Omicron 'less severe because it causes less lung damage'

Omicron 'less severe because it causes less lung damage'

January 1, 2022

Omicron leads to less severe disease than other Covid variants because it does not cause as much damage to the lungs, studies suggest

  • Study on mice found Omicron causes less lung damage than other variants 
  • Mice infected with the mutant strain had a tenth less of the virus in their lungs 
  • Separate paper on human lungs and bronchi cells found the same effect 

Omicron is less severe than previous Covid variants because it does not cause as much damage in the lungs, a spate of studies have suggested. 

A study by a consortium of US and Japanese scientists on hamsters and mice found those infected with Omicron had less lung damage, lost less weight and were less likely to die than those that had other variants.

It found mice infected with Omicron had a tenth less of the virus in their lungs compared to those with other variants. 

The findings backed up another paper by researchers at the University of Hong Kong, who studied human tissue in Omicron victims.

They found Omicron grew significantly more slowly in 12 lung samples than earlier strains of the virus.

Experts believe the fact the super mutant variant tends not to replicate as much in the lower parts of the lungs means it causes less significant damage, which could be behind its reduced severity.

Data from South Africa showed Omicron sufferers are up to 80 per cent less likely to end up in hospital than those with Delta. And a similar study by the UK Health and Security estimated the risk was 70 per cent less.

Roland Eils, a computational biologist at the Berlin Institute of Health, said there is an emerging theme in the literature suggesting the variant tends to stay outside the lungs.

A study by a consortium of US and Japanese scientists on Syrian mice found those infected with Omicron (right) had less lung damage, lost less weight and were less likely to die than those that had the Delta variant (left)

The findings backed up another paper by researchers at the University of Hong Kong, who studied human tissue in Omicron victims (purple bars). They found Omicron grew significantly more slowly in 12 lung samples than earlier strains of the virus

Their study, which has not been peer-reviewed and was published on pre-print website medRxiv , found that among the 10,547 Omicron cases identified between October 1 and November 30, 261 (2.5 per cent) were admitted to hospital. For comparison, among the 948 non-Omicron cases in the same period – almost all of which would have been Delta, which was behind 95 per cent of cases before Omicron emerged – 121 people were hospitalised (12.8 per cent). The researchers said shows that those who caught Omicron had a 80 per cent lower risk of requiring hospital care

People who catch Omicron are 80 per cent less likely to be hospitalised than those who get Delta, a major study from South Africa suggests.

The real-world analysis, of more than 160,000 people, showed Omicron sufferers were also 70 per cent less likely to be admitted to ICU or put on a ventilator compared to those with Delta. 

South African doctors have insisted for weeks that Omicron is milder since raising the alarm about it on November 24 and accused the UK of panicking about Omicron.

But the researchers at the National Institute for Communicable Diseases (NICD) who carried out the study, which has not yet been peer-reviewed, said it still doesn’t answer whether Omicron is intrinsically weaker than Delta.

‘It is difficult to disentangle the relative contribution of high levels of previous population immunity versus intrinsic lower virulence to the observed lower disease severity,’ the researchers concluded. 

Built-up immunity from three previous waves of the virus and vaccines are believed to be doing most of the heavy lifting in keeping patients out of hospital this time around.

Up to 70 per cent of South Africans are believed to have had Covid before and only around a quarter are double vaccinated, with boosters not widely available yet.  

He told the New York Times: ‘It’s fair to say that the idea of a disease that manifests itself primarily in the upper respiratory system is emerging.’

The first paper, which has been published as a pre-print on Research Gate and has not yet been peer-reviewed, looked at tissue samples in mice and hamsters.

Researchers assessed different parts of the lungs for mice infected with the Omicron variant as well as other strains of the virus including Delta.

They found those infected with Omicron experienced much less severe symptoms.

And they were particularly struck by how Syrian hamsters — a breed which is particularly susceptible to previous strains — had lower levels of illness with the variant. 

Dr Michael Diamond, a virologist at Washington University and a co-author of the study, said: ‘This was surprising, since every other variant has robustly infected these hamsters.’

Meanwhile, the second study by Hong Kong scientists — also not peer-reviewed — looked at Omicron in the lung cells of humans.

They found the virus replicates significantly less in the lower parts of the lung.

The researchers also studied cells in the bronchi — the tubes in the upper chest delivering air to the lungs — finding that the variant 70 times as apparent in those cells.

They said the virus’ prevalence in higher parts of the chest make it more transmissible because it is expelled more quickly in breath.

But they said its lower levels in the lungs could also be behind the reduced severity suggested by swathes of other studies.

They wrote: ‘These observations may suggest that Omicron may have reduced clinical severity but such interpretations need to be qualified because the disease severity of Covid-19 is determined not only by virus replication but also by dysregulated innate immune responses.’ 

It comes after a British analysis of more than a million cases of Omicron and Delta in recent weeks found the risk of hospitalisation with the now dominant variant is about one-third that of its predecessor.

Britain is experiencing a surge in Covid cases driven by the highly-transmissible variant, with record daily infections of 189,846 yesterday.

While hospital admissions have started to rise, the government has said it believes the new variant is milder than the Delta variant.

The number of patients needing mechanical ventilation beds has also remained steady through December, unlike previous peaks in the pandemic.

The analysis was published by the UK Health Security Agency, after it worked alongside Cambridge University MRC Biostatistics unit to analyse 528,176 Omicron cases and 573,012 Delta cases.

It also found that vaccines can work well against Omicron.

‘In this analysis, the risk of hospitalisation is lower for Omicron cases with symptomatic or asymptomatic infection after two and three doses of vaccine, with an 81 per cent … reduction in the risk of hospitalisation after three doses compared to unvaccinated Omicron cases,’ the UKHSA said.

Susan Hopkins, Chief Medical Adviser at UKHSA, said the analysis was in keeping with other encouraging signs on Omicron but said the health service could still struggle with such high transmission rates.

‘It remains too early to draw any definitive conclusions on hospital severity, and the increased transmissibility of Omicron and the rising cases in the over 60s population in England means it remains highly likely that there will be significant pressure on the NHS in coming weeks,’ she said.

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