A growing body of evidence indicates that the Omicron Covid variant is more likely to infect the throat than the lungs, which scientists believe may explain why it appears to be more infectious but less deadly than other versions of the virus. Six studies – four published since Christmas Eve – have found that Omicron does not damage people’s lungs as much as the Delta and other previous variants of Covid. The studies have yet to be peer-reviewed by other scientists.
“The result of all the mutations that make Omicron different from previous variants is that it may have altered its ability to infect different sorts of cells,” said Deenan Pillay, professor of virology at University College London.
“In essence, it looks to be more able to infect the upper respiratory tract – cells in the throat. So it would multiply in cells there more readily than in cells deep in the lung. This is really preliminary but the studies point in the same direction.”
If the virus produces more cells in the throat, that makes it more transmissible, which would help to explain the rapid spread of Omicron. A virus that is good at infecting lung tissue, on the other hand, will be potentially more dangerous but less transmissible.
Researchers from the University of Liverpool’s Molecular Virology Research Group published a pre-print on Boxing Day that shows Omicron leading to “less severe disease” in mice, according to Prof James Stewart. The paper showed that mice infected with Omicron lose less weight, have lower viral loads and experience less- severe pneumonia.
“It’s one piece of the jigsaw,” he said. “The animal model does suggest that the disease is less severe than Delta and the original Wuhan virus. It seems to get cleared faster and the animals recovered more rapidly, and that ties in with clinical data coming through.
“The early indications are that it’s good news, but that’s not a signal to drop our guard, because if you’re clinically vulnerable, the consequences are still not great – there are deaths from Omicron. Not everyone can rip their masks off and party.”
The Neyts Lab at Leuven University in Belgium found similar results in Syrian hamsters, with a lower viral load in the lungs compared with other variants. Prof Johan Neyts said this may be because the virus was better at infecting humans than hamsters, or that it was more likely to infect the upper respiratory tract, or that it provoked less-severe disease.
A further pre-print, submitted to Nature last week by researchers in the US, also found that mice with Omicron lost less weight and had a lower viral load. And researchers at the University of Glasgow’s Centre for Virus Research have found evidence that Omicron has changed the way it enters the body. Omicron was substantially likely to evade the immunity of people who had had two doses of the vaccine, but a booster dose gave a “partial restoration of immunity”.
The slew of Christmas research builds on a study from the University of Hong Kong last month showing less Omicron infection in the lungs, and on research led by Prof Ravi Gupta at the University of Cambridge, whose team investigated blood samples from vaccinated patients. They found Omicron able to evade vaccines, but less able to enter lung cells.
The latest scientific research comes amid a debate about how best to carry out home-testing. Last week, some scientists suggested that lateral flow tests (LFTs) might be more accurate if people took swabs from the throat as well as the nose. Prof Jennifer Rohn at University College London said that her experience of using LFTs was that she had tested negative using nose swabs but positive when taking a sample from her throat.
That seemed to be supported by a study from South Africa showing that saliva samples subjected to PCR tests were better than nasal swabs at detecting Omicron.
However, Prof Lawrence Young, virologist at the University of Warwick, said the study was not significant enough for conclusions to be drawn. “This is a small study on acutely symptomatic, non-hospitalised patients. On the one hand it confirms previous studies indicating that salivary testing could be useful as a more easily performed sampling approach. I don’t think this study is significant enough to conclude anything about the behaviour of Omicron.”
The UK Health Security Agency said there was no indication that rapid tests differed in their ability to detect the Omicron or Delta variants, although the tests were continuously monitored by researchers. “All lateral flow devices used by NHS Test and Trace have gone through rigorous validation and are proven to be highly effective at detecting Covid-19 in people,” a spokesperson said. “We are currently seeing high numbers of positive lateral flow device tests reported. This means we are detecting tens of thousands of cases that might otherwise have gone undetected.”