Just over a month since its discovery, we are still learning the real risks of the Omicron variant of SARS-CoV-2, the virus that causes COVID-19. The latest report from the Imperial College London COVID-19 response team suggests that the risk of reinfection from the new variant is 5.4 times higher than with Delta.  

Previous work had estimated that a prior infection gave an 85 percent protection against a second COVID infection from Delta over 6 months. This was estimated in healthcare workers. For Omicron, the value is 19 percent. The report is yet to be peer-reviewed. 

“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination. This level of immune evasion means that Omicron poses a major, imminent threat to public health,” Prof Neil Ferguson from Imperial College London said in a statement.  

The team also looked at the infection risk from the Omicron variant in people with two doses of the vaccine and in those who have received the booster. The data was collected by the U.K. Health Security Agency and the U.K. health service between November 29 and December 11. In people that received the standard vaccination the efficacy against symptomatic COVID appears to be at most 20 percent. Following the booster, the efficacy sits between 55 and 80 percent.  

“Quantifying reinfection risk and vaccine effectiveness against Omicron is essential for modelling the likely future trajectory of the Omicron wave and the potential impact of vaccination and other public health interventions,” Prof Azra Ghani from Imperial College London explained.  

The study also suggests that there is not much difference in severity between the Omicron and the Delta variant, but the team stressed that the data remains limited, especially with regards to hospitalization. The situation ought to be taken seriously. 

“The message from this study is clear – don’t think that if you’ve caught COVID before, you can’t fall ill with it again.  This preprint backs up previous studies that indicate reinfections are much more common with the Omicron variant as it appears to be able to more easily evade any prior immunity gained,” Prof Deborah Dunn-Walters, Chair of the British Society for Immunology COVID-19 Taskforce and Professor of Immunology at the University of Surrey, who was not involved in the study said in a statement.  

“Vaccination provides you with a safe and effective way to boost your immunity levels to COVID-19, and in particular to the Omicron variant, without falling ill with the disease.  This booster dose is particularly vital to get now as this research shows that three doses provide much better protection against Omicron than two vaccine doses alone.