Explanation:

- The sentence is discussing how a vaccine that reduces the number of symptomatic infections can lessen virus transmission.

- Let’s look at the options:

- Option 1: "incidents" – means events or occurrences, not suitable here.

- Option 2: "vigilance" – means alertness, not related to infections.

- Option 3: "indigent" – means poor or needy, irrelevant to context.

- Option 4: "incidence" – This means the rate or number of new cases, perfectly fitting the sentence about infections.

- Option 5: "decadent" – means morally or culturally declined, not logical here.

- So, the correct answer is option 4: incidence.

- “Incidence” fits because we often talk about the incidence of diseases or infections in medical and epidemiological contexts.