Pfizer’s (NSE:PFE) pneumococcal 13-valent conjugate vaccine Prevnar 13 is one of pharma’s bestsellers, earning nearly $6 billion last year. Now, the company has scored FDA approval for its Prevnar 20 vaccine for adults 18 and older. The vaccine is the first to offer protection against 20 serotypes of pneumococcal disease that cause the majority of pneumonia and…