My 4 month old has pertussis, and is vaccinated. how many injections are needed to be fully covered? i have a 2 year old at home i want to be safe.

Fully ?: Pertussis infection is a matter of exposure intensity up against level of protection. With at least one shot, weeks before, the risk of dying as 4mo can do drop to near zero, completing the series helps but anyone at any age can still get it regardless of shot status. A 2yo vaccinated kid is not at big risk but the whole household would commonly be treated with meds.