Small UK study investigates the effectiveness of a single MVA-BN dose against mpox disease

In a recent study posted to the Knowledge Hub* preprint server, researchers estimated the single-dose MVA-BN (modified vaccinia Ankara–Bavaria nordic) vaccine effectiveness (VE) against symptomatic mpox among high-risk gay, bisexual and other men who have sex with men (GBMSM) in England.

Study: Effectiveness of one dose of MVA-BN smallpox vaccine against monkeypox in England using the case-coverage method. Image Credit: photobyphotoboy/Shutterstock
Study: Effectiveness of one dose of MVA-BN smallpox vaccine against monkeypox in England using the case-coverage method. Image Credit: photobyphotoboy/Shutterstock

Background

In the United Kingdom (UK), mpox case counts in the current 2022 outbreak are rising, mainly among GBMSM individuals. To curtail mpox virus transmission, the MVA-BN vaccine, an attenuated vaccine licensed against smallpox, has been administered to high-risk individuals; data on MVA-BN VE against mpox are limited.

About the study

In the present observational study, researchers evaluated one-dose MVA-BN VE against mpox among GBMSM individuals residing in England.

VE estimates were obtained by adopting the case-coverage or screening approach, in which the vaccination rates among mpox cases were compared to the population coverage. The team used a previously reported calculation to estimate the total number of high-risk GBMSM individuals in England to be 89,240. The calculation was based on the predicted number of high-risk GBMSM individuals (n=48,500) visiting sexual care clinics regularly and 60% inflation (n=29,100), infrequent visitors, and an additional 15% (n=11,640) individuals who did not avail of sexual care services.

To consider uncertainty, sensitivity analyses were performed by reducing or elevating the GBMSM denominator by 20%. Data on vaccination sites, mainly sexual care clinics, with the everyday count of doses administered to GBMSM individuals, case contacts, and healthcare workers, obtained by the vaccination coverage reporting system established at the commencement of the vaccination rollout program by the National Health Service (NHS) of England.

Questionnaires were emailed or texted to confirmed case individuals and potential (Orthopoxvirus-positive) case individuals to obtain data on vaccination status, demographics, sexual orientation, and symptoms. Only questionnaires with the rash onset (or alternate proxy) date between 4 July and 9 October 2022 were assessed. Only cases in which the index dates were July 4, 2022, onward till the week beginning October 3, 2022, onward, were analyzed.

The team searched the PubMed database for relevant literature using keywords such as ‘MVA,’ ‘vaccine,’ and ‘monkeypox’ without any time limit using snowball sampling. In addition, the websites of regulatory authorities such as EMA (European Medicines Agency) and FDA (Food and Drug Administration) and preprints pertaining to MVA-BN VE studies during the ongoing 2022 outbreak were searched.

The numerator was based on the number of returned completed questionnaires wherein the diagnostic specimen collection date ranged between week 28 (11 July) and week 38 (25 September). The denominator was based on the total number of mpox cases to whom text messages with the questionnaire link were sent during the period. Logistic regression modeling was used for the analysis.

Results

The uptake of MVA-BN vaccines among GBMSM individuals elevated steadily from July 2022 onward and reached 47% by October 9 (week 40), 2022, with most vaccines administered by 22 August 2022 (week 34). Decreasing the denominator by 20.0% increased vaccination coverage by September-end to 63.0%, and increasing the denominator by 20% reduced the coverage to 42.0%. In total, 1,102 monkeypox virus-positive individuals responded to the questionnaires by 3 November 2022. Of these, 52 cases were excluded since they were of either females or heterosexual males, and 97 cases were excluded due to missing vaccination dates.

Further, only 460 reported cases had index dates beginning July 4, 2022, onward. As a result, 363 cases were considered for the final analysis. Between week 28 and week 38, 2,018 mpox cases were documented, of which 77% (n=1,545) of case individuals had texted their questionnaires. In total, 508 study questionnaires were filled out between 11 July and 25 September (33% response rate). Among 363 confirmed mpox cases, eight occurred ≥2.0 weeks after vaccination, 32 occurred within 13 days of vaccination, and the remaining individuals were not vaccinated.

The estimated VE ≥2.0 weeks post-singe-dose MVA-BN vaccination was 78%. A 20% reduction in the GBMSM individual denominator increased the population coverage, resulting in an elevated VE estimate of 85%. On the contrary, a 20% increase in the denominator lowered the VE estimate to 71%. 

One-dose MVA-BN VE within 13 days of vaccination was -4.0% (ranging between -30% to 23%, not different from zero for any of the case scenarios. Restricting the analysis to only case individuals below 50 years of age yielded an MVA-BN VE estimate of 74%. The findings showed that at-risk GBMSM individuals who were unvaccinated had a 14-fold higher risk of mpox.

Conclusion

Overall, the study findings showed that one-dose MVA-BN vaccination provided immense and rapid protection against mpox among high-risk GBMSM and provided a valuable tool for controlling the May 2022 mpox virus outbreak.

The findings showed that MVA-BN was effective after the initial 13 days of vaccination, and the median period for the mpox virus to incubate was 8.0 days to 9.0 days, indicating that MVA- BN vaccination was most likely to be effective when administered pre-exposure instead of administration for prophylaxis. However, behavioral alterations (e.g., refraining from sexual activities) post-MVA-BN vaccinations might have impacted the estimated VE values.

*Important notice

Knowledge Hub publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Pooja Toshniwal Paharia

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Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.

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