The NBA has completed a study with infectious disease specialists and testing manufacturers on the preseason antibody test results of 2,300 players and staff, citing further evidence of a need for booster doses of Covid-19 vaccines for league personnel, according to a copy of the study's findings obtained by ESPN.

The NBA's study found Moderna and Pfizer vaccines created higher levels of antibodies compared to Johnson and Johnson and detailed the decline of antibodies over time. The NBA wanted to understand the effectiveness of different vaccines, and what different levels of antibodies might mean for risk of infection.

The NBA is aware of 34 cases of fully vaccinated players or team staff becoming infected with a breakthrough case of Covid-19 through November 19, including 31 that had detectable levels of antibodies significantly lower than those observed in the remaining testing population, according to the report. Three of the 34 infections happened with "not detected" antibody levels, the study said.

The NBA has a 97% vaccination rate among its players, the league says. The NBA is using the findings of the report to encourage eligible players to follow the previous NBA/NBPA recommendation of getting a booster shot, and team doctors are expected to use these results to educate players on the importance of the additional vaccination protection.

Within the 2,388 people tested in the preseason, 75 produced "not detected" results, including 11% who had received the Johnson and Johnson vaccine. That percentage was considerable higher than those who received Pfizer (1%) and Moderna (0.2%).

Those results suggest that those received the J&J vaccine at least two months ago -- or a second dose of Pfizer or Moderna at least six months ago -- are at increased risk of breakthrough infections.