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How do vaccinated people spread Delta? What the science says
When early field data showed that vaccinating people cuts transmission of the SARS-CoV-2 virus, researchers were cautiously optimistic. But they warned that many of those studies, although promising, took place before the fast-spreading Delta variant proliferated worldwide. Now, reports from various countries seem to confirm what scientists feared after the variant tore through India with alarming speed in April and May: Delta is more likely than other variants to spread through vaccinated people.
Data from COVID-19 tests in the United States, the United Kingdom and Singapore are showing that vaccinated people who become infected with Delta SARS-CoV-2 can carry as much virus in their nose as do unvaccinated people. This means that despite the protection offered by vaccines, a proportion of vaccinated people can pass on Delta, possibly aiding its rise.
Delta coronavirus variant: scientists brace for impact
“People who have a Delta virus and happen to have ‘breakthrough’ infections can carry these really high levels of virus, and can unwittingly spread the virus to others,” says David O’Connor, a virologist at the University of Wisconsin–Madison.
The findings underscore the importance of protective measures such as wearing masks indoors to reduce transmission. Researchers stress that COVID-19 vaccines are protective against serious illness and death, but the data on Delta transmission show that “people who are vaccinated still need to take precautions”, O’Connor says.
Testing transmissibility
O’Connor and colleagues at the Madison and Dane County health department looked at infections in Wisconsin in June and July.
The team used PCR tests, which are widely used to confirm COVID-19 infections, to estimate the concentration of virus in nasal-fluid samples. The tests detect the virus’s genetic material by amplifying DNA until it is detectable as a fluorescent signal. The number of amplification cycles needed to get a signal — a measure called the cycle threshold value or Ct — serves as a proxy for viral concentration in the sample. The lower a sample’s Ct, the more viral genetic material present.
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Nature 596, 327-328 (2021)
doi: https://doi.org/10.1038/d41586-021-02187-1
References
Riemersma, K. K. et al. Preprint at medRxiv https://doi.org/10.1101/2021.07.31.21261387 (2021).
Brown, C. M. et al. MMWR Morb. Mortal. Wkly. Rep. 70 1059–1062 (2021).
Musser, J. M. et al. Preprint at medRxiv https://doi.org/10.1101/2021.07.19.21260808 (2021).
Chia, P. Y. et al. Preprint at medRxiv https://doi.org/10.1101/2021.07.28.21261295 (2021).
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