US, UK and Singapore reporting surge;inoculated still need to take precautions
Data from Covid-19 tests in the United States (US), the United Kingdom (UK) and Singapore show that vaccinated people 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, suggests a preprint study published on medRxiv. The three countries are reporting a fast surge in the infection.
The findings underscore the importance of protective measures such as wearing masks 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, said an article based on the study and published in the Nature.
“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,” said David O’Connor, a virologist at the University of Wisconsin–Madison.
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 said.
O’Connor and colleagues at the Madison and Dane County health department looked at infections in Wisconsin in June and July.
In a preprint study published on medRxiv on Aug 11, the researchers compared Ct values for 719 people between June 29 and July 31, during which 90 per cent of the 122 coronavirus samples they sequenced were the Delta variant. Of the 311 vaccinated people who tested positive for SAR-CoV-2 in that group, most had Ct values of less than 25, a level at which researchers expect the presence of infectious SARS-CoV-2. To confirm this, the team cultured 55 samples that had Ct values less than 25, from vaccinated and unvaccinated people, and detected infectious virus in nearly everyone. Most unvaccinated people also had Ct values below this level.
“The bottom line is, this can happen — it can be true that vaccinated people can spread the virus. But we do not yet know what their relative role in overall community spread is,” said co-author Thomas Friedrich, a virologist at the University of Wisconsin–Madison.
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, said the article quoting the study.
Data from Provincetown, Massachusetts, suggest similar findings. The article pointed out that an August report from the US Centers for Disease Control and Prevention (CDC) showed that following large gatherings in the beach town, nearly three-quarters of 469 new COVID-19 cases that occurred in the state were in vaccinated people. Both vaccinated and unvaccinated people had comparably low Ct values, indicating high viral loads, and of the 133 samples sequenced, 90% were identified as Delta. The findings prompted the CDC to update its guidance on 27 July and once again recommend that people in areas of high transmission wear masks indoors.
The Provincetown results were linked to big gatherings, but Wisconsin didn’t have similar activity, suggesting that small household gatherings could also help Delta to spread, Friedrich said.