I could be wrong, but I don’t think it’s due to negative vaccine effectiveness. Going through France data briefly shows that the unvaccinated group has a higher rate of Covid-19 than the vaccinated group.

Below is a screenshot, where, on 9 Jan 2022, for example, there were about 2,500 Covid cases per 100,000 unvaccinated people and 1,400 Covid cases per 100,000 vaccinated people. But I think this figure is not adjusted for confounders like age or sex. But at least it implies no negative vaccine effectiveness.

Source: https://www.gouvernement.fr/info-coronavirus/carte-et-donnees#situation_epidemiologique_-_tests_positifs_chez_les_personnes_vaccinees_et_non_vaccinees