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…