Virological and epidemiological investigations of a severe hand foot and mouth disease outbreak, Vietnam, 2023.

We report on the virological and epidemiological features of a severe hand foot and mouth disease outbreak in Vietnam in 2023. Enterovirus A71 (EV-A71) subgenogroup B5, first emerging around November 2022, and imported into Vietnam through a single introduction, was the primary cause. Convalescent plasma collected from patients infected with subgenogroups B5 in 2014 and 2023 could neutralize both contemporary viruses with comparable titers, suggesting no evidence of immune evasion. However, at one and six months before the start of the outbreak, only 6.7% of 0- < 3 year-old children in Ho Chi Minh City, an epicenter of the outbreak, had neutralizing antibodies against EV-A71. Seroprevalence increased as the outbreak progressed, coinciding with a drop in EV-A71 infected patients residing in Ho Chi Minh City, suggesting that serosurveillance could be a useful tool to assess the risk of pathogen emergence. Our collective findings have provided novel insights into the epidemiological factors driving EV-A71 emergence, and therefore would inform outbreak preparedness and response, and in particular the development of serosurveillance-based approach to assess the risk of pathogen emergence. EV-A71 infection represents an ongoing public health threat, currently affecting countries in the Asia-Pacific region, but with a high potential to become a global problem, and therefore should be closely monitored.