Pfizer’s (NSE:PFE) pneumococcal 13-valent conjugate vaccine Prevnar 13 brought in $5.85 billion last year, but Merck (NYSE:MRK) is hoping to increase its presence in the market with its investigational 15-valent pneumococcal conjugate vaccine. Although Merck has a pneumococcal vaccine on the market known as Pneumovax 23, its sales were $1.1 billion last year. While technically a…