New data from Imperial College London has found “no evidence” the Omicron variant generates less severe disease compared to prior SARS-CoV-2 variants. An analysis of an Omicron super spreader event in Norway is also offering crucial insights into the characteristics of this new variant and its ability to breakthrough pre-existing immunity.
The latest findings come from Imperial College’s COVID-19 Response Team, a collection of researchers who have been offering real-time modeling on the spread of SARS-CoV-2 since the beginning of the pandemic. Confirming the results of a recent Oxford lab study indicating two vaccine doses are not sufficient to protect from Omicron infection, the new real-world data found the new variant significantly evades immunity generated by both vaccine and previous infection.
Tracking around two weeks of case data in the UK the report estimates Omicron is linked to a 5.4 fold higher risk of reinfecting those who previously caught the virus compared to the Delta variant’s rate of reinfection. Looking at vaccine effectiveness the report estimates two vaccine doses offer between zero and 20 percent protection from symptomatic Omicron infection.
“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination,” says Neil Ferguson, one of the report’s authors. “This level of immune evasion means that Omicron poses a major, imminent threat to public health.”
While this new data on Omicron breaking through pre-existing immunity to SARS-CoV-2 is backed up by a growing body of evidence from around the world, it is still unclear exactly how severe the disease caused by this variant will be. Although preliminary signs from the first Omicron wave in South Africa offered indications this variant may be mild compared to previous variants, the new UK data suggests there is “no evidence” this is true.
“We find no evidence (for both risk of hospitalization attendance and symptom status) of Omicron having different severity from Delta, though data on hospitalizations are still very limited,” the researchers state in the report.
The huge challenge researchers currently face in ascertaining the severity of disease caused by Omicron is that this new variant is emerging into a world with fundamentally different levels of pre-existing population immunity compared to prior variants. It took scientists several months, for example, to establish clearly that the Delta variant caused more severe disease compared to earlier strains of SARS-CoV-2. And even then, Delta didn’t lead to greater peaks in mortality compared to prior variants because of previous infections and vaccine-induced immunity.
A recently published working paper from infectious disease experts Roby Bhattacharyya and William Hanage outlines the challenges in estimating Omicron’s severity when comparing current rates of hospitalizations and death to points earlier in the pandemic. The duo explain that because of prior infections and vaccination rates, it is likely the severity of Omicron will be “systemically underestimated.”
“While the lower observed IFR [infection fatality rate] in the early weeks of the Omicron wave in South Africa is better than the alternative, the most likely explanation lies in increased immunity among those being infected; more time and careful comparisons controlling for age, prior immunity, detection bias, lag period, hospital capacity, and numerous other factors will be required to infer anything about intrinsic virulence,” write Bhattacharyya and Hanage. “Our collective intuition on how population-level IFR relates to intrinsic severity of a variant needs to be recalibrated over time as immunity accrues, and far more so with a variant as immune-evasive as Omicron.”
Norway event highlights how fast Omicron can spread
Although the Omicron disease severity question may take a few more weeks to answer, investigations into early Omicron super spreader events are offering crucial insights into the characteristics of this new variant. A new study published in the journal Eurosurveillance has analyzed a massive infection event that took place in late November at a Christmas party in Norway.
In total, 117 people attended the event, including the index case who had recently returned from a trip to South Africa. Most attendees were two-dose vaccinated and had a confirmed negative PCR or rapid test within two days of the event.
By December 13, 81 people at the event had tested positive for SARS-CoV-2. The majority of those cases had started to develop symptoms within three days of the event, suggesting Omicron has a faster average incubation period than prior variants.
Only one of the positive cases was asymptomatic, which is another unusual characteristic of Omicron. The most common symptom reported was a cough, with stuffy nose, fatigue and sore throat appearing as common in most of the infected cases.
Up to December 13, none of the confirmed cases required hospitalization. Around half of the positive cases self-reported their symptoms as three on a severity scale of zero to five.
An early statement from the Norwegian Institute of Public Health (NIPH) noted the average age of the cohort was late 30s, so these findings cannot offer insights into the broader severity of Omicron infections in more vulnerable or elderly populations. The main takeaway from the analysis is just how transmissible Omicron is and how easily it can infect those with two doses of a COVID-19 vaccine.
“Even though most of the cases have not had a severe disease course at this time, almost all developed symptoms relatively quickly after the Christmas party,” the NIPH reported in a recent statement. “The attendees were young and fully vaccinated and would not typically develop serious illness after SARS-CoV-2 infection. It is therefore difficult to comment on the severity of the disease with this variant based on these preliminary results.”
Alexandra Hogan, an infectious disease researcher from Imperial College London, says the emerging data affirms the importance of three vaccine doses to protect against the spread of Omicron. While it will be some time before Omicron disease severity is understood, Hogan says it is clear Omicron will spread quickly in both unvaccinated and two-dose vaccinated populations over the coming weeks.
“While data are still emerging, the rapid spread of the Omicron variant is highly concerning,” says Hogan. “Substantial increases in infections and cases are predicted in the coming weeks, both in countries with ongoing virus circulation, and in those settings that have previously suppressed transmission and are now lifting restrictions.”
The new Imperial College report can be found here.
Source: Imperial College London
