This is a somewhat misleading headline. Everyone who participated in the study was unvaccinated.

> Eligible participants were personnel of the Swiss Armed Forces (SAF) who were aged 18–30 years with a positive or negative RT-PCR test for SARS-CoV-2 during their service between March 1, 2020, and Dec 31, 2020.

And the control group, if I am reading correctly, was all people who did not have COVID (nothing to do with vaccination status although some of them were vaccinated).

So this means really they were comparing the levels in people who had COVID vs. people who did not. The claim that the difference could be related to the vaccination status of the people in the control group seems to be a bit of a leap.