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COVID vaccines cut the risk of transmitting Delta — but not for long
The first study to look directly at how well vaccines prevent the spread of the Delta variant of SARS-CoV-2 brings good news and bad.
The study shows that people who become infected with the Delta variant are less likely to pass the virus to their close contacts if they have already had a COVID-19 vaccine than if they haven’t1. But that protective effect is relatively small, and dwindles alarmingly at three months after the receipt of the second shot.
The findings add to scientists’ understanding of the vaccination’s effect on curbing Delta’s spread, but are “both more and less encouraging”, says Marm Kilpatrick, an infectious-disease researcher at the University of California, Santa Cruz.
Previous studies have found that people infected with Delta have roughly the same levels of viral genetic materials in their noses regardless of whether they’d previously been vaccinated, suggesting that vaccinated and unvaccinated people might be equally infectious2. But studies also suggest that vaccinated people are less likely to spread the virus if they subsequently catch Delta: their levels of nasal virus drop faster than do those of unvaccinated infected people, and their nasal swabs contain smaller amounts of infectious virus3,4. The mutation that helps Delta spread like wildfire
The latest study examined the effect of vaccines on transmission more directly. It analysed testing data from 139,164 close contacts of 95,716 people infected with SARS-CoV-2 between January and August 2021 in the United Kingdom, when the Alpha and Delta variants were competing for dominance.
The authors found that although the vaccines did offer some protection against infection and onward transmission, Delta dampened that effect. A person who was fully vaccinated and then had a ‘breakthrough’ Delta infection was almost twice as likely to pass on the virus as someone who was infected with Alpha. And that was on top of the higher risk of having a breakthrough infection caused by Delta than one caused by Alpha.
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doi: https://doi.org/10.1038/d41586-021-02689-y
References
Eyre, D. W. et al. Preprint at medRxiv https://doi.org/10.1101/2021.09.28.21264260 (2021).
Brown, C. M. et al. MMWR Morb. Mortal. Wkly. Rep. 70 1059–1062 (2021).
Chia, P. Y. et al. Preprint at medRxiv https://doi.org/10.1101/2021.07.28.21261295 (2021).
Shamier, M. C. et al. Preprint at medRxiv https://doi.org/10.1101/2021.08.20.21262158 (2021).
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