What is the risk of SARS-CoV-2 transmission in vaccinated populations?
What is the risk of COVID-19 being passed on by people who have been vaccinated?
This review examined evidence on the risk of spread of COVID-19 from vaccinated people to others, who themselves may, or may not, be vaccinated. In total, 35 studies published in 2021 were included in this review.
Most of these studies were carried out in ways that mean results may appear to suggest a relationship where in fact one does not exist. In these cases, this means that the quality of the evidence is low and cannot be fully trusted.
Results
Early studies when the dominant strain of COVID was alpha, showed that spread of the virus from vaccinated people was reduced. Results from more recent studies are uncertain about whether there is a reduction in spread of the virus after vaccination. This could be due to the commonest strain of COVID changing from alpha to delta during this period.
For both alpha and delta strains the longer it is since someone received their vaccination, the more easily they could pass the virus on. Fully vaccinated people (i.e. those who had received booster vaccinations) were less likely to pass on the virus than those with partial vaccination.
Most of the studies looked at spread within household settings, so the results cannot be applied to other settings.
Possible Future Actions
- More research is needed to understand how the virus spreads, especially the delta strain (the commonest one in Wales at the time of this Review) and other strains of concern (including omicron).
- The spread of the virus in other settings such as schools, care homes, hospitals, workplaces and social venues, and in people at risk needs further study.
- Full vaccination, including boosters, should be encouraged.
- Until more is known, other preventative measures to reduce spread of the virus may still be needed.
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