Triple vaccinated are less contagious in omicron infection

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Corona vaccinated individuals are well protected from severe disease in the event of a breakthrough infection, but they are also less contagious than unvaccinated individuals. Unlike the delta variant, however, the omicron variant requires three doses of vaccine to effectively reduce the number of infectious virus particles. This was reported by Swiss researchers led by Isabella Eckerle and Benjamin Meyer of the University of Geneva in the journal Nature Medicine.

The study refers to the omicron subvariant BA.1, not the BA.2 subvariant, which is now dominant in Austria. “Our results underscore the positive effect of vaccination beyond individual protection against severe disease,” the researchers write. First, Der Spiegel reported on the study.

The scientists had taken swabs from the nasal mucosa of 565 Corona-infected people within the first five days after the onset of symptoms between April 2020 and February 2022. The study thus covers virtually the entire pandemic course to date: Infections with the original circulating variant and the delta and omicron variants were recorded. Some subjects were unvaccinated, while others were dually vaccinated or boosted. Almost all of those vaccinated had received an RNA vaccine.

The scientists then determined the so-called Ct value, which indicates how much genetic material of the virus is present in a sample. They also used cell culture experiments to determine the number of infectious virus particles in an instance. This is a better indicator of the infectiousness of a diseased person, as the researchers’ tests confirmed.

Data analysis showed that dual-vaccinated individuals had significantly fewer infectious particles in their upper respiratory tract during a delta infection than unvaccinated individuals, nearly one-fifth. In addition, according to the study, vaccinated individuals could fight off the virus more quickly. In omicron breakthrough infections, the amount of infectious viral particles after a double vaccination was as high as in unvaccinated persons. Only booster vaccination significantly reduced the level to about one-fifth.

Another result: During the omicron wave, the researchers found significantly lower amounts of infectious particles overall in the vaccinated subjects than during the delta wave – despite the higher infectiousness of the omicron variant. They conclude that omicron-infected individuals are not more contagious because they excrete more viruses. It is possible, they say, that the mechanism of entry of the virus into the cell is different.

The researchers point out that it is unknown what level of detected virus particles an infected person is contagious. The fluctuations in the amount of virus could also have been influenced by the respective quality of the smear or the fact that the samples were not taken from all test persons at the same time of infection.