Pfizer (NYSE: PFE) is enrolling subjects in a study combining an investigational pneumococcal conjugate vaccine with a booster dose of its Pfizer-BioNTech COVID-19 vaccine.

At present, COVID-19 and pneumococcal conjugate vaccines are among Pfizer’s bestsellers. The company expects COVID-19 vaccines to pull in $26 billion this year. Last year, the company’s Prevnar 13 pneumococcal 13-valent conjugate vaccine raked in $5.85 billion.

[Related: 50 of 2020’s best-selling pharmaceuticals]

The company intends to gather safety data from the co-administration of its 20-valent pneumococcal conjugate vaccine candidate (20vPnC) and its COVID-19 vaccine. The follow-up period for the study involving 600 adults will be six months. The secondary endpoint of the study is to characterize the immune response from both vaccines.

The company will recruit participants from its Phase 3 clinical trial for the Pfizer-BioNTech COVID-19 vaccine study.

Pfizer will randomize volunteers into three groups, with the first receiving both the 20vPnC vaccine and COVID-19 vaccine booster. The second group will receive 20vPnC and placebo, while the final group will receive a COVID-19 booster and placebo.