An Investigation of MMR‐Related Mumps Cluster Following Immunization Among Practical Nursing Students, Bangkok, Thailand, 2024

<b>Background:</b> Although MMR vaccination can induce mumps infections, clustered cases right after the vaccination are rarely reported. On September 10, 2024, the Department of Disease Control, Thailand, received a report of a cluster of practical nursing students (PNSs) with jaw and ear swelling following an MMR (L-Zagreb strain) vaccination. An investigation was conducted to confirm the outbreak, identify sources and risk factors, and recommend preventive measures. <b>Methods:</b> Active case findings were conducted among PNS and hospital service recipients who received the suspected vaccine batches. Suspected cases of MMR-related mumps infection were defined as individuals experiencing either jaw swelling/pain, testicular swelling/pain, or groin pain 12-25 days postvaccination. Confirmed cases had positive RT-PCR for mumps. <i>SH</i> gene sequencing determined mumps phylogenetics, while nanopore sequencing of the <i>NP</i> gene assessed polymorphisms. Group and in-depth interviews with vaccine suppliers, pharmacists, nurses, and PNS evaluated the vaccine cold chain and setting. A retrospective cohort study among PNS used questionnaires on demographics and vaccination history to identify risk factors, analyzed via multivariable logistic regression. Qualitative data underwent content analysis. <b>Results:</b> Two batches of MMR vaccine were suspected and immediately suspended. Of the 108 vaccinated PNS, 12 met the case definition (eight suspected and four confirmed), yielding an attack rate of 11.1% (12.5% in males and 10.9% in females). Among 61 hospital vaccine recipients who received the same vaccine batches, 30 were contactable, and none met the case definition. All cases had ear/jaw pain, with 41.7% experiencing sore throats and 33.3% myalgia, though none were severe. Three confirmed cases had genetic material aligning with the L-Zagreb strain (Accession AY685920). No <i>NP</i> gene polymorphisms were detected in vaccines, though specimen buccal swabs had insufficient genetic material. Two factors may link to the adverse event: prior MMR vaccination (aOR = 12.90, 95% CI: 1.39, 172.00) and a delay of over 15 min from vial retrieval to administration (aOR = 26.90, 95% CI: 4.20, 247.00). Vaccine supply, distribution, and storage met standards, but vaccine campaign registration and waiting time processes require improvement. <b>Conclusion:</b> MMR-related mumps infections were confirmed during PNS mass vaccination campaign. Potential risk factors include a history of prior MMR vaccination and delay time from vial retrieval to vaccine administration. Improvements are needed in vaccination campaigns, particularly in the registration system and expediting vaccination process flow.