Immunization Schedule for Infants

A 6-month-old infant is brought to the clinic for a routine wellness checkup. The nurse reviews the immunization history and notes the infant has received the following vaccinations: hepatitis B at birth, DTaP at 2 months, and IPV at 2 months. The nurse informs the parents that the next immunization needed is scheduled at 4 months. Which vaccine should the nurse confirm for the upcoming visit, keeping in mind the CDC's childhood immunization schedule?