“Methods: We performed a retrospective observational cohort study of contacts of SARS-CoV-2-infected index cases using contact testing data from England.
Results: … Transmission reductions declined over time since second vaccination, for Delta reaching similar levels to unvaccinated individuals by 12 weeks for ChAdOx1 and attenuating substantially for BNT162b2. Protection from vaccination in contacts also declined in the 3 months after second vaccination.
Duration of protection and transmission reductions
[H]igher probabilities of PCR-positive results in contacts 14 days after second vaccination for Delta vs. Alpha meant that by 12 weeks post second ChAdOx1 dose there was no evidence that onward Delta transmission rates differed between those not vaccinated and those having received two ChAdOx1 doses and the impact of BNT162b2 had also attenuated substantially.”
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