The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km, but also an unprecedented and much larger amount of water vapor into the stratosphere. Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground-based Water Vapor Millimeter Wave Spectrometer instruments began to measure recordhigh amounts of water vapor in the mesosphere over a wide range of latitudes. While there are indications that some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruption, variations in water vapor mixing ratios also depend on dynamical factors. The phase of the QBO in 2015 was similar to that in 2022, and we make use of this similarity in order to better understand what role dynamics played in establishing the unusually large 2022 water vapor mixing ratios, both in the upper and lower mesosphere. Plain Language Summary The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km. While the direct injection to 53 km was impressive, the quantity was insufficient to significantly affect the global mesospheric (∼50–80 km) water vapor budget. However, the Hunga Tonga eruption can also affect mesospheric water vapor through the gradual ascent of the unprecedented and much larger amount of water vapor that was directly injected into the stratosphere (∼15–50 km). Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground-based Water Vapor Millimeter Wave Spectrometer instruments began to measure recordhigh amounts of water vapor in the mesosphere. While some of this increase is probably caused by the rise of unusually wet air from the Hunga Tonga plume, determining the precise contribution of the plume is difficult because there are a number of other factors that also caused an increase in mesospheric water vapor in 2022.