In the month since we first heard about Omicron, the variant has been found in many countries around the world where it is driving the most dramatic spike in COVID-19 cases we have yet seen.


In the UK new cases rose by over 30,000 a day in the one week period from 14 December. As of 22 December, cases are nearly double the previous peak seen last January, with Omicron now the dominant variant. Similar trends are also seen in many other countries after the arrival of Omicron.

The four biggest questions surrounding Omicron are: is it more transmissible, is it more immune evading, does it cause different disease and do we need to do anything different to stop it. We do not have definitive answers to most of these questions, as we are still learning about the variant. Several studies have recently been published as non-peer reviewed pre-prints that may help to provide some answers.

The rapid spread of Omicron and the associated spike in COVID-19 cases in many countries suggests that the variant may be more transmissible. This concept is supported by pre-prints that show the variant is more infectious and has a shorter incubation period (the time between infection and the start of being contagious) than Delta. However, the rapid rise in Omicron cases may also be due to its ability to evade the protection of vaccine or infection-induced immunity.

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Recent research from South Africa that has not yet been peer-reviewed suggests that Omicron may be more immune-evading then Delta with increased risks of reinfection and breakthrough infections. This is beginning to be confirmed with laboratory experiments that have shown low to virtually no protection against infection with two vaccine doses.

Protection was somewhat restored in people that had an mRNA vaccine booster, though breakthrough infections of boosted individuals have been reported. Imperial College of London has modelled the vaccine efficacy of two doses of AstraZeneca as between 0-20 percent, rising to 55-80 percent after a third dose.

Both Pfizer and Moderna have stated that two doses of their vaccines are significantly less effective against Omicron. Though a third dose was expected to increase protection, even two doses will likely still provide some protection against severe disease.

There is also good evidence emerging that Omicron can evade immunity from a previous infection, even if that previous infection was with Delta. Though, it is not known if an Omicron infection would protect against a subsequent Delta infection. Modelling from Imperial estimates that Omicron is associated with a 5.41 fold increased risk of reinfection, compared to Delta.

Part of the reason for this immune evasion and possible increase in transmissibility is the large number of mutations Omicron has (more than twice the number of mutations seen in previous variants).

Some mutations in the Spike protein are known to affect virus transmissibility and immune evasion. However, many are wondering if some of these mutations in Omicron have changed the severity of COVID-19 disease.

Recent non-peer reviewed research suggests that the variant may change where in the lung it grows best, with Omicron showing less replication in the lungs than Delta but much higher replication in the bronchus. In addition, another pre-print study suggests that in lung cells, the variant is actually less transmissible than Delta. It is possible that if Omicron replicates less in the lungs then it might cause less severe disease.

An early report from South Africa has suggested the variant may produce more mild symptoms, a conclusion that has been met with much debate. Recent non-peer reviewed research from South Africa looked at over 160,000 COVID-19 cases and found that Omicron infections were less likely to result in hospitalisation or severe disease, compared to Delta.

However, the authors note that this may be because many of the Omicron cases are reinfections, with immunity from previous infections conferring some protection. However, similar trends are now being observed in the UK, with both Imperial College London and University of Edinburgh showing a reduction in the risks of hospitalisation with Omicron compared to Delta. None of this data has yet been peer reviewed and our full understanding of the clinical course of the disease and its severity will likely change as new and more full information becomes available over the coming weeks.

Even if the severity of Omicron is lower than that of Delta, we know the variant can still cause severe and fatal illness. With the current rapid rise in cases, even a small rate of severe infections could result in a large number of hospitalisations that threaten overburdening the NHS.

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In addition, it is expected that Omicron infections will still result in approximately 12 percent of cases going on to develop the long term debilitating symptoms of Long COVID, for both adults and children. At the current level of over 100,000 COVID-19 cases per day in the UK (as of 22 December), that would be 12,000 new cases of Long COVID every day.

This would be on top of the estimated 1.9 percent of the UK population that is already suffering from Long COVID. Because of this, even a ‘mild’ version of Omicron could result in significant, and long-term, impacts to health, the NHS and the economy, and requires our immediate action.

However, our actions to address Omicron can not rely solely on vaccine boosters, as vaccine uptake has lagged and the efficacy of the boosters in stopping infections is likely to be lower than we were seeing for Delta (though widespread uptake of the boosters will help reduce transmission and severe infections). So, as we continue with the booster roll out it is important to continue with existing preventative measures that remain highly effective, even for Omicron.

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It is likely that most testing methods (PCR and rapid antigen tests) will still detect Omicron. Masking, distancing, ventilation, testing, isolation/quarantine and vaccination have all been proven to reduce virus transmission. Together the use of these measures can control the spread of Omicron, reduce hospitalisations and Long COVID and minimise the burden on the NHS, without requiring a lockdown as has been proposed and recently adopted by the Netherlands.

Regardless, it is important to remember that Omicron will not be the last variant. As long as we continue to have COVID-19 infections around the world, we will continue to have virus evolution and risk further variants like Omicron. Therefore, there is a need to continue with our precautions taking while ensuring global vaccine availability.



Jeremy Rossman is a Senior Lecturer in Virology and President of Research-Aid Networks, University of Kent. His research focuses on the process of infectious disease outbreaks, and he has contributed to studies published in journals including PLoS Pathogens, Bioinformatics and Cell.