Scientists have assessed 750 samples from important COVID-19 superspreading clusters in Austria and found that on average 1,000 infectious virus particles are transmitted from one infected person to the next. According to the study, published in the journal Science Translational Medicine, these values are overall considerably higher than for other viruses such as HIV or noroviruses which cause gastroenteritis.
"Yet, occasionally we also found infected people who apparently came into contact with fewer virus particles and still became infected," said study co-author Andreas Bergthaler from the Austrian Academy of Sciences. "We suspect that parameters such as the application of protective measures, the transmission route, or the immune system may play a decisive role here," Bergthaler added.
The researchers believe reducing the viral load of infected individuals by a combination of measures such as mouth-nose protection, physical distance, and adequate indoor air exchange play a key role in preventing the spread of the virus. In the study, the scientists used mutation analysis to reconstruct the SARS-CoV-2 transmission among 76 cases of infection and uncover the cryptic link between two epidemiological clusters. According to the researchers, a special feature of the study is that they analysed a chain of eight consecutive transmissions.
"The transmission chain started with a returnee from Italy. Within 24 days, the SARS-CoV-2 virus spread in the greater Vienna region via public and social events in closed rooms," the scientists noted. "This example illustrates how contact tracing and virus mutation analysis together provide a strong pillar of modern pandemic control," Bergthaler added.
The scientists also observed the mutation behaviour of the coronavirus during the course of the disease in 31 patients. "Thanks to excellent epidemiological and our deep virus sequencing data, we could follow how the SARS-CoV-2 virus mutated in one individual and was then transmitted to others," Bergthaler said.