Since placebos don’t work on mosquitoes, I decided to do more research. The number one research study was in new Mexico and they stated:
Our study has confirmed earlier results that vitamin B1 is not a systemic insect repellent.
https://academic.oup.com/jinsectscience/article/15/1/140/2583458
The Efficacy of Some Commercially Available Insect Repellents for Aedes aegypti (Diptera: Culicidae) and Aedes albopictus (Diptera: Culicidae)
New Mexico State University researchers find some surprising results testing mosquito repellents
Mosquito repellent patches for the transdermal delivery of thiamine (vitamin B1) have been available to consumers for decades. The proposed mode-of-action is that the patch alters blood chemistry and human smell making the user less attractive to mosquitoes. Our results show that the Mosquito Skin Patch did not provide any repellency against Ae. aegypti or Ae. albopictus , which confirms a study from 1969 with similar results ( Khan et al. 1969 , Kahn et al. 1975 ).