of Silvia Turin
A Chinese study explains the mechanisms underlying the variant’s “competitive” advantage over other strains. Delta is most contagious during the early stage of infection: it replicates much more and sooner
Why is the Delta variant so rapidly “conquering” the whole world? It is more transmissible, in the order of 40-60% compared to the Alfa, but – for the first time – a study also sheds light on which mechanism may be the basis of this formidable contagiousness and what it means with respect to the risk we run of catching the virus.
Analysis of the first Delta outbreak in China
Chinese researchers from the Guangdong Provincial Center for Disease Control and Prevention rigorously examined all data related to the first Delta outbreak in China and found that the Delta can reproduce faster (people get infected first) e the viral loads detected are at least 1,000 times higher to those of the other variants. What does this mean in practice? Angela Rasmussen, researcher at the Organization for Vaccines and Infectious Diseases of the University of Saskatchewan, explains on Twitter: “Delta’s incubation period is shortened compared to previous viruses. Furthermore, infected people are spreading “a ton more viruses” than previous variants. This certainly explains the increased transmissibility. If people emit 1,000 times more viruses, the likelihood of close contact being exposed to an infectious dose is much higher. If people become contagious more quickly after exposure, they may have more opportunities to infect others. ‘
Reduced incubation days
The study, published in Virological, started with the first local infection of the Delta variant in mainland China recognized on May 21, 2021. From the first index case to the last reported on June 18, 2021, a total of 167 local infections. The data set of the quarantined subjects allowed scientists to determine the time interval between exposure and reaching the viral load detectable by PCR (molecular buffer, ed) in infected subjects; the test was performed every day. The researchers noted that the positivity was detectable on average after 4 days (IQR 3-5) from the exposure in the 2021 epidemic compared to the average 6 (IQR 5-8) of the 2020 strain. “Incubation” lasts less.
Viral load at least 1,000 times higher immediately
Compared to 2020 strains, relative viral loads in Delta variant infections were 1,260 times higher than the day the viruses were first detected. At the first detection, 80% of the samples Delta had this high value, while at the beginning of the pandemic it was 19%. “These data show that the Delta variant may be more contagious during the early stage of infection,” the Chinese researchers write.
What does not change
The study also shows that the transmission routes have not changed: the main ones mode of contagion continue to be a mix of inhaled aerosols and direct contact (with droplets of saliva). If there is more virus around, it will be easier for it to spread. There is no news on the greater severity of Delta (pathogenicity), its ability to infect causes more people to get sick, but not in a different way than the other variants and the greater viral load has not been shown to be related to a greater risk of incurring forms severe from Covid.
Who is at risk
The higher viral load can explain the reinfections (or infections) among vaccinated: A high enough dose of the virus can overcome vaccine-induced defenses enough to cause an infection, although vaccine-induced immunity will limit the severity of the disease (in most cases). “Unvaccinated people have no defenses against Delta – writes Angela Rasmussen -. Partially vaccinated people have low levels of neutralizing antibodies. It is easy for Delta to overcome antibodies only by the strength of numbers (viral load, ed). Ma fully vaccinated people have high titers of neutralizing antibodies and memory B and T cell responses. It is much more difficult for Delta to overwhelm these defenses simply by “throwing” more viruses at them. ”
July 17, 2021 (change July 17, 2021 | 07:57)
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