Omicron v Delta: How do variants compare on infectiousness and effect on Covid vaccines?

The new coronavirus variant Omicron has already prompted the return of mandatory mask rules to England and cases have rapidly increased since first detected.

Some 437 confirmed cases of the variant have been detected in the UK as of Tuesday while one expert has warned it could become our dominant strain within just weeks.

A full picture of the threat the variant poses is still to emerge. But what do we know about Omicron and how does it compare to Delta?

When they were discovered

Delta: It was first discovered in India last December before rapidly spreading to the UK. The variant is still the main dominant strain as it is in many countries.

Last week the ONS said 99 per cent of sequenced cases in the UK were Delta ending the week November 27.

Omicron: UK scientists became aware of Omicron on November 2 after samples were uploaded to a coronavirus variant tracking website from South Africa, Hong Kong and then Botswana.

However, it is likely the variant was in circulation earlier than that date but was not sequenced.

Mutations

Delta: Delta has 13 mutations, of which nine are in the ‘spike protein’. According to the ONS, scientists can differentiate Delta from Omicron by identifying patterns among three genes in positive results - the S gene, the ORF1ab gene and the N gene.

Delta infections usually test positive for all three genes, whereas the Omicron variant tests negative for the S gene.

Omicron: Omicron has the most mutations of any variant of SARS-CoV-2, the virus that causes COVID-19. It caused alarm among scientists because at least 32 of its mutations are in the spike protein.

The spike protein is part of the virus which helps it latch onto our cells, fitting cellular receptors “like a lock and key,” say Yale University researchers.

While Omicron has more mutations in total, Robert Garry, a virologist at Tulane University, told CNN that the number of mutations don’t automatically translate to a more dangerous virus.

How contagious are they?

Delta: The Delta variant was more than 2x as contagious as variants that came before it, according to the Centre for Disease Control in the United States.

The incubation period was also shorter than previous iterations of SARS-CoV-2, with a mean time of 4 days, meaning people became infectious sooner.

Omicron: Omicron’s transmissiblity compared with other variants, including Delta, according to the World Health Organisation.

However, Boris Johnson told Cabinet ministers on Tuesday “early indications were that it was more transmissible than Delta” which was later repeated by his official spokesperson publicly.

Anthony Fauci, the top US medical adviser, also said on Tuesday the variant was “clearly highly transmissible.”

How severe is illness?

Delta: According to a study published earlier this year, those who got the Delta variant were twice as likely to be hospitalised as a result of their illness than if they had the earlier Alpha variant.

However, the vast majority of those in the study, 70 per cent, were unvaccinated as the cases occurred very early on into the vaccine rollout.

Omicron: A full picture is yet to emerge, according to the WHO. But a study from South Africa suggests it may well be a milder illness than Delta. Dr Fauci has also spoken of “encouraging signals” about the severity of disease.

The study showed at the Steve Biko and Tshwane District Hospital Complex in Pretoria on December 2 only nine of the 42 patients on the Covid ward were being treated for the virus and were in need of oxygen.

However, scientists have warned that very high levels of even a mild illness could be challenging for the NHS if it results in enough hospitalisations.

For this reason, Dr Simon Clarke - a microbiologist at the University of Reading - told the Mail Online it was “entirely possible” Omicron could trigger a wave of hospital admissions on par with the peak in January 2021.

Vaccine escape

Delta: Two doses of either Oxford or Pfizer jabs provides at least the same level of protection as having had Covid-19 before, according to a study earlier this year.

Two doses of the Pfizer/BioNTech or Oxford/AstraZeneca vaccine were estimated to be 95 per cent and 92 per cent effective against hospitalisation with the Delta variant, respectively, according to the ONS.

Omicron: The World Health Organisation’s Dr Mike Ryan has said he expects that vaccinations should continue to work to protect against severe illness as a result of the variant.

There is some early data in a study of people who were double jabbed that Pfizer’s jab is much less effective against Omicron than earlier variants.

Studies on other vaccines’ effectiveness against Omicron have yet to report.

However, researcher Alex Sigal said Omicron’s ability to escape vaccine antibodies is “incomplete” and that the results were “better than I expected of Omicron” - meaning boosters could play a key role in keeping us safe.

And a team studying the effects of third doses said the body’s T cell immune response after a booster shot is such that it may still provide good protection.

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