How do vaccinated people spread the Delta variant of covid? After the speed with which the mutant crossed India in April and May, several works have tried to investigate this aspect. Reports from some countries seem to confirm that Delta is more likely to spread through vaccinated people than other variants. But the question is more complex than a statistic, and the key questions about their role in the contagion still remain open. To take stock of what science says about it is the journal ‘Nature’, which has published an overview of the studies conducted and the information that has emerged so far.
We start from the data, those of Covid tests in the US, the UK and Singapore, which show how vaccinated people are also infected with Delta and can carry as much virus in the nose as unvaccinated people. “This means – reads the online article – that, despite the protection offered” by shield injections, “a part of the vaccinated people can transmit Delta, possibly favoring its rise”. People with so-called ‘breakthrough’ infections caused by the Delta variant, “can carry these very high levels of the virus and may unknowingly spread the virus to others,” says David O’Connor, virologist. of the University of Wisconsin-Madison.
The observed results therefore underline the importance of measures such as wearing masks indoors. Researchers point out that Covid vaccines are protective against serious illness and death, and that Delta’s transmission data shows that “vaccinated people still have to take precautions,” says O’Connor, who along with colleagues from the County Health Department. Madison and Dane examined infections in Wisconsin in June and July, and estimated the concentration of the virus in nasal samples. In the study that appears preprint on ‘medRxiv’, not yet peer-reviewed, the experts compared the ‘Ct’ values (the lower they are, the more viral genetic material is present in the samples) for 719 people tested between June 29 and July 31 (90% Delta infected). Of the 311 positive vaccines, most had Ct values at a level at which the presence of infectious virus is expected.
And in fact, the team cultured 55 of these samples and detected infectious viruses in almost all of them. The bottom line, says co-author Thomas Friedrich, also a virologist at the University of Wisconsin-Madison, “is that it can happen: it may be true that vaccinated people can spread the virus. But we still don’t know what their relative role is. in the overall diffusion of the community “, specifies the expert. Data from Provincetown, Massachusetts, which photographs the effect of large beach gatherings, suggests similar results: an August report from the US Centers for Disease Control and Prevention showed that both vaccinated and uninhibited people vaccinated had values indicating high viral loads, and 90% of the sequenced samples were Delta.
In Texas, a team from Houston Methodist Hospital has sequenced and recorded Sars-CoV-2 variants for nearly all Covid cases in the hospital system: about 17% of those with Delta involve people vaccinated since March 2021, nearly 3 times the rate of ‘breakthrough’ infections compared to all other variants combined. Delta patients stayed in the hospital slightly longer than people infected with other variants. “There is potentially a slightly different biology for infection,” says James Musser, director of an infectious disease research center at the hospital.
However, people vaccinated and then affected by Delta could remain infectious for a shorter period, according to researchers from Singapore who observed the rapid decline in viral loads after a week in those who received the vaccine in one study. So much so that a co-author of the work, Barnaby Young, comes to the same conclusion, namely that “measures such as masks and hand hygiene are important for everyone”.
A massive analysis of the transmission of the Delta variant finally comes from the UK’s REACT-1 program, led by a team from Imperial College London, which tests more than 100,000 British volunteers every few weeks. The team performed Ct analyzes for the samples received in May, June and July, when Delta was rapidly replacing the other variants to become dominant in the country. The results suggested that among the positive people, those who had been vaccinated had a lower viral load on average than the unvaccinated. Paul Elliott, an epidemiologist at Imperial, says these results differ from other Ct studies because this study randomly sampled the population and also included people who tested positive without showing symptoms.
These data – along with an increase in cases in young people who do not yet have a full vaccination course – underscore the efficacy of dual vaccination against Delta, Elliott says. “We think it’s really important to double-dose as many people, and especially younger groups, as soon as possible,” he concludes.