Infection and Vaccine-Induced Immunity Drive Decline of Mpox in the Netherlands

 

 

Immunity gained through infections and post-exposure vaccinations rather than preventive measures has significantly reduced mpox transmission in the Netherlands in 2022. This conclusion comes from recent research by the Dutch National Institute for Public Health and the Environment (RIVM), published in Eurosurveillance. The study also highlights the potential impact of behavioral changes.

Mpox Cases in the Netherlands: A Timeline

The Netherlands documented its first mpox case on May 20, 2022, marking the start of a global outbreak. By the end of 2023, the country had recorded a total of 1,294 cases. Similar to trends observed globally, 94% of these cases were among men who have sex with men (MSM), with 99% of the cases occurring in males.

Preventive Vaccination Program and Its Impact

The Primary Preventive Vaccination (PPV) program was initiated on July 25, 2022. However, researchers observed that the decline in mpox cases among MSM began as early as July, suggesting the reduction in cases was not solely due to the vaccination program.

Effectiveness of Vaccination

Between July 25, 2022, and April 30, 2023, a total of 29,851 PPV doses were administered, achieving an overall vaccine uptake rate of 45.8%, with 35.4% of the target population fully vaccinated. The preventive vaccination’s effectiveness was estimated at 68.2%.

Researchers noted that the PPV program commenced during the peak of mpox transmission in the Netherlands. By the time the vaccinations took effect, the incidence had already declined to approximately five cases per day. This timing made it unlikely that the PPV program significantly contributed to the outbreak’s decline.

Role of Infection-Induced Immunity and Behavioral Changes

The study concluded that infection-acquired immunity played a major role in reducing mpox cases, particularly among those at highest risk. The researchers also pointed out that while behavioral changes likely contributed to the decline, the available data did not allow for an adequate study of this factor. However, based on other studies, the impact of behavior change should not be dismissed.