**Core Concept**
The ViPS vaccine is a conjugate vaccine used to protect against invasive pneumococcal disease (IPD), which is caused by *Streptococcus pneumoniae*. The vaccine schedule is designed to provide long-term protection against IPD, and booster doses are essential to maintain immunity.

**Why the Correct Answer is Right**
The ViPS vaccine schedule recommends booster doses every 3 years to maintain immunity against IPD. This is because the vaccine-induced antibodies against pneumococcal capsular polysaccharides decline over time, making individuals susceptible to IPD. Booster doses help to replenish these antibodies and provide continued protection. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend booster doses every 3 years for individuals who received the ViPS vaccine.

**Why Each Wrong Option is Incorrect**
**Option A:** Every year is not recommended as booster doses are not required annually to maintain immunity against IPD.
**Option B:** Every 2 years is also not recommended, as the current evidence suggests that booster doses are needed every 3 years to maintain optimal immunity.
**Option D:** Every 4 years is not the recommended schedule, as it may not provide adequate protection against IPD, especially in high-risk individuals.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the ViPS vaccine schedule may vary depending on the individual's risk factors, such as age, underlying medical conditions, and immunocompromised status. Healthcare providers should consult with local health authorities or guidelines to determine the most appropriate vaccination schedule for their patients.

**✓ Correct Answer: C. Every 3 year**