IntroductionSince the vaccine roll out, research has focused on vaccine safety and efficacy, with large clinical trials confirming that vaccines are generally effective against symptomatic COVID-19 infection. However, breakthrough infections can still occur, and the effectiveness of vaccines against transmission from infected vaccinated people to susceptible contacts is unclear.Health Technology Wales (HTW) collaborated with the Wales COVID-19 Evidence Centre to identify and examine evidence on the transmission risk of SARS-CoV-2 from vaccinated people to unvaccinated or vaccinated people.MethodsWe conducted a systematic literature search for evidence on vaccinated people exposed to SARS-CoV-2 in any setting. Outcome measures included transmission rate, cycle threshold (Ct) values and viral load. We identified a rapid review by the University of Calgary that was the main source of our outcome data. Nine studies published following the rapid review were also identified and included.ResultsIn total, 35 studies were included in this review: one randomized controlled trial (RCT), one post-hoc analysis of an RCT, 13 prospective cohort studies, 16 retrospective cohort studies and four case control studies.All studies reported a reduction in transmission of the B.1.1.7 (Alpha) variant from partial and fully vaccinated individuals. More recent evidence is uncertain on the effects of vaccination on transmission of the B.1.617.2 (Delta) variant. Overall, vaccine effectiveness in reducing transmission appears to increase with full vaccination, compared with partial vaccination. Most of the direct evidence is limited to transmission in household settings therefore, there is a gap in the evidence on risk of transmission in other settings. One UK study found protection against onward transmission waned within 3 months post second vaccination.ConclusionsEarly findings that focused on the alpha variant, showed a reduction in transmission from vaccinated people. There is limited evidence on the effectiveness of vaccination on transmission of the Delta variant, therefore alternative preventative measures to reduce transmission may still be required.

This is an update (literature search up to 15 March 2022) of a rapid review examining whether vaccination against SARS-CoV-2 (COVID-19) affects transmission of SARS-CoV-2. Streamlined systematic methodologies were used to accelerate the review process. The update identified 17 additional studies: 6 studies reported on transmission and 11 studies reported viral load. There was high heterogeneity across studies, which varied in design, participant characteristics and SARS-CoV-2 variants reported. Evidence from this update supports previous findings that that transmission of Omicron and Delta variants is lowest in booster-vaccinated people, followed by fully vaccinated people, with the highest rate of transmission in unvaccinated people. Additionally, some studies compared transmission between different variants or sub-variants; risk of transmission appears to be higher with Omicron than Delta, regardless of vaccination status.