Afkhami, S., D’Agostino, M. R., Zhang, A., Stacey, H. D., Marzok, A., Kang, A., Singh, R., Bavananthasivam, J., Ye, G., Luo, X., Wang, F., Ang, J. C., Zganiacz, A., Sankar, U., Kazhdan, N., Koenig, J. F. E., Phelps, A., Gameiro, S. F., Tang, S., … Xing, Z. (2022). Respiratory mucosal delivery of next-generation COVID-19 vaccine provides robust protection against both ancestral and variant strains of SARS-CoV-2. Cell, S0092867422001453. https://doi.org/10.1016/j.cell.2022.02.005

Prof. Akiko Iwasaki. (2022, January 27). Vaccines that reduce infection & disease are needed to combat the pandemic. Here, @tianyangmao @BenIsraelow et al. Describe our new mucosal booster strategy, Prime and Spike, to induce such immunity via nasal delivery of unadjuvanted spike vaccine 🧵 (1/) https://biorxiv.org/content/10.1101/2022.01.24.477597v1 https://t.co/bcB5MFph9F [Tweet]. @VirusesImmunity. https://twitter.com/VirusesImmunity/status/1486510697332842498

This is likely do to the reduced carbohydrate intake rather than increased fat intake. Since carbohydrates generally insulin sensitivity, it's likely that this additional insulin resistance is acting as a confounder (as well as, presumably, a standard deviation widener). Thus, I would expect similar results during hypocaloric carbohydrate restriction.