First Omicron Report of A Real World Infection: Highly Contagious with Flu-Like Symptoms

The SARS-CoV-2 Omicron variant is setting off a new wave of COVID-19 outbreaks around the world. Epidemiological studies are the basis for revealing the infectivity, immune escape and disease severity of Omicron variant, as well as the key to prevention and vaccination deployment.
On Dec.16th, 2021, Danish researchers published a paper of Epidemiological characterisation of the first 785 SARS-CoV-2 Omicron variant cases in Denmark, December 2021, and compared with those who infected with the Delta variant during the same period, which provided important information.
Two Omicron variant cases were firstly detected in Denmark on Nov.28th, 2021 in travellers returning from South Africa. Till Dec.9th,2021, 785 cases were registered in total.
Studies in Denmark show that Omicron cases in the country have increased by 40% daily since Dec. 4th. That means the data doubling in less than two days.

Figure 1. Sample date for SARS-CoV-2 Omicron cases by suspected place of infection, Denmark, 22 November–7 December 2021, data as at 9 December 2021 (n=785)

1. Demographic characteristics of infected cases
The ages of those infected ranged from 2 to 95, with a median age of 32;
55% were male;
77% had no travel history, indicating that the study group was closer to community infections.

2. Vaccination history and previous SARS-CoV-2 infection history of Omicron infected cases
A total of 83.1% of Omicron infected cases received either full or booster vaccination, with 76% receiving two doses and 7.1% receiving a third dose. Another 4.3% have previously infected with SARS-CoV-2.
Of those infected with Delta, only 53.2% infected cases received either full or booster vaccination. Only 0.9% have previously infected with SARS-CoV-2.

3. Condition of Omicron infected cases
Of those infected with Omicron who reported symptoms, 73% were symptomatic and 27% were asymptomatic.
Of those infected with Omicron, 1.2% were hospitalized, compared with 1.5% of those infected with Delta.
Among Omicron variant cases, 0.13% were in ICU, compared with 0.11% for Delta variant cases.
There were no deaths in Omicron variant cases, compared with 0.07% in Delta variant cases.

This is the first epidemiological study of the Omicron variant, and although the sample size is small, the tests done in Denmark are so well completed that they still provide crucial information.

This study continues to prove that:
1. Omicron variant has higher immune escape than Delta variant
83.1% of those infected with Omicron had received two or more doses of vaccine.
But for the first time, we can see that 50% of those infected with the Delta variant received more than two doses of vaccine.
While in Denmark, where 77.5% of the population is fully vaccinated, the spread of Omicron variant has not been interrupted.

2. Omicron variant is highly contagious
As this article and image showed above, once Omicron spreads, it doubles in less than two days.

3. 1.2% of Omicron variant cases were hospitalized and 0.13% required ICU admission.
So the symptoms of omicron-infected people in the real world are really mild.
But is it weaker than the previous strain?
Only 1.5% of delta-infected cases were hospitalized and 0.11% required ICU admission.
Statistical comparison was not made in this paper, and we suspect that the author was concerned about the unreliable conclusion due to the small sample size.
However, this result has shown that Omicron and Delta variant infected cases in this group have similar severity of disease; But don’t forget that the Omicron group had a higher percentage of vaccinated and previously infected people.
In fact, vaccination has led to generally milder cases of COVID-19 in the real world.

4. Do we have reason to relax?
By seeing this result, we may feel more emboldened to celebrate Christmas outside with family. But don’t forget the preconditions are full or booster vaccination, and wear masks and keep a distance.

More critically, according to one more paper published in Norway, the common symptoms of Omicron variant cases are completely different after vaccination. The most common symptoms were cough (83%), runny nose (78%), fatigue (74%), sore throat (72%) and headache (68%).

Table.Symptoms reported between 19 November and 3 December, by case status, in SARS-CoV-2 Omicron variant outbreak at Christmas party, Oslo, 26 November 2021 (n=110)

See, is it different from flu?

For vaccinators, it will undoubtedly reduce the attention to see a doctor after getting infected Omicron because of the not very different symptoms from flu, and make the epidemic prevention more difficult.

GENESIS provides Flu&COVID-19 Antigen Duo Rapid Test, which is intended for the simultaneous qualitative detection and differentiation of the nucleocapsid protein antigens from SARS-CoV-2, influenza A and influenza B in direct nasal specimens. Since the symptoms of Omicron infection are similar to flu, the product is designed to help people to know the infection symptoms are due to COVID-19 or flu virus in primary stage.

If you need to read the full text, refer to the two text links in the references. 

References:

1. https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2021.26.50.2101146#html_fulltext

2. https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2021.26.50.2101147#f1