Which of these vaccines requires two doses?

The varicella vaccine, which protects against chickenpox, typically requires two doses for full immunization. The first dose is administered around 12-15 months of age, and the second dose is usually given between 4-6 years of age. This two-dose schedule is designed to enhance the effectiveness of the vaccine and ensure long-lasting immunity to the varicella virus. In contrast, the other vaccines listed each have different dosage recommendations: the influenza vaccine is often given annually, the pneumococcal vaccine has a different schedule depending on the type and age, and the Td vaccine (tetanus and diphtheria) is generally a booster given every 10 years after the initial series, but does not require two doses in a primary series for adults.