Which statement about diphtheria incidence is true in populations with high vaccination coverage?

• A. It is rare due to vaccination
• B. It is transmitted by fecal-oral route
• C. It is caused by a gram-negative bacillus
• D. It is asymptomatic in all cases

Answer: (A)

Having high vaccination coverage makes diphtheria rare because the diphtheria toxoid vaccine stimulates antibodies that neutralize the toxin and prevent the toxin-mediated illness, which also reduces transmission. The vaccine regimen (DTaP in childhood with Tdap boosters later) creates herd immunity, offering protection even to those who aren’t fully vaccinated. Diphtheria is not spread by the fecal-oral route; it is transmitted mainly through respiratory droplets. The organism is a gram-positive bacillus, and while asymptomatic carriage can occur, vaccination greatly reduces the likelihood of symptomatic disease, making cases uncommon in highly vaccinated populations.