Adding to anxiety currently affecting the NBA amid a spike in COVID-19 cases, multiple players who previously tested positive for the coronavirus have recently tested positive a second time, sources told ESPN.

The CDC defines "reinfection" as a person getting infection, recovering and then later becoming infected again. There are ongoing studies about how long immunity may last, but the CDC says it expects some coronavirus reinfections.

The NBA has announced more than 100 positive tests since last summer, but the actual number since March is believed to be significantly higher. Several teams have more than 10 players who have tested positive at some point over the last nine months, sources told ESPN.

Because testing was less available and there were higher rates of false positives early in the pandemic, there is some level of uncertainty as to how many players had true positive cases early in 2020, especially during the league's three-month shutdown.

It is possible that some players who tested positive for the virus but were asymptomatic months ago were false positives. Some players have been tested for antibody levels to determine their level of immunity, but there currently isn't a league-wide procedure in place to regularly test for them.

Team and league doctors evaluate every positive test and player exposed on a case-by-case basis because the nature of the virus is still uncertain, league officials said. For example, players who have tested positive in the previous 90 days are sometimes handled differently that players who might've tested positive last summer because of the way the virus might still show up in their system.

The league office, National Basketball Players Association, teams and agents have been in talks in recent days to consider protocol changes to limit the spread that has caused three games to be postponed. The league has already placed players who previously had the virus -- Brooklyn's Kevin Durant and Miami's Bam Adebayo are two examples -- into week-long health and safety quarantines after exposed to an infected person over concern of reinfection or spreading the virus.

According to current CDC guidance, the duration for immunity following a COVID-19 infection is not yet understood. Some reinfections, based on knowledge of other coronaviruses, are expected but believed to be rare.

ESPN Reporter Tim MacMahon contributed to this story.