Dr Abdi Mahamud, the WHO’s Covid incident manager, said the agency was seeing ‘more and more studies’ hinting the new strain struggles to penetrate the lungs. He described it as ‘good news’.
Despite being ultra-infectious, Omicron has so far caused far fewer hospitalisations and deaths than previous waves.
Doctors believe rising levels of protection from vaccines and natural immunity has played the biggest role but an increasing number of studies suggest the variant is also intrinsically weaker.
Dr Mahamud told reporters in Geneva: ‘We are seeing more and more studies pointing out that Omicron is infecting the upper part of the body. Unlike the other ones, that could cause severe pneumonia.’
Increasingly, studies are suggesting that Omicron’s ability to spread so quickly might also be the reason it causes milder illness.
A study by researchers in Hong Kong found it multiplies 70 times faster in the airways compared to Delta, which could make people more infectious more quickly.
But it is 10 times slower than Delta at replicating in the lungs, where damage from the virus can lead to pneumonia and patients requiring ventilators.
The number of daily positive Covid tests recorded in England has exceeded 100,000 for nearly two weeks. However, the number of patients in hospital with the virus is a fraction of the level seen last winter, while deaths remain flat
The graphs show the amount of the coronavirus detected in human bronchial cells (left) and lung cells (right) 24 and 48 hours after coming into contact with the original strain of the virus (pink), Delta (orange) and Omicron (red). There was 70 times more Omicron recorded in the bronchus — the main pipe connecting the airways and lungs — compared to previous strains, but 10 times less virus in the lungs when compared to the original version and Delta. Experts from the University of Hong Kong said this suggests the virus is more transmissible but may cause less severe illness
People with severe Covid can suffer from pneumonia — swelling of the lung tissue — which leads to shortness of breath and a cough, with breathing difficulties becoming so severe for some that they require a ventilator.
UK hospitalisation levels are trending upwards, but the numbers in mechanical ventilation beds have remained static and doctors on the ground are reporting that Omicron infections appear to be less severe for many.
MailOnline analysis on Monday showed the proportion of Covid patients needing to be hooked up to ventilators in London has halved since Omicron took off.
NHS England statistics show 16 per cent of hospitalised patients required mechanical ventilation in late November when the Delta variant was still dominant.
But a month later, after it was replaced by Omicron, this had dropped to six per cent.
Dr Mahamud said Omicron data could be ‘good news’, but warned that, despite being linked with cold-like symptoms, it could cause chaos over the coming weeks in countries with low vaccination rates.
And promising studies from South Africa may be because the country is an ‘outlier’ due to its young population, he said.
Speaking to journalists in Geneva today, Dr Mahamud said Omicron’s high transmissibility means it will become dominant within weeks in many places, posing a threat in countries where a high portion of the population remains unvaccinated.
His remarks on the reduced risks of severe disease chime with other data including a study from South Africa which was one of the first countries where Omicron was detected.
And a study by South Africa’s National Institute for Communicable Diseases found people were 80 per cent less likely to be hospitalised and 70 per cent less likely to be admitted to ICU or be put on a ventilator.
The researchers found that 2.5 per cent of people who caught Omicron were admitted to hospital, compared to 12.8 per cent of those infected with Delta, equating to the 80 per cent drop.
Similar analysis by the UK Health Security Agency based on 132 people hospitalised with Omicron or Delta found those who caught the newer strain were 50 to 70 per cent less likely to be admitted to hospital.
And researchers in Hong Kong this month found Omicron multiplies 70 times faster than Delta in the airways and 10 times slower in the lungs, which could explain why it spreads faster but causes milder symptoms.
Higher viral loads nearer the throat means people are more likely to breathe out viral particles.
Delta was found to duplicate much quicker in the lungs, where more of the virus can lead to the most severe illness.
The finding may be the biological clue behind why doctors insist people infected with the strain only suffer cold-like symptoms.
However, Dr Mahamud also sounded a note of caution, calling South Africa an ‘outlier’ since it has a young population among other factors.
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.
Meanwhile, 82.5 per cent of over-12s are double-jabbed in the UK and 60 per cent are boosted, which studies have found to provide an extra layer of defence against Omicron.
Asked about whether an Omicron-specific vaccine was needed, Mahamud said it was too early to say but stressed that the decision required global coordination and should not be left to the commercial sector to decide alone.
Vaccine makers are already working on tweaking the current crop of jabs in case they are needed against Omicron.
But these won’t be ready for months and so far the current vaccines are holding up against severe Covid outcomes.
OMICRON’S SEVERITY: WHAT DO THE STUDIES SHOW?
Researchers: Hong Kong University
What they did: The researchers exposed lung tissue in a laboratory to the original Covid strain that was identified in Wuhan last year, along with the two variants, to compare how the viruses behave after infection.
What they found: Omicron multiplies 70 times faster than Delta in the bronchus — tubes connecting the windpipe and lungs — but 10 times slower in the lungs than predecessors. The finding may explain why the mutant virus is spreading at a ferocious pace, and also lends weight to the theory it is milder than the past variants.
What it means: Higher viral loads nearer the throat means people are more likely to breathe out viral particles. Delta was found to duplicate much quicker in the lungs, where more of the virus can lead to the most severe illness. The finding may be the biological clue behind why doctors insist people infected with the strain only suffer cold-like symptoms.
Researchers: Washington University
What they did: Researchers assessed different parts of the lungs for mice infected with the Omicron variant as well as other strains of the virus including Delta.
What they 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.
What it means: Scientists said it adds to the growing body of evidence that Omicron manifests itself in the upper respiratory system.
Researchers: South Africa’s National Institute for Communicable Diseases
What they did: The researchers examined 10,547 people infected with Omicron and 948 non-Omicron infections.
What they found: 2.5 per cent of those infected with Omicron were hospitalised, compared to 12.8 per cent of those infected with other strains. The team said this equated to Omicron being 80 per cent less likely to lead to hospitalisation than Delta.
What it means: The researchers said it still doesn’t answer whether Omicron is intrinsically weaker than Delta. 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.
Researchers: Imperial College London
What they did: Researchers analysed data on 56,000 Omicron cases and 269,000 Delta cases in the first to weeks of December.
What they found: Those who catch Omicron are 15 to 20 per cent less likely to be admitted to hospital and 40 to 45 per cent less likely to spend a night or more in hospital than those who catch Delta.
What it means: The team said the findings can help other countries plan for Omicron waves.
Researchers: University of Edinburgh
What they did: The team tracked more than 150,000 patients diagnosed with Covid, including 22,000 Omicron cases, most of whom were aged under 65.
What they found: The number of Omicron patients who needed to be hospitalised was 68 per cent lower than the expected number of admissions for Delta cases. If Omicron was as severe as Delta they would have seen 47 admissions but they only recorded 15.
What it means: The finding adds to research the variant is less severe, but researchers cautioned the high number of Omicron cases could mean the NHS is overwhelmed with more patients than last winter.