Severity of omicron variant of concern and effectiveness of vaccine boosters against symptomatic disease in Scotland

Plain English Summary

The Omicron variant of coronavirus (SARS-CoV-2 B1.1.529) was first identified by scientists in South Africa on 9 November 2021. It was labelled as a ‘variant of concern’ (VOC) by the World Health Organization in the same month, and became the dominant variant in Scotland on 17 December 2021. 

Find out more about VOCs [World Health Organization] 

This variant was shown to have many different genetic mutations, compared to previous variants of the virus that causes COVID-19. Experts suggested these mutations may cause changes in:  

• COVID-19 symptoms 

• The severity of the disease 

• How easily the virus spreads between people 

• The virus’ ability to escape immune protection given by vaccines or previous infections.  

Why did we carry out this research? 

Early evidence for Omicron suggested that common symptoms are milder than those seen with Delta for most people. Omicron infections appeared to result fewer people to be admitted to hospital than the Delta variant.  

We wanted to provide evidence on a national scale about the severity of disease in people infected with Omicron, as well as a better understanding of the protection given by COVID-19 vaccines and previous infections. 

These data have been made available to policymakers, and should help healthcare staff and members of the public to make more informed decisions in the Omicron wave of the pandemic. 

What data did we use? 

To carry out this work, we used data from the EAVE II project, which analyses healthcare information for 99% of the Scottish population in a secure database. All the information that could be used to identify a person is removed before we are allowed to analyse the data. 

We looked at data on PCR testing (‘real time polymerase chain reaction’ testing, RT-PCR), vaccination, and factors like age, sex and health history from 1 November to 19 December 2021. 

What is PCR? [National Human Genome Research Institute] 

PCR tests can be used to tell whether someone has a SARS-CoV-2 infection. For UK PCR tests done in the community, swab samples can be tested for the ‘S gene’. This is a section of genetic code that appears in some SARS-CoV-2 virus samples, but not others. 

As part of this work, we showed that S-gene positive and negative samples were very reliable indicators of Delta and Omicron variant infections, respectively. This allowed us to compare the different variants for almost everyone who tested positive in the study period. 

Find out more about EAVE II 

What did we find? 

People who tested positive for SARS-CoV-2 

162,946 positive PCR tests were recorded during the study period. 

We found that half (49.2%) of people who tested positive with the Omicron variant of SARS-CoV-2 were in the 20-39 age bracket. The number of infections with the Delta variant was highest in unvaccinated people, with almost a quarter of infections (23.2%) in children aged 0-11. Most Delta and Omicron infections were in people thought to be at lower risk of becoming very unwell with COVID-19. This includes those with fewer existing illnesses, as well as young people. 

People who were admitted to hospital with COVID-19 

We found that the number of people admitted to hospital with Omicron was about a third of what we would expect if the variant was as likely to result in hospitalisation as Delta. 

In the study period, 896 people were admitted to hospital with COVID-19 within 14 days of a positive test. Almost all of them had a Delta variant infection (95.5%). All of the 45 people who died with COVID-19 during the study period had a Delta variant infection. 

People aged 20-59 with the Omicron variant were admitted to hospital at a lower rate compared to the Delta variant.  

Risk of re-infection with SARS-CoV-2 

We found that a history of SARS-CoV-2 natural infection does not protect people as well, or for as long, against Omicron as it does against Delta.  

People who had a SARS-CoV-2 infection more than 90 days before were 93% less likely to become ill with a Delta variant infection, than those who had never tested positive before. 

However, people with a history of SARS-CoV-2 infection were only 75% less likely to become ill with the Omicron variant 28-90 days later, compared to those who had never tested positive before. This dropped further to 43% after 90 days. 

These first infections could be with the Alpha, Delta or original ‘Wuhan’ variant. 

Effectiveness of a third or booster vaccine dose 

We looked at how effective a third or booster vaccine was against symptomatic COVID-19 caused by the Omicron variant. To do this, we compared people with similar characteristics who had: 

• their second vaccine dose at least 25 weeks before having symptoms 

• a third or booster dose at least two weeks before having symptoms. 

Our analysis showed a 57% reduction in the likelihood of symptomatic Omicron infection for people who had a third or booster dose. This was lower than the reduction for symptomatic Delta infections, at 83% for people aged 16-49 and 88% for people aged 50 or older.   

We took previous positive tests into account when calculating these percentages, as natural infections can also provide some protection against future disease.  

What does this mean? 

Overall, our results indicate that the Omicron variant causes milder disease than Delta, but is also more likely to overcome immune protection offered by vaccination or previous infections. 

The data suggest that people are three times less likely to be admitted to hospital with the Omicron variant than with Delta.  

A third or booster vaccine dose gives people aged 16 or older good protection against the Omicron variant, but better protection against the Delta variant, which is still in circulation. 

Note

This plain English summary was created with the support and feedback of the EAVE II Public Advisory Group (PAG). 

To learn more about the PAG, see: Our EAVE II Public Advisory Group (PAG) | The University of Edinburgh

Pre-print

Results from this study were originally released in a pre-print article.

A full summary of this work is also available on our website.

Read our pre-print article