Measles — SAQ 2

Stem

A 3-year-old unvaccinated boy is diagnosed with measles on day 2 of the rash. He attends a daycare where 12 other children are unvaccinated or have only one dose of MMR. One of the daycare staff is 10 weeks pregnant and has no history of measles or MMR. Another child at the daycare is receiving chemotherapy for leukaemia. The index case's mother is anxious because she has read online that the MMR vaccine causes autism.

Questions

a) What is the single most important immediate public-health action, and why? (2 marks)

Notify public health immediately because measles is statutorily notifiable in virtually every country. Notification triggers contact tracing, outbreak investigation, and post-exposure prophylaxis. It is a clinical intervention, not just an administrative step. The index case should also be isolated with airborne precautions (single room, preferably negative pressure, FFP3/N95 for staff) for at least 4 days after the rash appeared.

b) Outline the post-exposure prophylaxis for the susceptible pregnant staff member. (2 marks)

The pregnant staff member is susceptible (no measles history, no MMR) and pregnancy is a contraindication to live MMR vaccine. She should receive human normal immunoglobulin (HNIG) 0.5 mL/kg IM (maximum 15 mL) within 6 days of first exposure. HNIG provides passive protection and may attenuate disease. She must be monitored for symptoms. MMR is contraindicated in pregnancy; she should receive MMR postpartum (two doses, at least 4 weeks apart, with contraception for 1 month before a future pregnancy). Measles is not teratogenic (unlike rubella) but increases the risk of miscarriage, preterm labour, and severe maternal pneumonia.

c) How would you manage the immunocompromised child receiving chemotherapy? (2 marks)

The immunocompromised child is at high risk for severe, atypical measles — potentially with no rash, giant-cell (Hecht) pneumonia, or measles inclusion-body encephalitis (MIBE). Live MMR is contraindicated. Give HNIG 0.5 mL/kg IM (max 15 mL) within 6 days of exposure. The child should be monitored closely; if measles develops, manage supportively with airborne isolation, consider IV ribavirin for severe or life-threatening disease, and admit early because deterioration can be rapid.

d) What advice would you give the mother about the MMR-autism concern? (2 marks)

The 1998 Wakefield Lancet paper linking MMR to autism was retracted in 2010 and the author's medical licence was struck off. Large, rigorous studies — including Hviid et al. 2019 in 657,461 Danish children (Annals of Internal Medicine) — found no association between MMR and autism, even in children with a family history of autism or other susceptibility factors. The adjusted hazard ratio was 0.93 (95% CI, 0.85 to 1.02). Delaying or refusing MMR leaves the child at risk of measles, mumps, and rubella, which can cause severe complications including pneumonia, encephalitis, SSPE, and immune amnesia.

e) What is the minimum two-dose MMR coverage needed for herd immunity, and why is measles so sensitive to small coverage gaps? (2 marks)

The herd-immunity threshold is approximately 92 to 95 percent two-dose coverage. With a basic reproduction number (R0) of 12 to 18, the herd-immunity threshold is H = 1 - 1/R0, giving about 91.7 to 94.4 percent. Because two-dose MMR effectiveness is about 97 percent, this means near-universal two-dose coverage is required. Measles is the first vaccine-preventable disease to resurge when coverage falls because its extreme contagiousness means that even small pockets of susceptibility are rapidly exploited.

References3ShowHide

1. [1]Rota PA, Moss WJ, Takeda M, et al. Measles. Nature Reviews Disease Primers, 2016.PMID 27411684
2. [7]World Health Organization. Measles vaccines: WHO position paper - Recommendations. Vaccine, 2019.PMID 28760612
3. [10]Hviid A, Hansen JV, Frisch M, Melbye M. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Annals of Internal Medicine, 2019.PMID 30831578