Delta variant, 74% infections from pre-symptomatic people: the study

The Delta variant of Covid could also run on the legs of people who are apparently fine, but in reality they already have the virus and have not yet discovered it. According to analysis of a Covid outbreak that occurred in Guangdong China, people infected with this mutant are more likely to spread the virus before developing symptoms than those infected with previous ‘versions’ of the coronavirus. And this could happen in 74% of cases. The work leading a group of scientists to this conclusion is cited in a feature published online in the journal ‘Nature’. Benjamin Cowling, an epidemiologist at the University of Hong Kong, co-author of the study not yet peer-reviewed and published on a preprint platform (‘medRxiv’), with colleagues considered the test data of 101 people infected between May and June in Guangdong and the data of their close contacts. Scientists found that on average, people had nearly 2 days (1.8) to release viral RNA before showing signs of Covid. A dangerous window and wider than the one calculated in the pre-Delta era (which was 0.8 days). The researchers also confirmed that people infected with Delta had a higher viral load than those infected with the original virus. And that index cases not vaccinated or with only the first dose active were more likely to transmit the infection to their contacts than those who had received the full double-dose course. The calculation reported in the paper would indicate that 74% of Delta infections occurred during the pre-symptomatic phase (73.9%), a higher percentage than the previous variants. This high rate could help “explain how this variant was able to become the dominant strain around the world,” says Barnaby Young, a physician at the National Center for Infectious Diseases in Singapore. VACCINATES WOULD HAVE 65% LESS CHANCE TO INFECT OTHERS The researchers also calculated the ‘base reproduction number’ (R0), which is the average number of people infected by a single infected in a susceptible population. For the Delta variant this index has a value of 6.4, much higher than that estimated for the original version of Sars-CoV-2 (i.e. 2-4) estimated for the original version of SARS-CoV-2, says Marm Kilpatrick, University of California, Santa Cruz. “Delta moves a little faster, but it is much more transmissible,” he notes. In the study, a small proportion of participants developed Delta breakthrough infections after 2 doses of inactivated virus-based Covid vaccine. But the shield product reduced the viral load at the peak of the infection. And it has been estimated that vaccinated people are also 65% less likely to infect someone else than unvaccinated. This reduction, Young concludes, reassures that “vaccines remain effective and a vital part of our response to the pandemic.”