Variant watch: Hospital risk for Omicron 40-45% lower, says UK study
The findings are the first encouraging scientific evidence linked to the variant of concern (VOC) discovered last month when it started tearing through parts of South Africa at a rate not seen with any other Sars-Cov-2 variant.
The Omicron variant is less likely to lead to severe disease in people who have taken vaccines or had Covid-19 in the past, two studies from the UK have said.
Taken together with findings from South Africa – all three studies were released on Wednesday – there is now sound scientific basis to conclude the variant is less virulent than others, especially Delta, which caused a devastating outbreak in India last summer and sparked new waves in other countries.
The findings are the first encouraging scientific evidence linked to the variant of concern (VOC) discovered last month when it started tearing through parts of South Africa at a rate not seen with any other Sars-Cov-2 variant. Scientists soon discovered it was also the most resistant configuration of the coronavirus, leading to higher odds of repeat and vaccine breakthrough infections.
If the Omicron variant was to be as virulent, or more, than Delta, the implications would have been dire, although its high transmissibility and resistance still pose a threat.
“Our analysis shows evidence of a moderate reduction in the risk of hospitalisation associated with the Omicron variant compared with the Delta variant. However, this appears to be offset by the reduced efficacy of vaccines against infection with the Omicron variant. Given the high transmissibility of the Omicron virus, there remains the potential for health services to face increasing demand if Omicron cases continue to grow at the rate that has been seen in recent weeks,” said professor Neil Ferguson of Imperial College London, which released the analysis of Omicron and Delta cases in England.
Two UK studies, similar severity findings
The Imperial College study included all RT-PCR-confirmed Covid-19 cases recorded between December 1-14 in England. It found that Omicron cases have, on average, a 15-20% reduced risk of needing to visit a hospital (the lowest level of severity) and an approximately 40-45% reduced risk of a hospitalisation resulting in a stay of one or more nights.
It also found that a past infection offered approximately a 50-70% reduction in hospitalisation risk compared. All of these comparisons were made against risks of hospitalisation seen with the Delta variant.
The researchers estimate that in unvaccinated people being infected for the first time, the risk of hospitalisation may be lowered by 0-30%, suggesting the severity in completely immune-naive people may not be very different from those who had a Delta infection for the first time, without any vaccine.
The other UK study was from Scotland. Although based on a small number of hospitalisations, the study made similar findings: those with Omicron infections were 68% less likely to need hospitalisation compared to people infected with the Delta variant.
Both reports, as well as the South African study, are yet to be peer-reviewed.
The Imperial College researchers also said in their study that Omicron infections in people with vaccination may be even less likely to require ICU admission or lead to death when compared to Delta variant, “given that remaining immune protection against more severe outcomes of infection are expected to be much higher than those against milder endpoints”.
Need for vaccines, boosters
The detailed findings corroborate lab studies that show people with booster doses have a more adequate immune response to counter the Omicron variant. In their real-world analysis, the Scotland report found a 57% reduction in the risk of symptomatic infection in people who were infected with the VOC compared to those who just had two doses at least 25 weeks prior.
The detailed Imperial College findings made similar findings. For instance, people with two doses of the AstraZeneca vaccine (used in India as the Covishield vaccine), had a higher risk ratio of 0.37 than those with three doses of the same vaccine (0.21). These risk ratios mean two doses reduced the risk of hospitalisation by 63% while three doses cut it by 79%.
Crucially, the report added, people who took the AstraZeneca vaccine had a lower risk in needing to visit a hospital if infected by the Omicron variant when compared to the equivalent risk in the case of a Delta variant infection. In the case of Pfizer-BioNTech or Moderna vaccines, the likelihood of requiring a hospital visit – defined as the lowest level of severity – were similar between Omicron and Delta infections.
The report also stressed on the need to vaccinate the unvaccinated, especially those who did not have a past infection. “The proportion of unvaccinated individuals infected is likely to be substantially higher. In that context, our finding that a previous infection reduces the risk of any hospitalisation by approximately 50% and the risk of a hospital stay of 1+ days by 61% (before adjustments for under ascertainment of reinfections) is significant,” the report said.