBA and the National Basketball Players Association said they were recommending the booster shots be received by those who are fully vaccinated, suggesting that it be done by Dec. 1 in most cases.

But with positivity rates rising in many areas of the country and the rate of COVID-19 community transmission in most NBA markets considered by the Centers for Disease Control and Prevention to be "high" or "substantial," the league and union felt the additional urgency is warranted. The NBA has seen an increased positivity rate among vaccinated players, team staff and family members of late, which is consistent with the trends in many places around the nation.

At least eight players are known to be in the league's health and safety protocols, meaning they are believed to have tested positive for COVID-19. Another who had been in the protocols, Philadelphia's Tobias Harris, played Thursday night in his first game back since testing positive.

Harris missed six games and had been dealing with symptoms.

"I'm working my way back into it," Harris said.

Other NBA players currently known to be in the league's protocols include Philadelphia's Joel Embiid, Matisse Thybulle and Isaiah Joe, San Antonio teammates Jakob Poeltl and Jock Landale, Cleveland teammates Kevin Love and Lauri Markkanen, and Chicago's Nikola Vucevic.

People who are fully vaccinated are still strongly protected against hospitalization and death from COVID-19. But immunity against infection can wane over time, and the extra-contagious delta variant is spreading widely. The NBA -- following the lead of U.S. health authorities -- wants to shore up protection in at-risk people who were vaccinated months ago.

The guidance from the league earlier in the week made clear that those who received the single-dose Johnson & Johnson vaccine more than two months ago should prioritize getting a Pfizer or Moderna booster quickly. Those who received the Pfizer or Moderna vaccine more than six months ago were also told earlier in the week to seek boosters.

About 97% of NBA players have been vaccinated against COVID-19, the league has said. The most notable holdout is Brooklyn's Kyrie Irving, a perennial All-Star who is not being allowed by the Nets to play until he gets vaccinated.

The Associated Press contributed to this report.