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16 December 2021

How likely am I to catch Omicron?

The new variant is expected to become the dominant strain in the UK – and already is in London.

By Harry Clarke-Ezzidio

The UK recorded its highest number of daily coronavirus cases since the pandemic began today (16 December), with 88,376 new cases identified.

This is up from the peak of last winter’s wave when 68,053 cases were detected on 8 January. It comes as the government’s Covid “Plan B” measures come into effect, while the NHS booster programme has been given a target of vaccinating one million people a day.

The highly transmissible Omicron variant is behind the rise, and in the run-up to Christmas, many are wondering how likely they are to catch it.

How many Omicron cases are there in the UK?

To confirm Omicron cases, PCR tests have to be sent to a special lab to be sequenced, which can take a couple of days. 

The latest government figure put the number of confirmed Omicron cases across the UK at 10,017 on 14 December.

However, Omicron also carries an easier to detect marker called “S gene target failure” (SGTF) which it shares with the now largely absent Alpha (Kent) variant. Applying the number of sequenced tests with SGTF to daily positive test totals shows that there could have been as many as 17,715 new cases on 12 December. That figure will of course have risen since.

It is also worth noting that this only includes positive tests – the number of infections is likely to be higher still, as many people won’t have been tested.

As a result of reporting lags between infection, testing and confirmation of a case, it’s hard to put a definitive number on how many people are catching Omicron. The UK Health Security Agency (UKHSA) estimated that on 13 December alone there were around 200,000 new Omicron infections, but this was based on a back-of-an-envelope calculation.

How fast does Omicron spread?

Though it is generally agreed that the Omicron variant is more transmissible than previous strains, experts are not yet sure how quickly it spreads among populations.

A recent study by the University of Hong Kong found that Omicron could grow 70 times faster than the Delta variant in the human bronchus (the tubes that carry air from your windpipe to your lungs). However, it was found to replicate more than ten times less efficiently in the human lung tissue, which researchers said “may suggest lower severity of disease”. 

Further evidence is needed to back up the findings. That said, it does match with early figures from South Africa, which show that while hospital admissions are not growing as fast as cases are rising.

The South African data also shows that a smaller proportion of patients are requiring ICU than in previous waves, indicating that the variant could be less severe.

It’s unclear whether this data can easily be applied to the situation in the UK. South Africa has far higher levels of “natural immunity” (people who have recovered from the disease), but the UK has mainly relied on immunity through vaccination. The UK also has an older population, which is more likely to be severely affected.

Some are already preparing for the worst: NHS bosses have been briefed that the maximum number of Covid surge beds in London hospitals will be full by January.

How do vaccines affect Omicron?

Early evidence shows that boosters, especially mRNA vaccine such as Pfizer and Moderna, are key.

A pre-print study from the Oxford Vaccine Group found that two doses of AstraZeneca did not offer significant protection against Omicron, and the protection offered from two Pfizer doses was significantly reduced against the variant. The authors found that this could lead to “increased breakthrough infections in previously infected or double vaccinated individuals, which could drive a further wave of infection”. But they said there was no evidence yet that Omicron would cause more severe disease.

A separate pre-print from the Balazs Laboratory at the Ragon Institute of MGH, MIT and Harvard found that Omicron was better at infecting cells than any other variant the lab had tested. However, the researchers also found that “individuals boosted with mRNA vaccines exhibited potent neutralisation of Omicron”, pointing to the importance of booster jabs.

Analysis from the UKHSA last week found that two weeks after a Pfizer booster dose, vaccine effectiveness against symptoms increased to around 71 per cent among those previously vaccinated with AstraZeneca and around 76 per cent among those who previously had Pfizer jabs.

How can I avoid catching Omicron?

It is hard to predict how likely an individual is to contract Omicron as it can depend on a number of factors, including age and vaccination status.

It is expected that Omicron will soon become the country's dominant strain – it already is in London, with SGTF detected in more than 60 per cent of tests.

The government has introduced a number of measures, including mask mandates in most indoor settings, an instruction to work from home “if possible”, and a requirement to provide your vaccination status or a negative test to enter large venues.

The Prime Minister has warned a “tidal wave” of infections will hit the country over the Christmas period. In a televised address on 12 December, Boris Johnson said: “We have the vaccines to protect our people. So, let’s do it, let’s get boosted now."

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