Effectiveness of historical smallpox vaccination against mpox clade II in men in Denmark, France, the Netherlands and Spain, 2022.

BackgroundIn 2022, a global monkeypox virus (MPXV) clade II epidemic occurred mainly among men who have sex with men. Until early 1980s, European smallpox vaccination programmes were part of worldwide smallpox eradication efforts. Having received smallpox vaccine > 20 years ago may provide some cross-protection against MPXV.AimTo assess the effectiveness of historical smallpox vaccination against laboratory-confirmed mpox in 2022 in Europe.MethodsEuropean countries with sufficient data on case vaccination status and historical smallpox vaccination coverage were included. We selected mpox cases born in these countries during the height of the national smallpox vaccination campaigns (latest 1971), male, with date of onset before 1 August 2022. We estimated vaccine effectiveness (VE) and corresponding 95% CI for each country using logistic regression as per the Farrington screening method. We calculated a pooled estimate using a random effects model.ResultsIn Denmark, France, the Netherlands and Spain, historical smallpox vaccination coverage was high (80-90%) until the end of the 1960s. VE estimates varied widely (40-80%, I2 = 82%), possibly reflecting different booster strategies. The pooled VE estimate was 70% (95% CI: 23-89%).ConclusionOur findings suggest residual cross-protection by historical smallpox vaccination against mpox caused by MPXV clade II in men with high uncertainty and heterogeneity. Individuals at high-risk of exposure should be offered mpox vaccination, following national recommendations, regardless of prior smallpox vaccine history, until further evidence becomes available. There is an urgent need to conduct similar studies in sub-Saharan countries currently affected by the MPXV clade I outbreak.

Factors potentially contributing to the decline of the mpox outbreak in the Netherlands, 2022 and 2023.

BackgroundIn 2022 and 2023, a global outbreak of mpox affected mostly gay, bisexual and other men having sex with men (GBMSM). Outbreak control in the Netherlands included isolation, quarantine, post-exposure prophylaxis vaccination and primary preventive vaccination (PPV).AimWe describe the course of the outbreak, the vaccination programme, vaccine effectiveness (VE) of full vaccination against symptomatic disease, and trends in behaviour to generate hypotheses about factors that influenced the outbreak's decline.MethodsIn this observational study, we collected data from public health services on notified cases, number of PPV invitations and PPV doses administered. We calculated PPV uptake and coverage. Trends in behavioural data of GBMSM visiting sexual health centres were analysed for all consultations in 2022. We estimated VE using the screening method.ResultsUntil 31 December 2023, 1,294 mpox cases were reported. The outbreak peaked in early July 2022 and then declined sharply. PPV started on 25 July 2022; in total 29,851 doses were administered, 45.8% received at least one dose, 35.4% were fully vaccinated. The estimated VE was 68.2% (95% CI 4.3-89.5%). We did not observe an evident decrease in high-risk behaviour.DiscussionIt is unlikely that PPV was a driver of the outbreak's decline, as incidence started to decline well before the start of the PPV programme. The possible impact of behavioural change could not be demonstrated with the available indicators, however, the data had limitations, hampering interpretation. We hypothesise that infection-induced immunity in high-risk groups was an important factor explaining the decline.