A district reports a measles outbreak despite 92% measles vaccination coverage. The MOST likely explanation is:

• A Cold chain failure causing vaccine potency loss — vaccine efficacy reduced to <80%
• B Waning immunity — measles vaccination requires annual boosters
• C Intrinsic failure rate of measles vaccine is 20% even with ideal storage
• D High R₀ of measles requiring >95% coverage for herd immunity ✓

Explanation

Measles has one of the highest R₀ values (12–18) of all infectious diseases, requiring herd immunity threshold of approximately 92–95% to prevent outbreaks. At 92% coverage, a district may still be below the HIT, particularly if unvaccinated individuals are geographically clustered (herd immunity gaps). Primary vaccine failure rate of the measles vaccine is approximately 5% (95% efficacy with a single dose) and 3% after two doses. Annual boosters are not required — two-dose schedule provides long-lasting immunity.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.