"The negative VE estimates in the Omicron period could have several plausible explanations. First, this could be due to behavioural confounding, where vaccinated CYP begin to act more freely and take less caution. This is how they can eventually get more symptomatic infections, as a net result of modest vaccine protection against symptomatic infection, waning of the effectiveness, and their behaviour. Initially, the vaccine effect is still dominant, but as it wanes over time, the behaviour effect might cause negative VE. Second, at this stage of the pandemic it is plausible that there were more CYP who acquired natural immunity among the unvaccinated group than among the vaccinated. Natural immunity may be either more robust, more durable, or both, in comparison to vaccine induced immunity against the omicron variant, leading to negative VE over time."
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