This brief article is an updated warning about the threat created by the Omicron variant. It is assessed with 15, 22 and 23 December 2021 information. It concerns a series of indications related to the risks of hospitalisation, to vaccination and to incubation period for the Omicron variant.
Since the Omicron variant was identified, early studies are carried out as quickly as possible to allow us understanding the threat better and thus improving our answers.
This warning follows up on the previous one, issued on 18 December, and completes and updates it.
The early assessments that have been recently published and we use here are:
- UK Health Security Agency, SARS-CoV-2 variants of concern and variants under investigation in England, Technical briefing 33, 23 December 2021 – Summary.
- Neil Ferguson, Azra Ghani, et al., “Report 50 – Hospitalisation risk for Omicron cases in England”, Imperial College London, 22 December 2021 – with a Summary by Emily Head, Dr Sabine L. van Elsland, “Some reduction in hospitalisation for Omicron v Delta in England: early analysis“, Ibid,).
- Brandal Lin T., et al.., “Outbreak caused by the SARS-CoV-2 Omicron variant in Norway, November to December 2021“, EuroSurveillance, 2021;26(50): pii=2101147, received: 10 Dec 2021; Accepted: 15 Dec 2021
- For reference – Delta and prior variants:
- Rebecca Grant et al., Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: Results from a nationwide case-control study in France, The Lancet, 25 Nov 2021
- Sheikh A, McMenamin J, et al.., “SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness“, The Lancet. 2021 Jun 26;397(10293):2461-2462. doi: 10.1016/S0140-6736(21)01358-1. Epub 2021 Jun 14. PMID: 34139198; PMCID: PMC8201647.
Other studies are to be expected. Monitoring must of course continue.
Collectively, the very high infectious power of Omicron continues making it a very serious threat to societies.
At individual level, the situation, compared with the Delta variant, may be slightly less dangerous, notably for vaccinated people and only as far as hospitalisations are concerned. Yet, the bad news regarding the booster dose also introduces some new concern. Reduced incubation period and the danger of not understanding how tests function must also be taken into account, especially before Christmas and New Year.
Summarised current findings on hospitalisation
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The two assessments on hospitalisation risks are for the UK, however, considering past waves, they are indications of what could happen elsewhere in the world. We should, however, consider notably previous immunity acquired within the population before applying British findings to other countries.
It should be stressed, furthermore, that these assessments are made on a small number of cases, thus a host of factors could lead to changes. Also, we still do not have any assessment of lethality. We do not know either – and cannot know – about both possible complications created by Omicron after the primary disease nor about the risks involved in terms of Long COVID. Cautiousness thus must remain the norm.
In a nutshell, the British findings are as follows:
Reduced rate of hospitalisation compared with Delta variant
“… the risk of hospital admission for an identified case with Omicron is reduced compared to a case of Delta…UK Health Security Agency, Technical briefing 33, 23 December 2021
An individual with Omicron is estimated to be between 31 and 45% less likely to attend [Accident and Emergency] A&E compared to Delta, and 50 to 70% less likely to be admitted to hospital.”.
Imperial College London estimates are less optimistic, but still encouraging. Always comparing with Delta,
The estimates suggest that Omicron cases have, on average, a 20-25% reduced risk of any hospitalisation and an approximately 40-45% reduced risk of a hospitalisation resulting in a stay of one or more nights.Report 50, Imperial College London, 22 December 2021
The length of stay in hospital also appears to be reduced, however, data are insufficient for any thorough assessment (ICL).
But how dangerous was and still is Delta?
According to the Scottish study,
“Risk of COVID-19 hospital admission was approximately doubled in those with the Delta VOC when compared to the Alpha VOC, with risk of admission particularly increased in those with five or more relevant comorbidities.”Sheikh A, McMenamin J, et al.., “SARS-CoV-2 Delta VOC in Scotland
Thus, if we have around 50% less risk of hospitalisation with Omicron than with Delta, which itself had a risk of hospitalisation that doubled (actually 1.85) compared with Alpha, we are back to a rate of hospitalisation similar to what we had with the Alpha variant. This means the third wave (see Towards a Covid-19 Fifth Wave), but we then need to look at immunity, both natural and related to vaccination. As a reminder there was hardly any vaccination for the third wave.
Previous immunity and Omicron hospitalisations
Imperial College London gives us a more detailed picture of hospitalisation rates for Omicron.
People with no previously acquired immunity
If you have not been vaccinated, nor been previously infected with the SRAS-CoV-2, the prospects are slightly better than with Delta, but not that much:
0-30% reduced risk of any hospitalisation [compared with Delta]Report 50, Imperial College London, 22 December 2021
People with existing immunity
“… the estimated reduction in risk of hospitalisation due to previous infection is… around 55-70% reduction“.Report 50, Imperial College London, 22 December 2021
Vaccination and Omicron
The (relatively) Good
The efficacy of the booster dose is confirmed by the studies.
“Vaccine efficacy analysis continues to show lower effectiveness for symptomatic Omicron disease. There is evidence that protection against symptomatic disease wanes after the second dose of vaccine, and then improves after the booster…. There are insufficient severe cases of Omicron as yet to analyse vaccine effectiveness against hospitalisation, but this is more likely to be sustained, particularly after a booster.”UK Health Security Agency, Technical briefing 33, 23 December 2021
There is however bad news that is broken out by the UK Health Security Agency and that confirms or explains Israel decision to recommend a 4th booster shot:
“…the latest data suggests this extra protection starts to wane more rapidly, being about 15 to 25% lower from 10 weeks after the booster dose.”UK Health Security Agency, Technical briefing 33, 23 December 2021
Thus, after a booster dose, one needs to wait 2 weeks before to be protected against Omicron, and this protection will start diminishing at an unknown rate 10 weeks after the shot, i.e. 2,5 months. As a whole, the booster dose only offers a full protection for 2 months.
“Intriguingly” to use ICL words, and although researchers caution about not relying too much on these findings, it would seem that a two course vaccination with Astrazeneca protects better from infection with Omicron than a two course vaccination with Pfizer or Moderna.
“Hazard ratios for hospital attendance with Omicron for PF/MD are similar to those seen for Delta in those vaccination categories, while Omicron hazard ratios are generally lower than for Delta for the AZ vaccination categories. Given the limited samples sizes to date, we caution about over-interpreting these trends, but they are compatible with previous findings…”Report 50, Imperial College London, 22 December 2021
Some elements on Omicron and incubation period
Incubation is the delay between the moment when an individual is infected and the time when s/he will show symptom – and most of the time become infectious thus dangerous to others.
With the initial strain of the SARS-CoV2, the picture was slightly more complex as people could become contagious before becoming symptomatic (see Dynamics of contagion and the COVID-19 Second Wave). In the very few studies we found on Delta, Omicron and incubation, there is no mention of this factor. Indeed, for example the Nov 2021 study on Delta specifies that
The incubation period was defined as the number of days between the single contact and the onset of symptoms.Grant et al., Impact of SARS-CoV-2 Delta variant on incubation
The Norwegian Christmas Party Story
The story of the Norwegian 2021 Christmas party cluster is worth narrating in full, as it highlights perfectly well the risks that not understanding tests and incubation entail
“The company Christmas party held on 26 November 2021 and that one of the attendees had returned from South Africa on 24 November 2021….Brandal Lin T., et al., “Outbreak caused by the SARS-CoV-2 Omicron variant in Norway, 10 Dec 2021
The closed event was held in a separate room (ca 145 m2) in a restaurant in Oslo from 18:00 to 22:30, after which the venue was opened to the public from 22:30 to 03:00. A pre-party had been arranged for the Christmas party attendees at a separate venue, after which they were transported by private buses to the restaurant. Although there were no restrictions in place for events at the time in Norway, all attendees of the party were reported to be fully vaccinated and had been asked by the organiser to perform a rapid antigen self-test….”
On 1st December the attendee having been in South Africa tested positive. Out of the 117 people attending the Christmas party, 110 accepted to be surveyed. Out of the 110 attendees, by 13 December, 66 were confirmed cases and 15 probable cases. Hence, the attack rate of Omicron was estimated to be 81/110 = 78%. The average age of the attendees was 38 years old.
Thus, to summarise, 117 people think they are very careful and all healthy, and, as a result gather happily at an event. Then, reality strikes, 81 of them are finally infected with Omicron within three weeks.
On the bright side, none of these people needed hospitalisation by 13 December 2021. Of course, this does not mean that they did not then lead to chains of contamination that triggered hospitalisation. This does not mean either that all similar contaminations would never imply hospitalisation.
Tests only protect others … when they are positive
What does the above story highlights?
Tests only protect others … when they are positive. If you test positive, then you know you are infectious and can contaminate others. Thus you isolate, and start caring about yourself. It is absolutely critical to use tests to identify and then limit contamination as early as possible.
However, if you test is negative, then this only means that at the time of testing you were not infectious. But you can very well be in one hour, two hours or six or whenever until the end of the longest possible incubation period. Ideally, if you want to be really sure and really protect others, you would need to permanently test yourself. The device we would need would be more akin to heart monitoring than to COVID tests.
Omicron and incubation
Omicron possible incubation period
Out of Norway’s study, we have first elements regarding the incubation for Omicron.
The incubation period for symptomatic cases ranged from 0 to 8 days with a median of 3 days (interquartile range: 3−4).Brandal Lin T., et al., “Outbreak caused by the SARS-CoV-2 Omicron variant in Norway, 10 Dec 2021
Comparison with Delta and prior variants
The incubation period for Delta was estimated to be 4,3 days and for prior variants (original, Alpha, Beta, etc.) to be 5 days.
Finally, we found that the mean incubation period was shorter for Delta compared to non-Delta infections (4.3 and 5.0 days, respectively)….Grant et al., Impact of SARS-CoV-2 Delta variant on incubation
We calculated the incubation period to be shorter for Delta (mean (SD) = 4.3 (2.4) days; median (IQR)= 4 (3-5)), compared to non-Delta infections (mean (SD) = 5.0 (2.4) days; median (IQR)= 5 (3-7)) (P < 0.001). Among non-Delta infections, the mean (SD) incubation time was 5.0 (2.3) days for Alpha, median (IQR)= 5 (3-7); 5.1 (2.7) for Beta/Gamma median (IQR)= 5 (3-7); and 5.1 (2.5) for non-VOC median (IQR)= 5 (3-7).
As the time for incubation is reduced, then it is much more difficult to stop contamination. Indeed, the human process that have been implemented will likely be slower than the spread of the infection.
The good news may be that if the longest length of the incubation period is confirmed to remain at 8 days and if there are no outliers, then possible quarantines can also be shortened. This is probably what we see happening, for example in England and probably in France. However, note that the scientific data upon which to ground this decision are still very sparse. The necessity to keep the various flows of a country going are also a major consideration in the decision regarding length of isolation period.
At the individual level, as long as there is no certainty regarding the length of the incubation period, even if you are authorised to end your isolation period, it would be safer for your loved ones and in general for people you will meet, if you were wearing a N95/FFP2 face mask and anyway reducing as much as possible contacts.